tor-browser

pngrutil.c (162313B)

---

/* pngrutil.c - utilities to read a PNG file
*
* Copyright (c) 2018-2025 Cosmin Truta
* Copyright (c) 1998-2002,2004,2006-2018 Glenn Randers-Pehrson
* Copyright (c) 1996-1997 Andreas Dilger
* Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.
*
* This code is released under the libpng license.
* For conditions of distribution and use, see the disclaimer
* and license in png.h
*
* This file contains routines that are only called from within
* libpng itself during the course of reading an image.
*/

#include "pngpriv.h"

#ifdef PNG_READ_SUPPORTED

/* The minimum 'zlib' stream is assumed to be just the 2 byte header, 5 bytes
* minimum 'deflate' stream, and the 4 byte checksum.
*/
#define LZ77Min  (2U+5U+4U)

#ifdef PNG_READ_INTERLACING_SUPPORTED
/* Arrays to facilitate interlacing - use pass (0 - 6) as index. */

/* Start of interlace block */
static const png_byte png_pass_start[7] = {0, 4, 0, 2, 0, 1, 0};
/* Offset to next interlace block */
static const png_byte png_pass_inc[7] = {8, 8, 4, 4, 2, 2, 1};
/* Start of interlace block in the y direction */
static const png_byte png_pass_ystart[7] = {0, 0, 4, 0, 2, 0, 1};
/* Offset to next interlace block in the y direction */
static const png_byte png_pass_yinc[7] = {8, 8, 8, 4, 4, 2, 2};

/* TODO: Move these arrays to a common utility module to avoid duplication. */
#endif

png_uint_32 PNGAPI
png_get_uint_31(png_const_structrp png_ptr, png_const_bytep buf)
{
  png_uint_32 uval = png_get_uint_32(buf);

  if (uval > PNG_UINT_31_MAX)
     png_error(png_ptr, "PNG unsigned integer out of range");

  return uval;
}

#ifdef PNG_READ_INT_FUNCTIONS_SUPPORTED
/* NOTE: the read macros will obscure these definitions, so that if
* PNG_USE_READ_MACROS is set the library will not use them internally,
* but the APIs will still be available externally.
*
* The parentheses around "PNGAPI function_name" in the following three
* functions are necessary because they allow the macros to co-exist with
* these (unused but exported) functions.
*/

/* Grab an unsigned 32-bit integer from a buffer in big-endian format. */
png_uint_32 (PNGAPI
png_get_uint_32)(png_const_bytep buf)
{
  png_uint_32 uval =
      ((png_uint_32)(*(buf    )) << 24) +
      ((png_uint_32)(*(buf + 1)) << 16) +
      ((png_uint_32)(*(buf + 2)) <<  8) +
      ((png_uint_32)(*(buf + 3))      ) ;

  return uval;
}

/* Grab a signed 32-bit integer from a buffer in big-endian format.  The
* data is stored in the PNG file in two's complement format and there
* is no guarantee that a 'png_int_32' is exactly 32 bits, therefore
* the following code does a two's complement to native conversion.
*/
png_int_32 (PNGAPI
png_get_int_32)(png_const_bytep buf)
{
  png_uint_32 uval = png_get_uint_32(buf);
  if ((uval & 0x80000000) == 0) /* non-negative */
     return (png_int_32)uval;

  uval = (uval ^ 0xffffffff) + 1;  /* 2's complement: -x = ~x+1 */
  if ((uval & 0x80000000) == 0) /* no overflow */
     return -(png_int_32)uval;
  /* The following has to be safe; this function only gets called on PNG data
   * and if we get here that data is invalid.  0 is the most safe value and
   * if not then an attacker would surely just generate a PNG with 0 instead.
   */
  return 0;
}

/* Grab an unsigned 16-bit integer from a buffer in big-endian format. */
png_uint_16 (PNGAPI
png_get_uint_16)(png_const_bytep buf)
{
  /* ANSI-C requires an int value to accommodate at least 16 bits so this
   * works and allows the compiler not to worry about possible narrowing
   * on 32-bit systems.  (Pre-ANSI systems did not make integers smaller
   * than 16 bits either.)
   */
  unsigned int val =
      ((unsigned int)(*buf) << 8) +
      ((unsigned int)(*(buf + 1)));

  return (png_uint_16)val;
}

#endif /* READ_INT_FUNCTIONS */

/* Read and check the PNG file signature */
void /* PRIVATE */
png_read_sig(png_structrp png_ptr, png_inforp info_ptr)
{
  size_t num_checked, num_to_check;

  /* Exit if the user application does not expect a signature. */
  if (png_ptr->sig_bytes >= 8)
     return;

  num_checked = png_ptr->sig_bytes;
  num_to_check = 8 - num_checked;

#ifdef PNG_IO_STATE_SUPPORTED
  png_ptr->io_state = PNG_IO_READING | PNG_IO_SIGNATURE;
#endif

  /* The signature must be serialized in a single I/O call. */
  png_read_data(png_ptr, &(info_ptr->signature[num_checked]), num_to_check);
  png_ptr->sig_bytes = 8;

  if (png_sig_cmp(info_ptr->signature, num_checked, num_to_check) != 0)
  {
     if (num_checked < 4 &&
         png_sig_cmp(info_ptr->signature, num_checked, num_to_check - 4) != 0)
        png_error(png_ptr, "Not a PNG file");
     else
        png_error(png_ptr, "PNG file corrupted by ASCII conversion");
  }
  if (num_checked < 3)
     png_ptr->mode |= PNG_HAVE_PNG_SIGNATURE;
}

/* This function is called to verify that a chunk name is valid.
* Do this using the bit-whacking approach from contrib/tools/pngfix.c
*
* Copied from libpng 1.7.
*/
static int
check_chunk_name(png_uint_32 name)
{
  png_uint_32 t;

  /* Remove bit 5 from all but the reserved byte; this means
   * every 8-bit unit must be in the range 65-90 to be valid.
   * So bit 5 must be zero, bit 6 must be set and bit 7 zero.
   */
  name &= ~PNG_U32(32,32,0,32);
  t = (name & ~0x1f1f1f1fU) ^ 0x40404040U;

  /* Subtract 65 for each 8-bit quantity, this must not
   * overflow and each byte must then be in the range 0-25.
   */
  name -= PNG_U32(65,65,65,65);
  t |= name;

  /* Subtract 26, handling the overflow which should set the
   * top three bits of each byte.
   */
  name -= PNG_U32(25,25,25,26);
  t |= ~name;

  return (t & 0xe0e0e0e0U) == 0U;
}

/* Read the chunk header (length + type name).
* Put the type name into png_ptr->chunk_name, and return the length.
*/
png_uint_32 /* PRIVATE */
png_read_chunk_header(png_structrp png_ptr)
{
  png_byte buf[8];
  png_uint_32 chunk_name, length;

#ifdef PNG_IO_STATE_SUPPORTED
  png_ptr->io_state = PNG_IO_READING | PNG_IO_CHUNK_HDR;
#endif

  /* Read the length and the chunk name.  png_struct::chunk_name is immediately
   * updated even if they are detectably wrong.  This aids error message
   * handling by allowing png_chunk_error to be used.
   */
  png_read_data(png_ptr, buf, 8);
  length = png_get_uint_31(png_ptr, buf);
  png_ptr->chunk_name = chunk_name = PNG_CHUNK_FROM_STRING(buf+4);

  /* Reset the crc and run it over the chunk name. */
  png_reset_crc(png_ptr);
  png_calculate_crc(png_ptr, buf + 4, 4);

  png_debug2(0, "Reading chunk typeid = 0x%lx, length = %lu",
      (unsigned long)png_ptr->chunk_name, (unsigned long)length);

  /* Sanity check the length (first by <= 0x80) and the chunk name.  An error
   * here indicates a broken stream and libpng has no recovery from this.
   */
  if (buf[0] >= 0x80U)
     png_chunk_error(png_ptr, "bad header (invalid length)");

  /* Check to see if chunk name is valid. */
  if (!check_chunk_name(chunk_name))
     png_chunk_error(png_ptr, "bad header (invalid type)");

#ifdef PNG_IO_STATE_SUPPORTED
  png_ptr->io_state = PNG_IO_READING | PNG_IO_CHUNK_DATA;
#endif

  return length;
}

/* Read data, and (optionally) run it through the CRC. */
void /* PRIVATE */
png_crc_read(png_structrp png_ptr, png_bytep buf, png_uint_32 length)
{
  if (png_ptr == NULL)
     return;

  png_read_data(png_ptr, buf, length);
  png_calculate_crc(png_ptr, buf, length);
}

/* Compare the CRC stored in the PNG file with that calculated by libpng from
* the data it has read thus far.
*/
static int
png_crc_error(png_structrp png_ptr, int handle_as_ancillary)
{
  png_byte crc_bytes[4];
  png_uint_32 crc;
  int need_crc = 1;

  /* There are four flags two for ancillary and two for critical chunks.  The
   * default setting of these flags is all zero.
   *
   * PNG_FLAG_CRC_ANCILLARY_USE
   * PNG_FLAG_CRC_ANCILLARY_NOWARN
   *  USE+NOWARN: no CRC calculation (implemented here), else;
   *  NOWARN:     png_chunk_error on error (implemented in png_crc_finish)
   *  else:       png_chunk_warning on error (implemented in png_crc_finish)
   *              This is the default.
   *
   *    I.e. NOWARN without USE produces png_chunk_error.  The default setting
   *    where neither are set does the same thing.
   *
   * PNG_FLAG_CRC_CRITICAL_USE
   * PNG_FLAG_CRC_CRITICAL_IGNORE
   *  IGNORE: no CRC calculation (implemented here), else;
   *  USE:    png_chunk_warning on error (implemented in png_crc_finish)
   *  else:   png_chunk_error on error (implemented in png_crc_finish)
   *          This is the default.
   *
   * This arose because of original mis-implementation and has persisted for
   * compatibility reasons.
   *
   * TODO: the flag names are internal so maybe this can be changed to
   * something comprehensible.
   */
  if (handle_as_ancillary || PNG_CHUNK_ANCILLARY(png_ptr->chunk_name) != 0)
  {
     if ((png_ptr->flags & PNG_FLAG_CRC_ANCILLARY_MASK) ==
         (PNG_FLAG_CRC_ANCILLARY_USE | PNG_FLAG_CRC_ANCILLARY_NOWARN))
        need_crc = 0;
  }

  else /* critical */
  {
     if ((png_ptr->flags & PNG_FLAG_CRC_CRITICAL_IGNORE) != 0)
        need_crc = 0;
  }

#ifdef PNG_IO_STATE_SUPPORTED
  png_ptr->io_state = PNG_IO_READING | PNG_IO_CHUNK_CRC;
#endif

  /* The chunk CRC must be serialized in a single I/O call. */
  png_read_data(png_ptr, crc_bytes, 4);

  if (need_crc != 0)
  {
     crc = png_get_uint_32(crc_bytes);
     return crc != png_ptr->crc;
  }

  else
     return 0;
}

/* Optionally skip data and then check the CRC.  Depending on whether we
* are reading an ancillary or critical chunk, and how the program has set
* things up, we may calculate the CRC on the data and print a message.
* Returns '1' if there was a CRC error, '0' otherwise.
*
* There is one public version which is used in most places and another which
* takes the value for the 'critical' flag to check.  This allows PLTE and IEND
* handling code to ignore the CRC error and removes some confusing code
* duplication.
*/
static int
png_crc_finish_critical(png_structrp png_ptr, png_uint_32 skip,
     int handle_as_ancillary)
{
  /* The size of the local buffer for inflate is a good guess as to a
   * reasonable size to use for buffering reads from the application.
   */
  while (skip > 0)
  {
     png_uint_32 len;
     png_byte tmpbuf[PNG_INFLATE_BUF_SIZE];

     len = (sizeof tmpbuf);
     if (len > skip)
        len = skip;
     skip -= len;

     png_crc_read(png_ptr, tmpbuf, len);
  }

  /* If 'handle_as_ancillary' has been requested and this is a critical chunk
   * but PNG_FLAG_CRC_CRITICAL_IGNORE was set then png_read_crc did not, in
   * fact, calculate the CRC so the ANCILLARY settings should not be used
   * instead.
   */
  if (handle_as_ancillary &&
      (png_ptr->flags & PNG_FLAG_CRC_CRITICAL_IGNORE) != 0)
     handle_as_ancillary = 0;

  /* TODO: this might be more comprehensible if png_crc_error was inlined here.
   */
  if (png_crc_error(png_ptr, handle_as_ancillary) != 0)
  {
     /* See above for the explanation of how the flags work. */
     if (handle_as_ancillary || PNG_CHUNK_ANCILLARY(png_ptr->chunk_name) != 0 ?
         (png_ptr->flags & PNG_FLAG_CRC_ANCILLARY_NOWARN) == 0 :
         (png_ptr->flags & PNG_FLAG_CRC_CRITICAL_USE) != 0)
        png_chunk_warning(png_ptr, "CRC error");

     else
        png_chunk_error(png_ptr, "CRC error");

     return 1;
  }

  return 0;
}

int /* PRIVATE */
png_crc_finish(png_structrp png_ptr, png_uint_32 skip)
{
  return png_crc_finish_critical(png_ptr, skip, 0/*critical handling*/);
}

#if defined(PNG_READ_iCCP_SUPPORTED) || defined(PNG_READ_iTXt_SUPPORTED) ||\
   defined(PNG_READ_pCAL_SUPPORTED) || defined(PNG_READ_sCAL_SUPPORTED) ||\
   defined(PNG_READ_sPLT_SUPPORTED) || defined(PNG_READ_tEXt_SUPPORTED) ||\
   defined(PNG_READ_zTXt_SUPPORTED) || defined(PNG_READ_eXIf_SUPPORTED) ||\
   defined(PNG_SEQUENTIAL_READ_SUPPORTED)
/* Manage the read buffer; this simply reallocates the buffer if it is not small
* enough (or if it is not allocated).  The routine returns a pointer to the
* buffer; if an error occurs and 'warn' is set the routine returns NULL, else
* it will call png_error on failure.
*/
static png_bytep
png_read_buffer(png_structrp png_ptr, png_alloc_size_t new_size)
{
  png_bytep buffer = png_ptr->read_buffer;

  if (new_size > png_chunk_max(png_ptr)) return NULL;

  if (buffer != NULL && new_size > png_ptr->read_buffer_size)
  {
     png_ptr->read_buffer = NULL;
     png_ptr->read_buffer_size = 0;
     png_free(png_ptr, buffer);
     buffer = NULL;
  }

  if (buffer == NULL)
  {
     buffer = png_voidcast(png_bytep, png_malloc_base(png_ptr, new_size));

     if (buffer != NULL)
     {
#        ifndef PNG_NO_MEMZERO /* for detecting UIM bugs **only** */
           memset(buffer, 0, new_size); /* just in case */
#        endif
        png_ptr->read_buffer = buffer;
        png_ptr->read_buffer_size = new_size;
     }
  }

  return buffer;
}
#endif /* READ_iCCP|iTXt|pCAL|sCAL|sPLT|tEXt|zTXt|eXIf|SEQUENTIAL_READ */

/* png_inflate_claim: claim the zstream for some nefarious purpose that involves
* decompression.  Returns Z_OK on success, else a zlib error code.  It checks
* the owner but, in final release builds, just issues a warning if some other
* chunk apparently owns the stream.  Prior to release it does a png_error.
*/
static int
png_inflate_claim(png_structrp png_ptr, png_uint_32 owner)
{
  if (png_ptr->zowner != 0)
  {
     char msg[64];

     PNG_STRING_FROM_CHUNK(msg, png_ptr->zowner);
     /* So the message that results is "<chunk> using zstream"; this is an
      * internal error, but is very useful for debugging.  i18n requirements
      * are minimal.
      */
     (void)png_safecat(msg, (sizeof msg), 4, " using zstream");
#if PNG_RELEASE_BUILD
     png_chunk_warning(png_ptr, msg);
     png_ptr->zowner = 0;
#else
     png_chunk_error(png_ptr, msg);
#endif
  }

  /* Implementation note: unlike 'png_deflate_claim' this internal function
   * does not take the size of the data as an argument.  Some efficiency could
   * be gained by using this when it is known *if* the zlib stream itself does
   * not record the number; however, this is an illusion: the original writer
   * of the PNG may have selected a lower window size, and we really must
   * follow that because, for systems with with limited capabilities, we
   * would otherwise reject the application's attempts to use a smaller window
   * size (zlib doesn't have an interface to say "this or lower"!).
   *
   * inflateReset2 was added to zlib 1.2.4; before this the window could not be
   * reset, therefore it is necessary to always allocate the maximum window
   * size with earlier zlibs just in case later compressed chunks need it.
   */
  {
     int ret; /* zlib return code */
#if ZLIB_VERNUM >= 0x1240
     int window_bits = 0;

# if defined(PNG_SET_OPTION_SUPPORTED) && defined(PNG_MAXIMUM_INFLATE_WINDOW)
     if (((png_ptr->options >> PNG_MAXIMUM_INFLATE_WINDOW) & 3) ==
         PNG_OPTION_ON)
     {
        window_bits = 15;
        png_ptr->zstream_start = 0; /* fixed window size */
     }

     else
     {
        png_ptr->zstream_start = 1;
     }
# endif

#endif /* ZLIB_VERNUM >= 0x1240 */

     /* Set this for safety, just in case the previous owner left pointers to
      * memory allocations.
      */
     png_ptr->zstream.next_in = NULL;
     png_ptr->zstream.avail_in = 0;
     png_ptr->zstream.next_out = NULL;
     png_ptr->zstream.avail_out = 0;

     if ((png_ptr->flags & PNG_FLAG_ZSTREAM_INITIALIZED) != 0)
     {
#if ZLIB_VERNUM >= 0x1240
        ret = inflateReset2(&png_ptr->zstream, window_bits);
#else
        ret = inflateReset(&png_ptr->zstream);
#endif
     }

     else
     {
#if ZLIB_VERNUM >= 0x1240
        ret = inflateInit2(&png_ptr->zstream, window_bits);
#else
        ret = inflateInit(&png_ptr->zstream);
#endif

        if (ret == Z_OK)
           png_ptr->flags |= PNG_FLAG_ZSTREAM_INITIALIZED;
     }

#ifdef PNG_DISABLE_ADLER32_CHECK_SUPPORTED
     if (((png_ptr->options >> PNG_IGNORE_ADLER32) & 3) == PNG_OPTION_ON)
        /* Turn off validation of the ADLER32 checksum in IDAT chunks */
        ret = inflateValidate(&png_ptr->zstream, 0);
#endif

     if (ret == Z_OK)
        png_ptr->zowner = owner;

     else
        png_zstream_error(png_ptr, ret);

     return ret;
  }

#ifdef window_bits
# undef window_bits
#endif
}

#if ZLIB_VERNUM >= 0x1240
/* Handle the start of the inflate stream if we called inflateInit2(strm,0);
* in this case some zlib versions skip validation of the CINFO field and, in
* certain circumstances, libpng may end up displaying an invalid image, in
* contrast to implementations that call zlib in the normal way (e.g. libpng
* 1.5).
*/
int /* PRIVATE */
png_zlib_inflate(png_structrp png_ptr, int flush)
{
  if (png_ptr->zstream_start && png_ptr->zstream.avail_in > 0)
  {
     if ((*png_ptr->zstream.next_in >> 4) > 7)
     {
        png_ptr->zstream.msg = "invalid window size (libpng)";
        return Z_DATA_ERROR;
     }

     png_ptr->zstream_start = 0;
  }

  return inflate(&png_ptr->zstream, flush);
}
#endif /* Zlib >= 1.2.4 */

#ifdef PNG_READ_COMPRESSED_TEXT_SUPPORTED
#if defined(PNG_READ_zTXt_SUPPORTED) || defined (PNG_READ_iTXt_SUPPORTED)
/* png_inflate now returns zlib error codes including Z_OK and Z_STREAM_END to
* allow the caller to do multiple calls if required.  If the 'finish' flag is
* set Z_FINISH will be passed to the final inflate() call and Z_STREAM_END must
* be returned or there has been a problem, otherwise Z_SYNC_FLUSH is used and
* Z_OK or Z_STREAM_END will be returned on success.
*
* The input and output sizes are updated to the actual amounts of data consumed
* or written, not the amount available (as in a z_stream).  The data pointers
* are not changed, so the next input is (data+input_size) and the next
* available output is (output+output_size).
*/
static int
png_inflate(png_structrp png_ptr, png_uint_32 owner, int finish,
   /* INPUT: */ png_const_bytep input, png_uint_32p input_size_ptr,
   /* OUTPUT: */ png_bytep output, png_alloc_size_t *output_size_ptr)
{
  if (png_ptr->zowner == owner) /* Else not claimed */
  {
     int ret;
     png_alloc_size_t avail_out = *output_size_ptr;
     png_uint_32 avail_in = *input_size_ptr;

     /* zlib can't necessarily handle more than 65535 bytes at once (i.e. it
      * can't even necessarily handle 65536 bytes) because the type uInt is
      * "16 bits or more".  Consequently it is necessary to chunk the input to
      * zlib.  This code uses ZLIB_IO_MAX, from pngpriv.h, as the maximum (the
      * maximum value that can be stored in a uInt.)  It is possible to set
      * ZLIB_IO_MAX to a lower value in pngpriv.h and this may sometimes have
      * a performance advantage, because it reduces the amount of data accessed
      * at each step and that may give the OS more time to page it in.
      */
     png_ptr->zstream.next_in = PNGZ_INPUT_CAST(input);
     /* avail_in and avail_out are set below from 'size' */
     png_ptr->zstream.avail_in = 0;
     png_ptr->zstream.avail_out = 0;

     /* Read directly into the output if it is available (this is set to
      * a local buffer below if output is NULL).
      */
     if (output != NULL)
        png_ptr->zstream.next_out = output;

     do
     {
        uInt avail;
        Byte local_buffer[PNG_INFLATE_BUF_SIZE];

        /* zlib INPUT BUFFER */
        /* The setting of 'avail_in' used to be outside the loop; by setting it
         * inside it is possible to chunk the input to zlib and simply rely on
         * zlib to advance the 'next_in' pointer.  This allows arbitrary
         * amounts of data to be passed through zlib at the unavoidable cost of
         * requiring a window save (memcpy of up to 32768 output bytes)
         * every ZLIB_IO_MAX input bytes.
         */
        avail_in += png_ptr->zstream.avail_in; /* not consumed last time */

        avail = ZLIB_IO_MAX;

        if (avail_in < avail)
           avail = (uInt)avail_in; /* safe: < than ZLIB_IO_MAX */

        avail_in -= avail;
        png_ptr->zstream.avail_in = avail;

        /* zlib OUTPUT BUFFER */
        avail_out += png_ptr->zstream.avail_out; /* not written last time */

        avail = ZLIB_IO_MAX; /* maximum zlib can process */

        if (output == NULL)
        {
           /* Reset the output buffer each time round if output is NULL and
            * make available the full buffer, up to 'remaining_space'
            */
           png_ptr->zstream.next_out = local_buffer;
           if ((sizeof local_buffer) < avail)
              avail = (sizeof local_buffer);
        }

        if (avail_out < avail)
           avail = (uInt)avail_out; /* safe: < ZLIB_IO_MAX */

        png_ptr->zstream.avail_out = avail;
        avail_out -= avail;

        /* zlib inflate call */
        /* In fact 'avail_out' may be 0 at this point, that happens at the end
         * of the read when the final LZ end code was not passed at the end of
         * the previous chunk of input data.  Tell zlib if we have reached the
         * end of the output buffer.
         */
        ret = PNG_INFLATE(png_ptr, avail_out > 0 ? Z_NO_FLUSH :
            (finish ? Z_FINISH : Z_SYNC_FLUSH));
     } while (ret == Z_OK);

     /* For safety kill the local buffer pointer now */
     if (output == NULL)
        png_ptr->zstream.next_out = NULL;

     /* Claw back the 'size' and 'remaining_space' byte counts. */
     avail_in += png_ptr->zstream.avail_in;
     avail_out += png_ptr->zstream.avail_out;

     /* Update the input and output sizes; the updated values are the amount
      * consumed or written, effectively the inverse of what zlib uses.
      */
     if (avail_out > 0)
        *output_size_ptr -= avail_out;

     if (avail_in > 0)
        *input_size_ptr -= avail_in;

     /* Ensure png_ptr->zstream.msg is set (even in the success case!) */
     png_zstream_error(png_ptr, ret);
     return ret;
  }

  else
  {
     /* This is a bad internal error.  The recovery assigns to the zstream msg
      * pointer, which is not owned by the caller, but this is safe; it's only
      * used on errors!
      */
     png_ptr->zstream.msg = PNGZ_MSG_CAST("zstream unclaimed");
     return Z_STREAM_ERROR;
  }
}

/*
* Decompress trailing data in a chunk.  The assumption is that read_buffer
* points at an allocated area holding the contents of a chunk with a
* trailing compressed part.  What we get back is an allocated area
* holding the original prefix part and an uncompressed version of the
* trailing part (the malloc area passed in is freed).
*/
static int
png_decompress_chunk(png_structrp png_ptr,
   png_uint_32 chunklength, png_uint_32 prefix_size,
   png_alloc_size_t *newlength /* must be initialized to the maximum! */,
   int terminate /*add a '\0' to the end of the uncompressed data*/)
{
  /* TODO: implement different limits for different types of chunk.
   *
   * The caller supplies *newlength set to the maximum length of the
   * uncompressed data, but this routine allocates space for the prefix and
   * maybe a '\0' terminator too.  We have to assume that 'prefix_size' is
   * limited only by the maximum chunk size.
   */
  png_alloc_size_t limit = png_chunk_max(png_ptr);

  if (limit >= prefix_size + (terminate != 0))
  {
     int ret;

     limit -= prefix_size + (terminate != 0);

     if (limit < *newlength)
        *newlength = limit;

     /* Now try to claim the stream. */
     ret = png_inflate_claim(png_ptr, png_ptr->chunk_name);

     if (ret == Z_OK)
     {
        png_uint_32 lzsize = chunklength - prefix_size;

        ret = png_inflate(png_ptr, png_ptr->chunk_name, 1/*finish*/,
            /* input: */ png_ptr->read_buffer + prefix_size, &lzsize,
            /* output: */ NULL, newlength);

        if (ret == Z_STREAM_END)
        {
           /* Use 'inflateReset' here, not 'inflateReset2' because this
            * preserves the previously decided window size (otherwise it would
            * be necessary to store the previous window size.)  In practice
            * this doesn't matter anyway, because png_inflate will call inflate
            * with Z_FINISH in almost all cases, so the window will not be
            * maintained.
            */
           if (inflateReset(&png_ptr->zstream) == Z_OK)
           {
              /* Because of the limit checks above we know that the new,
               * expanded, size will fit in a size_t (let alone an
               * png_alloc_size_t).  Use png_malloc_base here to avoid an
               * extra OOM message.
               */
              png_alloc_size_t new_size = *newlength;
              png_alloc_size_t buffer_size = prefix_size + new_size +
                  (terminate != 0);
              png_bytep text = png_voidcast(png_bytep, png_malloc_base(png_ptr,
                  buffer_size));

              if (text != NULL)
              {
                 memset(text, 0, buffer_size);

                 ret = png_inflate(png_ptr, png_ptr->chunk_name, 1/*finish*/,
                     png_ptr->read_buffer + prefix_size, &lzsize,
                     text + prefix_size, newlength);

                 if (ret == Z_STREAM_END)
                 {
                    if (new_size == *newlength)
                    {
                       if (terminate != 0)
                          text[prefix_size + *newlength] = 0;

                       if (prefix_size > 0)
                          memcpy(text, png_ptr->read_buffer, prefix_size);

                       {
                          png_bytep old_ptr = png_ptr->read_buffer;

                          png_ptr->read_buffer = text;
                          png_ptr->read_buffer_size = buffer_size;
                          text = old_ptr; /* freed below */
                       }
                    }

                    else
                    {
                       /* The size changed on the second read, there can be no
                        * guarantee that anything is correct at this point.
                        * The 'msg' pointer has been set to "unexpected end of
                        * LZ stream", which is fine, but return an error code
                        * that the caller won't accept.
                        */
                       ret = PNG_UNEXPECTED_ZLIB_RETURN;
                    }
                 }

                 else if (ret == Z_OK)
                    ret = PNG_UNEXPECTED_ZLIB_RETURN; /* for safety */

                 /* Free the text pointer (this is the old read_buffer on
                  * success)
                  */
                 png_free(png_ptr, text);

                 /* This really is very benign, but it's still an error because
                  * the extra space may otherwise be used as a Trojan Horse.
                  */
                 if (ret == Z_STREAM_END &&
                     chunklength - prefix_size != lzsize)
                    png_chunk_benign_error(png_ptr, "extra compressed data");
              }

              else
              {
                 /* Out of memory allocating the buffer */
                 ret = Z_MEM_ERROR;
                 png_zstream_error(png_ptr, Z_MEM_ERROR);
              }
           }

           else
           {
              /* inflateReset failed, store the error message */
              png_zstream_error(png_ptr, ret);
              ret = PNG_UNEXPECTED_ZLIB_RETURN;
           }
        }

        else if (ret == Z_OK)
           ret = PNG_UNEXPECTED_ZLIB_RETURN;

        /* Release the claimed stream */
        png_ptr->zowner = 0;
     }

     else /* the claim failed */ if (ret == Z_STREAM_END) /* impossible! */
        ret = PNG_UNEXPECTED_ZLIB_RETURN;

     return ret;
  }

  else
  {
     /* Application/configuration limits exceeded */
     png_zstream_error(png_ptr, Z_MEM_ERROR);
     return Z_MEM_ERROR;
  }
}
#endif /* READ_zTXt || READ_iTXt */
#endif /* READ_COMPRESSED_TEXT */

#ifdef PNG_READ_iCCP_SUPPORTED
/* Perform a partial read and decompress, producing 'avail_out' bytes and
* reading from the current chunk as required.
*/
static int
png_inflate_read(png_structrp png_ptr, png_bytep read_buffer, uInt read_size,
   png_uint_32p chunk_bytes, png_bytep next_out, png_alloc_size_t *out_size,
   int finish)
{
  if (png_ptr->zowner == png_ptr->chunk_name)
  {
     int ret;

     /* next_in and avail_in must have been initialized by the caller. */
     png_ptr->zstream.next_out = next_out;
     png_ptr->zstream.avail_out = 0; /* set in the loop */

     do
     {
        if (png_ptr->zstream.avail_in == 0)
        {
           if (read_size > *chunk_bytes)
              read_size = (uInt)*chunk_bytes;
           *chunk_bytes -= read_size;

           if (read_size > 0)
              png_crc_read(png_ptr, read_buffer, read_size);

           png_ptr->zstream.next_in = read_buffer;
           png_ptr->zstream.avail_in = read_size;
        }

        if (png_ptr->zstream.avail_out == 0)
        {
           uInt avail = ZLIB_IO_MAX;
           if (avail > *out_size)
              avail = (uInt)*out_size;
           *out_size -= avail;

           png_ptr->zstream.avail_out = avail;
        }

        /* Use Z_SYNC_FLUSH when there is no more chunk data to ensure that all
         * the available output is produced; this allows reading of truncated
         * streams.
         */
        ret = PNG_INFLATE(png_ptr, *chunk_bytes > 0 ?
            Z_NO_FLUSH : (finish ? Z_FINISH : Z_SYNC_FLUSH));
     }
     while (ret == Z_OK && (*out_size > 0 || png_ptr->zstream.avail_out > 0));

     *out_size += png_ptr->zstream.avail_out;
     png_ptr->zstream.avail_out = 0; /* Should not be required, but is safe */

     /* Ensure the error message pointer is always set: */
     png_zstream_error(png_ptr, ret);
     return ret;
  }

  else
  {
     png_ptr->zstream.msg = PNGZ_MSG_CAST("zstream unclaimed");
     return Z_STREAM_ERROR;
  }
}
#endif /* READ_iCCP */

/* CHUNK HANDLING */
/* Read and check the IDHR chunk */
static png_handle_result_code
png_handle_IHDR(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_byte buf[13];
  png_uint_32 width, height;
  int bit_depth, color_type, compression_type, filter_type;
  int interlace_type;

  png_debug(1, "in png_handle_IHDR");

  /* Length and position are checked by the caller. */

  png_ptr->mode |= PNG_HAVE_IHDR;

  png_crc_read(png_ptr, buf, 13);
  png_crc_finish(png_ptr, 0);

  width = png_get_uint_31(png_ptr, buf);
  height = png_get_uint_31(png_ptr, buf + 4);
  bit_depth = buf[8];
  color_type = buf[9];
  compression_type = buf[10];
  filter_type = buf[11];
  interlace_type = buf[12];

#ifdef PNG_READ_APNG_SUPPORTED
  png_ptr->first_frame_width = width;
  png_ptr->first_frame_height = height;
#endif

  /* Set internal variables */
  png_ptr->width = width;
  png_ptr->height = height;
  png_ptr->bit_depth = (png_byte)bit_depth;
  png_ptr->interlaced = (png_byte)interlace_type;
  png_ptr->color_type = (png_byte)color_type;
#ifdef PNG_MNG_FEATURES_SUPPORTED
  png_ptr->filter_type = (png_byte)filter_type;
#endif
  png_ptr->compression_type = (png_byte)compression_type;

  /* Find number of channels */
  switch (png_ptr->color_type)
  {
     default: /* invalid, png_set_IHDR calls png_error */
     case PNG_COLOR_TYPE_GRAY:
     case PNG_COLOR_TYPE_PALETTE:
        png_ptr->channels = 1;
        break;

     case PNG_COLOR_TYPE_RGB:
        png_ptr->channels = 3;
        break;

     case PNG_COLOR_TYPE_GRAY_ALPHA:
        png_ptr->channels = 2;
        break;

     case PNG_COLOR_TYPE_RGB_ALPHA:
        png_ptr->channels = 4;
        break;
  }

  /* Set up other useful info */
  png_ptr->pixel_depth = (png_byte)(png_ptr->bit_depth * png_ptr->channels);
  png_ptr->rowbytes = PNG_ROWBYTES(png_ptr->pixel_depth, png_ptr->width);
  png_debug1(3, "bit_depth = %d", png_ptr->bit_depth);
  png_debug1(3, "channels = %d", png_ptr->channels);
  png_debug1(3, "rowbytes = %lu", (unsigned long)png_ptr->rowbytes);

  /* Rely on png_set_IHDR to completely validate the data and call png_error if
   * it's wrong.
   */
  png_set_IHDR(png_ptr, info_ptr, width, height, bit_depth,
      color_type, interlace_type, compression_type, filter_type);

  return handled_ok;
  PNG_UNUSED(length)
}

/* Read and check the palette */
/* TODO: there are several obvious errors in this code when handling
* out-of-place chunks and there is much over-complexity caused by trying to
* patch up the problems.
*/
static png_handle_result_code
png_handle_PLTE(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_const_charp errmsg = NULL;

  png_debug(1, "in png_handle_PLTE");

  /* 1.6.47: consistency.  This used to be especially treated as a critical
   * error even in an image which is not colour mapped, there isn't a good
   * justification for treating some errors here one way and others another so
   * everything uses the same logic.
   */
  if ((png_ptr->mode & PNG_HAVE_PLTE) != 0)
     errmsg = "duplicate";

  else if ((png_ptr->mode & PNG_HAVE_IDAT) != 0)
     errmsg = "out of place";

  else if ((png_ptr->color_type & PNG_COLOR_MASK_COLOR) == 0)
     errmsg = "ignored in grayscale PNG";

  else if (length > 3*PNG_MAX_PALETTE_LENGTH || (length % 3) != 0)
     errmsg = "invalid";

  /* This drops PLTE in favour of tRNS or bKGD because both of those chunks
   * can have an effect on the rendering of the image whereas PLTE only matters
   * in the case of an 8-bit display with a decoder which controls the palette.
   *
   * The alternative here is to ignore the error and store the palette anyway;
   * destroying the tRNS will definately cause problems.
   *
   * NOTE: the case of PNG_COLOR_TYPE_PALETTE need not be considered because
   * the png_handle_ routines for the three 'after PLTE' chunks tRNS, bKGD and
   * hIST all check for a preceding PLTE in these cases.
   */
  else if (png_ptr->color_type != PNG_COLOR_TYPE_PALETTE &&
           (png_has_chunk(png_ptr, tRNS) || png_has_chunk(png_ptr, bKGD)))
     errmsg = "out of place";

  else
  {
     /* If the palette has 256 or fewer entries but is too large for the bit
      * depth we don't issue an error to preserve the behavior of previous
      * libpng versions. We silently truncate the unused extra palette entries
      * here.
      */
     const unsigned max_palette_length =
        (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE) ?
           1U << png_ptr->bit_depth : PNG_MAX_PALETTE_LENGTH;

     /* The cast is safe because 'length' is less than
      * 3*PNG_MAX_PALETTE_LENGTH
      */
     const unsigned num = (length > 3U*max_palette_length) ?
        max_palette_length : (unsigned)length / 3U;

     unsigned i, j;
     png_byte buf[3*PNG_MAX_PALETTE_LENGTH];
     png_color palette[PNG_MAX_PALETTE_LENGTH];

     /* Read the chunk into the buffer then read to the end of the chunk. */
     png_crc_read(png_ptr, buf, num*3U);
     png_crc_finish_critical(png_ptr, length - 3U*num,
           /* Handle as ancillary if PLTE is optional: */
           png_ptr->color_type != PNG_COLOR_TYPE_PALETTE);

     for (i = 0U, j = 0U; i < num; i++)
     {
        palette[i].red = buf[j++];
        palette[i].green = buf[j++];
        palette[i].blue = buf[j++];
     }

     /* A valid PLTE chunk has been read */
     png_ptr->mode |= PNG_HAVE_PLTE;

     /* TODO: png_set_PLTE has the side effect of setting png_ptr->palette to
      * its own copy of the palette.  This has the side effect that when
      * png_start_row is called (this happens after any call to
      * png_read_update_info) the info_ptr palette gets changed.  This is
      * extremely unexpected and confusing.
      *
      * REVIEW: there have been consistent bugs in the past about gamma and
      * similar transforms to colour mapped images being useless because the
      * modified palette cannot be accessed because of the above.
      *
      * CONSIDER: Fix this by not sharing the palette in this way.  But does
      * this completely fix the problem?
      */
     png_set_PLTE(png_ptr, info_ptr, palette, num);
     return handled_ok;
  }

  /* Here on error: errmsg is non NULL. */
  if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_error(png_ptr, errmsg);
  }

  else /* not critical to this image */
  {
     png_crc_finish_critical(png_ptr, length, 1/*handle as ancillary*/);
     png_chunk_benign_error(png_ptr, errmsg);
  }

  /* Because PNG_UNUSED(errmsg) does not work if all the uses are compiled out
   * (this does happen).
   */
  return errmsg != NULL ? handled_error : handled_error;
}

/* On read the IDAT chunk is always handled specially, even if marked for
* unknown handling (this is allowed), so:
*/
#define png_handle_IDAT NULL

static png_handle_result_code
png_handle_IEND(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_debug(1, "in png_handle_IEND");

  png_ptr->mode |= (PNG_AFTER_IDAT | PNG_HAVE_IEND);

  if (length != 0)
     png_chunk_benign_error(png_ptr, "invalid");

  png_crc_finish_critical(png_ptr, length, 1/*handle as ancillary*/);

  return handled_ok;
  PNG_UNUSED(info_ptr)
}

#ifdef PNG_READ_gAMA_SUPPORTED
static png_handle_result_code
png_handle_gAMA(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_uint_32 ugamma;
  png_byte buf[4];

  png_debug(1, "in png_handle_gAMA");

  png_crc_read(png_ptr, buf, 4);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  ugamma = png_get_uint_32(buf);

  if (ugamma > PNG_UINT_31_MAX)
  {
     png_chunk_benign_error(png_ptr, "invalid");
     return handled_error;
  }

  png_set_gAMA_fixed(png_ptr, info_ptr, (png_fixed_point)/*SAFE*/ugamma);

#ifdef PNG_READ_GAMMA_SUPPORTED
     /* PNGv3: chunk precedence for gamma is cICP, [iCCP], sRGB, gAMA.  gAMA is
      * at the end of the chain so simply check for an unset value.
      */
     if (png_ptr->chunk_gamma == 0)
        png_ptr->chunk_gamma = (png_fixed_point)/*SAFE*/ugamma;
#endif /*READ_GAMMA*/

  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_gAMA NULL
#endif

#ifdef PNG_READ_sBIT_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_sBIT(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  unsigned int truelen, i;
  png_byte sample_depth;
  png_byte buf[4];

  png_debug(1, "in png_handle_sBIT");

  if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
  {
     truelen = 3;
     sample_depth = 8;
  }

  else
  {
     truelen = png_ptr->channels;
     sample_depth = png_ptr->bit_depth;
  }

  if (length != truelen)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "bad length");
     return handled_error;
  }

  buf[0] = buf[1] = buf[2] = buf[3] = sample_depth;
  png_crc_read(png_ptr, buf, truelen);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  for (i=0; i<truelen; ++i)
  {
     if (buf[i] == 0 || buf[i] > sample_depth)
     {
        png_chunk_benign_error(png_ptr, "invalid");
        return handled_error;
     }
  }

  if ((png_ptr->color_type & PNG_COLOR_MASK_COLOR) != 0)
  {
     png_ptr->sig_bit.red = buf[0];
     png_ptr->sig_bit.green = buf[1];
     png_ptr->sig_bit.blue = buf[2];
     png_ptr->sig_bit.alpha = buf[3];
  }

  else /* grayscale */
  {
     png_ptr->sig_bit.gray = buf[0];
     png_ptr->sig_bit.red = buf[0];
     png_ptr->sig_bit.green = buf[0];
     png_ptr->sig_bit.blue = buf[0];
     png_ptr->sig_bit.alpha = buf[1];
  }

  png_set_sBIT(png_ptr, info_ptr, &(png_ptr->sig_bit));
  return handled_ok;
}
#else
#  define png_handle_sBIT NULL
#endif

#ifdef PNG_READ_cHRM_SUPPORTED
static png_int_32
png_get_int_32_checked(png_const_bytep buf, int *error)
{
  png_uint_32 uval = png_get_uint_32(buf);
  if ((uval & 0x80000000) == 0) /* non-negative */
     return (png_int_32)uval;

  uval = (uval ^ 0xffffffff) + 1;  /* 2's complement: -x = ~x+1 */
  if ((uval & 0x80000000) == 0) /* no overflow */
     return -(png_int_32)uval;

  /* This version of png_get_int_32 has a way of returning the error to the
   * caller, so:
   */
  *error = 1;
  return 0; /* Safe */
}

static png_handle_result_code /* PRIVATE */
png_handle_cHRM(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  int error = 0;
  png_xy xy;
  png_byte buf[32];

  png_debug(1, "in png_handle_cHRM");

  png_crc_read(png_ptr, buf, 32);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  xy.whitex = png_get_int_32_checked(buf +  0, &error);
  xy.whitey = png_get_int_32_checked(buf +  4, &error);
  xy.redx   = png_get_int_32_checked(buf +  8, &error);
  xy.redy   = png_get_int_32_checked(buf + 12, &error);
  xy.greenx = png_get_int_32_checked(buf + 16, &error);
  xy.greeny = png_get_int_32_checked(buf + 20, &error);
  xy.bluex  = png_get_int_32_checked(buf + 24, &error);
  xy.bluey  = png_get_int_32_checked(buf + 28, &error);

  if (error)
  {
     png_chunk_benign_error(png_ptr, "invalid");
     return handled_error;
  }

  /* png_set_cHRM may complain about some of the values but this doesn't matter
   * because it was a cHRM and it did have vaguely (if, perhaps, ridiculous)
   * values.  Ridiculousity will be checked if the values are used later.
   */
  png_set_cHRM_fixed(png_ptr, info_ptr, xy.whitex, xy.whitey, xy.redx, xy.redy,
        xy.greenx, xy.greeny, xy.bluex, xy.bluey);

  /* We only use 'chromaticities' for RGB to gray */
#  ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
     /* There is no need to check sRGB here, cICP is NYI and iCCP is not
      * supported so just check mDCV.
      */
     if (!png_has_chunk(png_ptr, mDCV))
     {
        png_ptr->chromaticities = xy;
     }
#  endif /* READ_RGB_TO_GRAY */

  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_cHRM NULL
#endif

#ifdef PNG_READ_sRGB_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_sRGB(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_byte intent;

  png_debug(1, "in png_handle_sRGB");

  png_crc_read(png_ptr, &intent, 1);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  /* This checks the range of the "rendering intent" because it is specified in
   * the PNG spec itself; the "reserved" values will result in the chunk not
   * being accepted, just as they do with the various "reserved" values in
   * IHDR.
   */
  if (intent > 3/*PNGv3 spec*/)
  {
     png_chunk_benign_error(png_ptr, "invalid");
     return handled_error;
  }

  png_set_sRGB(png_ptr, info_ptr, intent);
  /* NOTE: png_struct::chromaticities is not set here because the RGB to gray
   * coefficients are known without a need for the chromaticities.
   */

#ifdef PNG_READ_GAMMA_SUPPORTED
     /* PNGv3: chunk precedence for gamma is cICP, [iCCP], sRGB, gAMA.  iCCP is
      * not supported by libpng so the only requirement is to check for cICP
      * setting the gamma (this is NYI, but this check is safe.)
      */
     if (!png_has_chunk(png_ptr, cICP) || png_ptr->chunk_gamma == 0)
        png_ptr->chunk_gamma = PNG_GAMMA_sRGB_INVERSE;
#endif /*READ_GAMMA*/

  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_sRGB NULL
#endif /* READ_sRGB */

#ifdef PNG_READ_iCCP_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_iCCP(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
/* Note: this does not properly handle profiles that are > 64K under DOS */
{
  png_const_charp errmsg = NULL; /* error message output, or no error */
  int finished = 0; /* crc checked */

  png_debug(1, "in png_handle_iCCP");

  /* PNGv3: allow PNG files with both sRGB and iCCP because the PNG spec only
   * ever said that there "should" be only one, not "shall" and the PNGv3
   * colour chunk precedence rules give a handling for this case anyway.
   */
  {
     uInt read_length, keyword_length;
     char keyword[81];

     /* Find the keyword; the keyword plus separator and compression method
      * bytes can be at most 81 characters long.
      */
     read_length = 81; /* maximum */
     if (read_length > length)
        read_length = (uInt)/*SAFE*/length;

     png_crc_read(png_ptr, (png_bytep)keyword, read_length);
     length -= read_length;

     if (length < LZ77Min)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "too short");
        return handled_error;
     }

     keyword_length = 0;
     while (keyword_length < 80 && keyword_length < read_length &&
        keyword[keyword_length] != 0)
        ++keyword_length;

     /* TODO: make the keyword checking common */
     if (keyword_length >= 1 && keyword_length <= 79)
     {
        /* We only understand '0' compression - deflate - so if we get a
         * different value we can't safely decode the chunk.
         */
        if (keyword_length+1 < read_length &&
           keyword[keyword_length+1] == PNG_COMPRESSION_TYPE_BASE)
        {
           read_length -= keyword_length+2;

           if (png_inflate_claim(png_ptr, png_iCCP) == Z_OK)
           {
              Byte profile_header[132]={0};
              Byte local_buffer[PNG_INFLATE_BUF_SIZE];
              png_alloc_size_t size = (sizeof profile_header);

              png_ptr->zstream.next_in = (Bytef*)keyword + (keyword_length+2);
              png_ptr->zstream.avail_in = read_length;
              (void)png_inflate_read(png_ptr, local_buffer,
                  (sizeof local_buffer), &length, profile_header, &size,
                  0/*finish: don't, because the output is too small*/);

              if (size == 0)
              {
                 /* We have the ICC profile header; do the basic header checks.
                  */
                 png_uint_32 profile_length = png_get_uint_32(profile_header);

                 if (png_icc_check_length(png_ptr, keyword, profile_length) !=
                     0)
                 {
                    /* The length is apparently ok, so we can check the 132
                     * byte header.
                     */
                    if (png_icc_check_header(png_ptr, keyword, profile_length,
                             profile_header, png_ptr->color_type) != 0)
                    {
                       /* Now read the tag table; a variable size buffer is
                        * needed at this point, allocate one for the whole
                        * profile.  The header check has already validated
                        * that none of this stuff will overflow.
                        */
                       png_uint_32 tag_count =
                          png_get_uint_32(profile_header + 128);
                       png_bytep profile = png_read_buffer(png_ptr,
                             profile_length);

                       if (profile != NULL)
                       {
                          memcpy(profile, profile_header,
                              (sizeof profile_header));

                          size = 12 * tag_count;

                          (void)png_inflate_read(png_ptr, local_buffer,
                              (sizeof local_buffer), &length,
                              profile + (sizeof profile_header), &size, 0);

                          /* Still expect a buffer error because we expect
                           * there to be some tag data!
                           */
                          if (size == 0)
                          {
                             if (png_icc_check_tag_table(png_ptr,
                                      keyword, profile_length, profile) != 0)
                             {
                                /* The profile has been validated for basic
                                 * security issues, so read the whole thing in.
                                 */
                                size = profile_length - (sizeof profile_header)
                                    - 12 * tag_count;

                                (void)png_inflate_read(png_ptr, local_buffer,
                                    (sizeof local_buffer), &length,
                                    profile + (sizeof profile_header) +
                                    12 * tag_count, &size, 1/*finish*/);

                                if (length > 0 && !(png_ptr->flags &
                                    PNG_FLAG_BENIGN_ERRORS_WARN))
                                   errmsg = "extra compressed data";

                                /* But otherwise allow extra data: */
                                else if (size == 0)
                                {
                                   if (length > 0)
                                   {
                                      /* This can be handled completely, so
                                       * keep going.
                                       */
                                      png_chunk_warning(png_ptr,
                                          "extra compressed data");
                                   }

                                   png_crc_finish(png_ptr, length);
                                   finished = 1;

                                   /* Steal the profile for info_ptr. */
                                   if (info_ptr != NULL)
                                   {
                                      png_free_data(png_ptr, info_ptr,
                                          PNG_FREE_ICCP, 0);

                                      info_ptr->iccp_name = png_voidcast(char*,
                                          png_malloc_base(png_ptr,
                                          keyword_length+1));
                                      if (info_ptr->iccp_name != NULL)
                                      {
                                         memcpy(info_ptr->iccp_name, keyword,
                                             keyword_length+1);
                                         info_ptr->iccp_proflen =
                                             profile_length;
                                         info_ptr->iccp_profile = profile;
                                         png_ptr->read_buffer = NULL; /*steal*/
                                         info_ptr->free_me |= PNG_FREE_ICCP;
                                         info_ptr->valid |= PNG_INFO_iCCP;
                                      }

                                      else
                                         errmsg = "out of memory";
                                   }

                                   /* else the profile remains in the read
                                    * buffer which gets reused for subsequent
                                    * chunks.
                                    */

                                   if (errmsg == NULL)
                                   {
                                      png_ptr->zowner = 0;
                                      return handled_ok;
                                   }
                                }
                                if (errmsg == NULL)
                                   errmsg = png_ptr->zstream.msg;
                             }
                             /* else png_icc_check_tag_table output an error */
                          }
                          else /* profile truncated */
                             errmsg = png_ptr->zstream.msg;
                       }

                       else
                          errmsg = "out of memory";
                    }

                    /* else png_icc_check_header output an error */
                 }

                 /* else png_icc_check_length output an error */
              }

              else /* profile truncated */
                 errmsg = png_ptr->zstream.msg;

              /* Release the stream */
              png_ptr->zowner = 0;
           }

           else /* png_inflate_claim failed */
              errmsg = png_ptr->zstream.msg;
        }

        else
           errmsg = "bad compression method"; /* or missing */
     }

     else
        errmsg = "bad keyword";
  }

  /* Failure: the reason is in 'errmsg' */
  if (finished == 0)
     png_crc_finish(png_ptr, length);

  if (errmsg != NULL) /* else already output */
     png_chunk_benign_error(png_ptr, errmsg);

  return handled_error;
}
#else
#  define png_handle_iCCP NULL
#endif /* READ_iCCP */

#ifdef PNG_READ_sPLT_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_sPLT(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
/* Note: this does not properly handle chunks that are > 64K under DOS */
{
  png_bytep entry_start, buffer;
  png_sPLT_t new_palette;
  png_sPLT_entryp pp;
  png_uint_32 data_length;
  int entry_size, i;
  png_uint_32 skip = 0;
  png_uint_32 dl;
  size_t max_dl;

  png_debug(1, "in png_handle_sPLT");

#ifdef PNG_USER_LIMITS_SUPPORTED
  if (png_ptr->user_chunk_cache_max != 0)
  {
     if (png_ptr->user_chunk_cache_max == 1)
     {
        png_crc_finish(png_ptr, length);
        return handled_error;
     }

     if (--png_ptr->user_chunk_cache_max == 1)
     {
        png_warning(png_ptr, "No space in chunk cache for sPLT");
        png_crc_finish(png_ptr, length);
        return handled_error;
     }
  }
#endif

  buffer = png_read_buffer(png_ptr, length+1);
  if (buffer == NULL)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "out of memory");
     return handled_error;
  }


  /* WARNING: this may break if size_t is less than 32 bits; it is assumed
   * that the PNG_MAX_MALLOC_64K test is enabled in this case, but this is a
   * potential breakage point if the types in pngconf.h aren't exactly right.
   */
  png_crc_read(png_ptr, buffer, length);

  if (png_crc_finish(png_ptr, skip) != 0)
     return handled_error;

  buffer[length] = 0;

  for (entry_start = buffer; *entry_start; entry_start++)
     /* Empty loop to find end of name */ ;

  ++entry_start;

  /* A sample depth should follow the separator, and we should be on it  */
  if (length < 2U || entry_start > buffer + (length - 2U))
  {
     png_warning(png_ptr, "malformed sPLT chunk");
     return handled_error;
  }

  new_palette.depth = *entry_start++;
  entry_size = (new_palette.depth == 8 ? 6 : 10);
  /* This must fit in a png_uint_32 because it is derived from the original
   * chunk data length.
   */
  data_length = length - (png_uint_32)(entry_start - buffer);

  /* Integrity-check the data length */
  if ((data_length % (unsigned int)entry_size) != 0)
  {
     png_warning(png_ptr, "sPLT chunk has bad length");
     return handled_error;
  }

  dl = (png_uint_32)(data_length / (unsigned int)entry_size);
  max_dl = PNG_SIZE_MAX / (sizeof (png_sPLT_entry));

  if (dl > max_dl)
  {
     png_warning(png_ptr, "sPLT chunk too long");
     return handled_error;
  }

  new_palette.nentries = (png_int_32)(data_length / (unsigned int)entry_size);

  new_palette.entries = (png_sPLT_entryp)png_malloc_warn(png_ptr,
      (png_alloc_size_t) new_palette.nentries * (sizeof (png_sPLT_entry)));

  if (new_palette.entries == NULL)
  {
     png_warning(png_ptr, "sPLT chunk requires too much memory");
     return handled_error;
  }

  for (i = 0; i < new_palette.nentries; i++)
  {
     pp = new_palette.entries + i;

     if (new_palette.depth == 8)
     {
        pp->red = *entry_start++;
        pp->green = *entry_start++;
        pp->blue = *entry_start++;
        pp->alpha = *entry_start++;
     }

     else
     {
        pp->red   = png_get_uint_16(entry_start); entry_start += 2;
        pp->green = png_get_uint_16(entry_start); entry_start += 2;
        pp->blue  = png_get_uint_16(entry_start); entry_start += 2;
        pp->alpha = png_get_uint_16(entry_start); entry_start += 2;
     }

     pp->frequency = png_get_uint_16(entry_start); entry_start += 2;
  }

  /* Discard all chunk data except the name and stash that */
  new_palette.name = (png_charp)buffer;

  png_set_sPLT(png_ptr, info_ptr, &new_palette, 1);

  png_free(png_ptr, new_palette.entries);
  return handled_ok;
}
#else
#  define png_handle_sPLT NULL
#endif /* READ_sPLT */

#ifdef PNG_READ_tRNS_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_tRNS(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_byte readbuf[PNG_MAX_PALETTE_LENGTH];

  png_debug(1, "in png_handle_tRNS");

  if (png_ptr->color_type == PNG_COLOR_TYPE_GRAY)
  {
     png_byte buf[2];

     if (length != 2)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "invalid");
        return handled_error;
     }

     png_crc_read(png_ptr, buf, 2);
     png_ptr->num_trans = 1;
     png_ptr->trans_color.gray = png_get_uint_16(buf);
  }

  else if (png_ptr->color_type == PNG_COLOR_TYPE_RGB)
  {
     png_byte buf[6];

     if (length != 6)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "invalid");
        return handled_error;
     }

     png_crc_read(png_ptr, buf, length);
     png_ptr->num_trans = 1;
     png_ptr->trans_color.red = png_get_uint_16(buf);
     png_ptr->trans_color.green = png_get_uint_16(buf + 2);
     png_ptr->trans_color.blue = png_get_uint_16(buf + 4);
  }

  else if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
  {
     if ((png_ptr->mode & PNG_HAVE_PLTE) == 0)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "out of place");
        return handled_error;
     }

     if (length > (unsigned int) png_ptr->num_palette ||
        length > (unsigned int) PNG_MAX_PALETTE_LENGTH ||
        length == 0)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "invalid");
        return handled_error;
     }

     png_crc_read(png_ptr, readbuf, length);
     png_ptr->num_trans = (png_uint_16)length;
  }

  else
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "invalid with alpha channel");
     return handled_error;
  }

  if (png_crc_finish(png_ptr, 0) != 0)
  {
     png_ptr->num_trans = 0;
     return handled_error;
  }

  /* TODO: this is a horrible side effect in the palette case because the
   * png_struct ends up with a pointer to the tRNS buffer owned by the
   * png_info.  Fix this.
   */
  png_set_tRNS(png_ptr, info_ptr, readbuf, png_ptr->num_trans,
      &(png_ptr->trans_color));
  return handled_ok;
}
#else
#  define png_handle_tRNS NULL
#endif

#ifdef PNG_READ_bKGD_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_bKGD(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  unsigned int truelen;
  png_byte buf[6];
  png_color_16 background;

  png_debug(1, "in png_handle_bKGD");

  if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
  {
     if ((png_ptr->mode & PNG_HAVE_PLTE) == 0)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "out of place");
        return handled_error;
     }

     truelen = 1;
  }

  else if ((png_ptr->color_type & PNG_COLOR_MASK_COLOR) != 0)
     truelen = 6;

  else
     truelen = 2;

  if (length != truelen)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "invalid");
     return handled_error;
  }

  png_crc_read(png_ptr, buf, truelen);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  /* We convert the index value into RGB components so that we can allow
   * arbitrary RGB values for background when we have transparency, and
   * so it is easy to determine the RGB values of the background color
   * from the info_ptr struct.
   */
  if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
  {
     background.index = buf[0];

     if (info_ptr != NULL && info_ptr->num_palette != 0)
     {
        if (buf[0] >= info_ptr->num_palette)
        {
           png_chunk_benign_error(png_ptr, "invalid index");
           return handled_error;
        }

        background.red = (png_uint_16)png_ptr->palette[buf[0]].red;
        background.green = (png_uint_16)png_ptr->palette[buf[0]].green;
        background.blue = (png_uint_16)png_ptr->palette[buf[0]].blue;
     }

     else
        background.red = background.green = background.blue = 0;

     background.gray = 0;
  }

  else if ((png_ptr->color_type & PNG_COLOR_MASK_COLOR) == 0) /* GRAY */
  {
     if (png_ptr->bit_depth <= 8)
     {
        if (buf[0] != 0 || buf[1] >= (unsigned int)(1 << png_ptr->bit_depth))
        {
           png_chunk_benign_error(png_ptr, "invalid gray level");
           return handled_error;
        }
     }

     background.index = 0;
     background.red =
     background.green =
     background.blue =
     background.gray = png_get_uint_16(buf);
  }

  else
  {
     if (png_ptr->bit_depth <= 8)
     {
        if (buf[0] != 0 || buf[2] != 0 || buf[4] != 0)
        {
           png_chunk_benign_error(png_ptr, "invalid color");
           return handled_error;
        }
     }

     background.index = 0;
     background.red = png_get_uint_16(buf);
     background.green = png_get_uint_16(buf + 2);
     background.blue = png_get_uint_16(buf + 4);
     background.gray = 0;
  }

  png_set_bKGD(png_ptr, info_ptr, &background);
  return handled_ok;
}
#else
#  define png_handle_bKGD NULL
#endif

#ifdef PNG_READ_cICP_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_cICP(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_byte buf[4];

  png_debug(1, "in png_handle_cICP");

  png_crc_read(png_ptr, buf, 4);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  png_set_cICP(png_ptr, info_ptr, buf[0], buf[1],  buf[2], buf[3]);

  /* We only use 'chromaticities' for RGB to gray */
#  ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
     if (!png_has_chunk(png_ptr, mDCV))
     {
        /* TODO: png_ptr->chromaticities = chromaticities; */
     }
#  endif /* READ_RGB_TO_GRAY */

#ifdef PNG_READ_GAMMA_SUPPORTED
     /* PNGv3: chunk precedence for gamma is cICP, [iCCP], sRGB, gAMA.  cICP is
      * at the head so simply set the gamma if it can be determined.  If not
      * chunk_gamma remains unchanged; sRGB and gAMA handling check it for
      * being zero.
      */
     /* TODO: set png_struct::chunk_gamma when possible */
#endif /*READ_GAMMA*/

  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_cICP NULL
#endif

#ifdef PNG_READ_cLLI_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_cLLI(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_byte buf[8];

  png_debug(1, "in png_handle_cLLI");

  png_crc_read(png_ptr, buf, 8);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  /* The error checking happens here, this puts it in just one place: */
  png_set_cLLI_fixed(png_ptr, info_ptr, png_get_uint_32(buf),
        png_get_uint_32(buf+4));
  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_cLLI NULL
#endif

#ifdef PNG_READ_mDCV_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_mDCV(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_xy chromaticities;
  png_byte buf[24];

  png_debug(1, "in png_handle_mDCV");

  png_crc_read(png_ptr, buf, 24);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  /* The error checking happens here, this puts it in just one place.  The
   * odd /50000 scaling factor makes it more difficult but the (x.y) values are
   * only two bytes so a <<1 is safe.
   *
   * WARNING: the PNG specification defines the cHRM chunk to **start** with
   * the white point (x,y).  The W3C PNG v3 specification puts the white point
   * **after* R,G,B.  The x,y values in mDCV are also scaled by 50,000 and
   * stored in just two bytes, whereas those in cHRM are scaled by 100,000 and
   * stored in four bytes.  This is very, very confusing.  These APIs remove
   * the confusion by copying the existing, well established, API.
   */
  chromaticities.redx   = png_get_uint_16(buf+ 0U) << 1; /* red x */
  chromaticities.redy   = png_get_uint_16(buf+ 2U) << 1; /* red y */
  chromaticities.greenx = png_get_uint_16(buf+ 4U) << 1; /* green x */
  chromaticities.greeny = png_get_uint_16(buf+ 6U) << 1; /* green y */
  chromaticities.bluex  = png_get_uint_16(buf+ 8U) << 1; /* blue x */
  chromaticities.bluey  = png_get_uint_16(buf+10U) << 1; /* blue y */
  chromaticities.whitex = png_get_uint_16(buf+12U) << 1; /* white x */
  chromaticities.whitey = png_get_uint_16(buf+14U) << 1; /* white y */

  png_set_mDCV_fixed(png_ptr, info_ptr,
        chromaticities.whitex, chromaticities.whitey,
        chromaticities.redx, chromaticities.redy,
        chromaticities.greenx, chromaticities.greeny,
        chromaticities.bluex, chromaticities.bluey,
        png_get_uint_32(buf+16U), /* peak luminance */
        png_get_uint_32(buf+20U));/* minimum perceivable luminance */

  /* We only use 'chromaticities' for RGB to gray */
#  ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
     png_ptr->chromaticities = chromaticities;
#  endif /* READ_RGB_TO_GRAY */

  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_mDCV NULL
#endif

#ifdef PNG_READ_eXIf_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_eXIf(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_bytep buffer = NULL;

  png_debug(1, "in png_handle_eXIf");

  buffer = png_read_buffer(png_ptr, length);

  if (buffer == NULL)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "out of memory");
     return handled_error;
  }

  png_crc_read(png_ptr, buffer, length);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  /* PNGv3: the code used to check the byte order mark at the start for MM or
   * II, however PNGv3 states that the the first 4 bytes should be checked.
   * The caller ensures that there are four bytes available.
   */
  {
     png_uint_32 header = png_get_uint_32(buffer);

     /* These numbers are copied from the PNGv3 spec: */
     if (header != 0x49492A00 && header != 0x4D4D002A)
     {
        png_chunk_benign_error(png_ptr, "invalid");
        return handled_error;
     }
  }

  png_set_eXIf_1(png_ptr, info_ptr, length, buffer);
  return handled_ok;
}
#else
#  define png_handle_eXIf NULL
#endif

#ifdef PNG_READ_hIST_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_hIST(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  unsigned int num, i;
  png_uint_16 readbuf[PNG_MAX_PALETTE_LENGTH];

  png_debug(1, "in png_handle_hIST");

  /* This cast is safe because the chunk definition limits the length to a
   * maximum of 1024 bytes.
   *
   * TODO: maybe use png_uint_32 anyway, not unsigned int, to reduce the
   * casts.
   */
  num = (unsigned int)length / 2 ;

  if (length != num * 2 ||
      num != (unsigned int)png_ptr->num_palette ||
      num > (unsigned int)PNG_MAX_PALETTE_LENGTH)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "invalid");
     return handled_error;
  }

  for (i = 0; i < num; i++)
  {
     png_byte buf[2];

     png_crc_read(png_ptr, buf, 2);
     readbuf[i] = png_get_uint_16(buf);
  }

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  png_set_hIST(png_ptr, info_ptr, readbuf);
  return handled_ok;
}
#else
#  define png_handle_hIST NULL
#endif

#ifdef PNG_READ_pHYs_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_pHYs(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_byte buf[9];
  png_uint_32 res_x, res_y;
  int unit_type;

  png_debug(1, "in png_handle_pHYs");

  png_crc_read(png_ptr, buf, 9);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  res_x = png_get_uint_32(buf);
  res_y = png_get_uint_32(buf + 4);
  unit_type = buf[8];
  png_set_pHYs(png_ptr, info_ptr, res_x, res_y, unit_type);
  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_pHYs NULL
#endif

#ifdef PNG_READ_oFFs_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_oFFs(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_byte buf[9];
  png_int_32 offset_x, offset_y;
  int unit_type;

  png_debug(1, "in png_handle_oFFs");

  png_crc_read(png_ptr, buf, 9);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  offset_x = png_get_int_32(buf);
  offset_y = png_get_int_32(buf + 4);
  unit_type = buf[8];
  png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, unit_type);
  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_oFFs NULL
#endif

#ifdef PNG_READ_pCAL_SUPPORTED
/* Read the pCAL chunk (described in the PNG Extensions document) */
static png_handle_result_code /* PRIVATE */
png_handle_pCAL(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_int_32 X0, X1;
  png_byte type, nparams;
  png_bytep buffer, buf, units, endptr;
  png_charpp params;
  int i;

  png_debug(1, "in png_handle_pCAL");
  png_debug1(2, "Allocating and reading pCAL chunk data (%u bytes)",
      length + 1);

  buffer = png_read_buffer(png_ptr, length+1);

  if (buffer == NULL)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "out of memory");
     return handled_error;
  }

  png_crc_read(png_ptr, buffer, length);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  buffer[length] = 0; /* Null terminate the last string */

  png_debug(3, "Finding end of pCAL purpose string");
  for (buf = buffer; *buf; buf++)
     /* Empty loop */ ;

  endptr = buffer + length;

  /* We need to have at least 12 bytes after the purpose string
   * in order to get the parameter information.
   */
  if (endptr - buf <= 12)
  {
     png_chunk_benign_error(png_ptr, "invalid");
     return handled_error;
  }

  png_debug(3, "Reading pCAL X0, X1, type, nparams, and units");
  X0 = png_get_int_32((png_bytep)buf+1);
  X1 = png_get_int_32((png_bytep)buf+5);
  type = buf[9];
  nparams = buf[10];
  units = buf + 11;

  png_debug(3, "Checking pCAL equation type and number of parameters");
  /* Check that we have the right number of parameters for known
   * equation types.
   */
  if ((type == PNG_EQUATION_LINEAR && nparams != 2) ||
      (type == PNG_EQUATION_BASE_E && nparams != 3) ||
      (type == PNG_EQUATION_ARBITRARY && nparams != 3) ||
      (type == PNG_EQUATION_HYPERBOLIC && nparams != 4))
  {
     png_chunk_benign_error(png_ptr, "invalid parameter count");
     return handled_error;
  }

  else if (type >= PNG_EQUATION_LAST)
  {
     png_chunk_benign_error(png_ptr, "unrecognized equation type");
  }

  for (buf = units; *buf; buf++)
     /* Empty loop to move past the units string. */ ;

  png_debug(3, "Allocating pCAL parameters array");

  params = png_voidcast(png_charpp, png_malloc_warn(png_ptr,
      nparams * (sizeof (png_charp))));

  if (params == NULL)
  {
     png_chunk_benign_error(png_ptr, "out of memory");
     return handled_error;
  }

  /* Get pointers to the start of each parameter string. */
  for (i = 0; i < nparams; i++)
  {
     buf++; /* Skip the null string terminator from previous parameter. */

     png_debug1(3, "Reading pCAL parameter %d", i);

     for (params[i] = (png_charp)buf; buf <= endptr && *buf != 0; buf++)
        /* Empty loop to move past each parameter string */ ;

     /* Make sure we haven't run out of data yet */
     if (buf > endptr)
     {
        png_free(png_ptr, params);
        png_chunk_benign_error(png_ptr, "invalid data");
        return handled_error;
     }
  }

  png_set_pCAL(png_ptr, info_ptr, (png_charp)buffer, X0, X1, type, nparams,
      (png_charp)units, params);

  /* TODO: BUG: png_set_pCAL calls png_chunk_report which, in this case, calls
   * png_benign_error and that can error out.
   *
   * png_read_buffer needs to be allocated with space for both nparams and the
   * parameter strings.  Not hard to do.
   */
  png_free(png_ptr, params);
  return handled_ok;
}
#else
#  define png_handle_pCAL NULL
#endif

#ifdef PNG_READ_sCAL_SUPPORTED
/* Read the sCAL chunk */
static png_handle_result_code /* PRIVATE */
png_handle_sCAL(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_bytep buffer;
  size_t i;
  int state;

  png_debug(1, "in png_handle_sCAL");
  png_debug1(2, "Allocating and reading sCAL chunk data (%u bytes)",
      length + 1);

  buffer = png_read_buffer(png_ptr, length+1);

  if (buffer == NULL)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "out of memory");
     return handled_error;
  }

  png_crc_read(png_ptr, buffer, length);
  buffer[length] = 0; /* Null terminate the last string */

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  /* Validate the unit. */
  if (buffer[0] != 1 && buffer[0] != 2)
  {
     png_chunk_benign_error(png_ptr, "invalid unit");
     return handled_error;
  }

  /* Validate the ASCII numbers, need two ASCII numbers separated by
   * a '\0' and they need to fit exactly in the chunk data.
   */
  i = 1;
  state = 0;

  if (png_check_fp_number((png_const_charp)buffer, length, &state, &i) == 0 ||
      i >= length || buffer[i++] != 0)
     png_chunk_benign_error(png_ptr, "bad width format");

  else if (PNG_FP_IS_POSITIVE(state) == 0)
     png_chunk_benign_error(png_ptr, "non-positive width");

  else
  {
     size_t heighti = i;

     state = 0;
     if (png_check_fp_number((png_const_charp)buffer, length,
         &state, &i) == 0 || i != length)
        png_chunk_benign_error(png_ptr, "bad height format");

     else if (PNG_FP_IS_POSITIVE(state) == 0)
        png_chunk_benign_error(png_ptr, "non-positive height");

     else
     {
        /* This is the (only) success case. */
        png_set_sCAL_s(png_ptr, info_ptr, buffer[0],
            (png_charp)buffer+1, (png_charp)buffer+heighti);
        return handled_ok;
     }
  }

  return handled_error;
}
#else
#  define png_handle_sCAL NULL
#endif

#ifdef PNG_READ_tIME_SUPPORTED
static png_handle_result_code /* PRIVATE */
png_handle_tIME(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_byte buf[7];
  png_time mod_time;

  png_debug(1, "in png_handle_tIME");

  /* TODO: what is this doing here?  It should be happened in pngread.c and
   * pngpread.c, although it could be moved to png_handle_chunk below and
   * thereby avoid some code duplication.
   */
  if ((png_ptr->mode & PNG_HAVE_IDAT) != 0)
     png_ptr->mode |= PNG_AFTER_IDAT;

  png_crc_read(png_ptr, buf, 7);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  mod_time.second = buf[6];
  mod_time.minute = buf[5];
  mod_time.hour = buf[4];
  mod_time.day = buf[3];
  mod_time.month = buf[2];
  mod_time.year = png_get_uint_16(buf);

  png_set_tIME(png_ptr, info_ptr, &mod_time);
  return handled_ok;
  PNG_UNUSED(length)
}
#else
#  define png_handle_tIME NULL
#endif

#ifdef PNG_READ_tEXt_SUPPORTED
/* Note: this does not properly handle chunks that are > 64K under DOS */
static png_handle_result_code /* PRIVATE */
png_handle_tEXt(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_text  text_info;
  png_bytep buffer;
  png_charp key;
  png_charp text;
  png_uint_32 skip = 0;

  png_debug(1, "in png_handle_tEXt");

#ifdef PNG_USER_LIMITS_SUPPORTED
  if (png_ptr->user_chunk_cache_max != 0)
  {
     if (png_ptr->user_chunk_cache_max == 1)
     {
        png_crc_finish(png_ptr, length);
        return handled_error;
     }

     if (--png_ptr->user_chunk_cache_max == 1)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "no space in chunk cache");
        return handled_error;
     }
  }
#endif

  buffer = png_read_buffer(png_ptr, length+1);

  if (buffer == NULL)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "out of memory");
     return handled_error;
  }

  png_crc_read(png_ptr, buffer, length);

  if (png_crc_finish(png_ptr, skip) != 0)
     return handled_error;

  key = (png_charp)buffer;
  key[length] = 0;

  for (text = key; *text; text++)
     /* Empty loop to find end of key */ ;

  if (text != key + length)
     text++;

  text_info.compression = PNG_TEXT_COMPRESSION_NONE;
  text_info.key = key;
  text_info.lang = NULL;
  text_info.lang_key = NULL;
  text_info.itxt_length = 0;
  text_info.text = text;
  text_info.text_length = strlen(text);

  if (png_set_text_2(png_ptr, info_ptr, &text_info, 1) == 0)
     return handled_ok;

  png_chunk_benign_error(png_ptr, "out of memory");
  return handled_error;
}
#else
#  define png_handle_tEXt NULL
#endif

#ifdef PNG_READ_zTXt_SUPPORTED
/* Note: this does not correctly handle chunks that are > 64K under DOS */
static png_handle_result_code /* PRIVATE */
png_handle_zTXt(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_const_charp errmsg = NULL;
  png_bytep       buffer;
  png_uint_32     keyword_length;

  png_debug(1, "in png_handle_zTXt");

#ifdef PNG_USER_LIMITS_SUPPORTED
  if (png_ptr->user_chunk_cache_max != 0)
  {
     if (png_ptr->user_chunk_cache_max == 1)
     {
        png_crc_finish(png_ptr, length);
        return handled_error;
     }

     if (--png_ptr->user_chunk_cache_max == 1)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "no space in chunk cache");
        return handled_error;
     }
  }
#endif

  /* Note, "length" is sufficient here; we won't be adding
   * a null terminator later.  The limit check in png_handle_chunk should be
   * sufficient.
   */
  buffer = png_read_buffer(png_ptr, length);

  if (buffer == NULL)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "out of memory");
     return handled_error;
  }

  png_crc_read(png_ptr, buffer, length);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  /* TODO: also check that the keyword contents match the spec! */
  for (keyword_length = 0;
     keyword_length < length && buffer[keyword_length] != 0;
     ++keyword_length)
     /* Empty loop to find end of name */ ;

  if (keyword_length > 79 || keyword_length < 1)
     errmsg = "bad keyword";

  /* zTXt must have some LZ data after the keyword, although it may expand to
   * zero bytes; we need a '\0' at the end of the keyword, the compression type
   * then the LZ data:
   */
  else if (keyword_length + 3 > length)
     errmsg = "truncated";

  else if (buffer[keyword_length+1] != PNG_COMPRESSION_TYPE_BASE)
     errmsg = "unknown compression type";

  else
  {
     png_alloc_size_t uncompressed_length = PNG_SIZE_MAX;

     /* TODO: at present png_decompress_chunk imposes a single application
      * level memory limit, this should be split to different values for iCCP
      * and text chunks.
      */
     if (png_decompress_chunk(png_ptr, length, keyword_length+2,
         &uncompressed_length, 1/*terminate*/) == Z_STREAM_END)
     {
        png_text text;

        if (png_ptr->read_buffer == NULL)
          errmsg="Read failure in png_handle_zTXt";
        else
        {
           /* It worked; png_ptr->read_buffer now looks like a tEXt chunk
            * except for the extra compression type byte and the fact that
            * it isn't necessarily '\0' terminated.
            */
           buffer = png_ptr->read_buffer;
           buffer[uncompressed_length+(keyword_length+2)] = 0;

           text.compression = PNG_TEXT_COMPRESSION_zTXt;
           text.key = (png_charp)buffer;
           text.text = (png_charp)(buffer + keyword_length+2);
           text.text_length = uncompressed_length;
           text.itxt_length = 0;
           text.lang = NULL;
           text.lang_key = NULL;

           if (png_set_text_2(png_ptr, info_ptr, &text, 1) == 0)
              return handled_ok;

           errmsg = "out of memory";
        }
     }

     else
        errmsg = png_ptr->zstream.msg;
  }

  png_chunk_benign_error(png_ptr, errmsg);
  return handled_error;
}
#else
#  define png_handle_zTXt NULL
#endif

#ifdef PNG_READ_iTXt_SUPPORTED
/* Note: this does not correctly handle chunks that are > 64K under DOS */
static png_handle_result_code /* PRIVATE */
png_handle_iTXt(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  png_const_charp errmsg = NULL;
  png_bytep buffer;
  png_uint_32 prefix_length;

  png_debug(1, "in png_handle_iTXt");

#ifdef PNG_USER_LIMITS_SUPPORTED
  if (png_ptr->user_chunk_cache_max != 0)
  {
     if (png_ptr->user_chunk_cache_max == 1)
     {
        png_crc_finish(png_ptr, length);
        return handled_error;
     }

     if (--png_ptr->user_chunk_cache_max == 1)
     {
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, "no space in chunk cache");
        return handled_error;
     }
  }
#endif

  buffer = png_read_buffer(png_ptr, length+1);

  if (buffer == NULL)
  {
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "out of memory");
     return handled_error;
  }

  png_crc_read(png_ptr, buffer, length);

  if (png_crc_finish(png_ptr, 0) != 0)
     return handled_error;

  /* First the keyword. */
  for (prefix_length=0;
     prefix_length < length && buffer[prefix_length] != 0;
     ++prefix_length)
     /* Empty loop */ ;

  /* Perform a basic check on the keyword length here. */
  if (prefix_length > 79 || prefix_length < 1)
     errmsg = "bad keyword";

  /* Expect keyword, compression flag, compression type, language, translated
   * keyword (both may be empty but are 0 terminated) then the text, which may
   * be empty.
   */
  else if (prefix_length + 5 > length)
     errmsg = "truncated";

  else if (buffer[prefix_length+1] == 0 ||
     (buffer[prefix_length+1] == 1 &&
     buffer[prefix_length+2] == PNG_COMPRESSION_TYPE_BASE))
  {
     int compressed = buffer[prefix_length+1] != 0;
     png_uint_32 language_offset, translated_keyword_offset;
     png_alloc_size_t uncompressed_length = 0;

     /* Now the language tag */
     prefix_length += 3;
     language_offset = prefix_length;

     for (; prefix_length < length && buffer[prefix_length] != 0;
        ++prefix_length)
        /* Empty loop */ ;

     /* WARNING: the length may be invalid here, this is checked below. */
     translated_keyword_offset = ++prefix_length;

     for (; prefix_length < length && buffer[prefix_length] != 0;
        ++prefix_length)
        /* Empty loop */ ;

     /* prefix_length should now be at the trailing '\0' of the translated
      * keyword, but it may already be over the end.  None of this arithmetic
      * can overflow because chunks are at most 2^31 bytes long, but on 16-bit
      * systems the available allocation may overflow.
      */
     ++prefix_length;

     if (compressed == 0 && prefix_length <= length)
        uncompressed_length = length - prefix_length;

     else if (compressed != 0 && prefix_length < length)
     {
        uncompressed_length = PNG_SIZE_MAX;

        /* TODO: at present png_decompress_chunk imposes a single application
         * level memory limit, this should be split to different values for
         * iCCP and text chunks.
         */
        if (png_decompress_chunk(png_ptr, length, prefix_length,
            &uncompressed_length, 1/*terminate*/) == Z_STREAM_END)
           buffer = png_ptr->read_buffer;

        else
           errmsg = png_ptr->zstream.msg;
     }

     else
        errmsg = "truncated";

     if (errmsg == NULL)
     {
        png_text text;

        buffer[uncompressed_length+prefix_length] = 0;

        if (compressed == 0)
           text.compression = PNG_ITXT_COMPRESSION_NONE;

        else
           text.compression = PNG_ITXT_COMPRESSION_zTXt;

        text.key = (png_charp)buffer;
        text.lang = (png_charp)buffer + language_offset;
        text.lang_key = (png_charp)buffer + translated_keyword_offset;
        text.text = (png_charp)buffer + prefix_length;
        text.text_length = 0;
        text.itxt_length = uncompressed_length;

        if (png_set_text_2(png_ptr, info_ptr, &text, 1) == 0)
           return handled_ok;

        errmsg = "out of memory";
     }
  }

  else
     errmsg = "bad compression info";

  if (errmsg != NULL)
     png_chunk_benign_error(png_ptr, errmsg);
  return handled_error;
}
#else
#  define png_handle_iTXt NULL
#endif

#ifdef PNG_READ_APNG_SUPPORTED
void /* PRIVATE */
png_handle_acTL(png_structp png_ptr, png_infop info_ptr, png_uint_32 length)
{
   png_byte data[8];
   png_uint_32 num_frames;
   png_uint_32 num_plays;
   png_uint_32 didSet;

   png_debug(1, "in png_handle_acTL");

   if ((png_ptr->mode & PNG_HAVE_IHDR) == 0)
   {
       png_error(png_ptr, "Missing IHDR before acTL");
   }
   else if ((png_ptr->mode & PNG_HAVE_IDAT) != 0)
   {
       png_warning(png_ptr, "Invalid acTL after IDAT skipped");
       png_crc_finish(png_ptr, length);
       return;
   }
   else if ((png_ptr->mode & PNG_HAVE_acTL) != 0)
   {
       png_warning(png_ptr, "Duplicate acTL skipped");
       png_crc_finish(png_ptr, length);
       return;
   }
   else if (length != 8)
   {
       png_warning(png_ptr, "acTL with invalid length skipped");
       png_crc_finish(png_ptr, length);
       return;
   }

   png_crc_read(png_ptr, data, 8);
   png_crc_finish(png_ptr, 0);

   num_frames = png_get_uint_31(png_ptr, data);
   num_plays = png_get_uint_31(png_ptr, data + 4);

   /* the set function will do error checking on num_frames */
   didSet = png_set_acTL(png_ptr, info_ptr, num_frames, num_plays);
   if (didSet != 0)
       png_ptr->mode |= PNG_HAVE_acTL;
}

void /* PRIVATE */
png_handle_fcTL(png_structp png_ptr, png_infop info_ptr, png_uint_32 length)
{
   png_byte data[22];
   png_uint_32 width;
   png_uint_32 height;
   png_uint_32 x_offset;
   png_uint_32 y_offset;
   png_uint_16 delay_num;
   png_uint_16 delay_den;
   png_byte dispose_op;
   png_byte blend_op;

   png_debug(1, "in png_handle_fcTL");

   png_ensure_sequence_number(png_ptr, length);

   if ((png_ptr->mode & PNG_HAVE_IHDR) == 0)
   {
       png_error(png_ptr, "Missing IHDR before fcTL");
   }
   else if ((png_ptr->mode & PNG_HAVE_IDAT) != 0)
   {
       /* for any frames other then the first this message may be misleading,
       * but correct. PNG_HAVE_IDAT is unset before the frame head is read
       * i can't think of a better message */
       png_warning(png_ptr, "Invalid fcTL after IDAT skipped");
       png_crc_finish(png_ptr, length-4);
       return;
   }
   else if ((png_ptr->mode & PNG_HAVE_fcTL) != 0)
   {
       png_warning(png_ptr, "Duplicate fcTL within one frame skipped");
       png_crc_finish(png_ptr, length-4);
       return;
   }
   else if (length != 26)
   {
       png_warning(png_ptr, "fcTL with invalid length skipped");
       png_crc_finish(png_ptr, length-4);
       return;
   }

   png_crc_read(png_ptr, data, 22);
   png_crc_finish(png_ptr, 0);

   width = png_get_uint_31(png_ptr, data);
   height = png_get_uint_31(png_ptr, data + 4);
   x_offset = png_get_uint_31(png_ptr, data + 8);
   y_offset = png_get_uint_31(png_ptr, data + 12);
   delay_num = png_get_uint_16(data + 16);
   delay_den = png_get_uint_16(data + 18);
   dispose_op = data[20];
   blend_op = data[21];

   if (png_ptr->num_frames_read == 0 && (x_offset != 0 || y_offset != 0))
   {
       png_warning(png_ptr, "fcTL for the first frame must have zero offset");
       return;
   }

   if (info_ptr != NULL)
   {
       if (png_ptr->num_frames_read == 0 &&
           (width != info_ptr->width || height != info_ptr->height))
       {
           png_warning(png_ptr, "size in first frame's fcTL must match "
                              "the size in IHDR");
           return;
       }

       /* The set function will do more error checking */
       png_set_next_frame_fcTL(png_ptr, info_ptr, width, height,
                               x_offset, y_offset, delay_num, delay_den,
                               dispose_op, blend_op);

       png_read_reinit(png_ptr, info_ptr);

       png_ptr->mode |= PNG_HAVE_fcTL;
   }
}

void /* PRIVATE */
png_have_info(png_structp png_ptr, png_infop info_ptr)
{
   if ((info_ptr->valid & PNG_INFO_acTL) != 0 &&
       (info_ptr->valid & PNG_INFO_fcTL) == 0)
   {
       png_ptr->apng_flags |= PNG_FIRST_FRAME_HIDDEN;
       info_ptr->num_frames++;
   }
}

void /* PRIVATE */
png_handle_fdAT(png_structp png_ptr, png_infop info_ptr, png_uint_32 length)
{
   png_ensure_sequence_number(png_ptr, length);

   /* This function is only called from png_read_end(), png_read_info(),
   * and png_push_read_chunk() which means that:
   * - the user doesn't want to read this frame
   * - or this is an out-of-place fdAT
   * in either case it is safe to ignore the chunk with a warning */
   png_warning(png_ptr, "ignoring fdAT chunk");
   png_crc_finish(png_ptr, length - 4);
   PNG_UNUSED(info_ptr)
}

void /* PRIVATE */
png_ensure_sequence_number(png_structp png_ptr, png_uint_32 length)
{
   png_byte data[4];
   png_uint_32 sequence_number;

   if (length < 4)
       png_error(png_ptr, "invalid fcTL or fdAT chunk found");

   png_crc_read(png_ptr, data, 4);
   sequence_number = png_get_uint_31(png_ptr, data);

   if (sequence_number != png_ptr->next_seq_num)
       png_error(png_ptr, "fcTL or fdAT chunk with out-of-order sequence "
                          "number found");

   png_ptr->next_seq_num++;
}
#endif /* READ_APNG */

#ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
/* Utility function for png_handle_unknown; set up png_ptr::unknown_chunk */
static int
png_cache_unknown_chunk(png_structrp png_ptr, png_uint_32 length)
{
  const png_alloc_size_t limit = png_chunk_max(png_ptr);

  if (png_ptr->unknown_chunk.data != NULL)
  {
     png_free(png_ptr, png_ptr->unknown_chunk.data);
     png_ptr->unknown_chunk.data = NULL;
  }

  if (length <= limit)
  {
     PNG_CSTRING_FROM_CHUNK(png_ptr->unknown_chunk.name, png_ptr->chunk_name);
     /* The following is safe because of the PNG_SIZE_MAX init above */
     png_ptr->unknown_chunk.size = (size_t)length/*SAFE*/;
     /* 'mode' is a flag array, only the bottom four bits matter here */
     png_ptr->unknown_chunk.location = (png_byte)png_ptr->mode/*SAFE*/;

     if (length == 0)
        png_ptr->unknown_chunk.data = NULL;

     else
     {
        /* Do a 'warn' here - it is handled below. */
        png_ptr->unknown_chunk.data = png_voidcast(png_bytep,
            png_malloc_warn(png_ptr, length));
     }
  }

  if (png_ptr->unknown_chunk.data == NULL && length > 0)
  {
     /* This is benign because we clean up correctly */
     png_crc_finish(png_ptr, length);
     png_chunk_benign_error(png_ptr, "unknown chunk exceeds memory limits");
     return 0;
  }

  else
  {
     if (length > 0)
        png_crc_read(png_ptr, png_ptr->unknown_chunk.data, length);
     png_crc_finish(png_ptr, 0);
     return 1;
  }
}
#endif /* READ_UNKNOWN_CHUNKS */

/* Handle an unknown, or known but disabled, chunk */
png_handle_result_code /*PRIVATE*/
png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
   png_uint_32 length, int keep)
{
  png_handle_result_code handled = handled_discarded; /* the default */

  png_debug(1, "in png_handle_unknown");

#ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
  /* NOTE: this code is based on the code in libpng-1.4.12 except for fixing
   * the bug which meant that setting a non-default behavior for a specific
   * chunk would be ignored (the default was always used unless a user
   * callback was installed).
   *
   * 'keep' is the value from the png_chunk_unknown_handling, the setting for
   * this specific chunk_name, if PNG_HANDLE_AS_UNKNOWN_SUPPORTED, if not it
   * will always be PNG_HANDLE_CHUNK_AS_DEFAULT and it needs to be set here.
   * This is just an optimization to avoid multiple calls to the lookup
   * function.
   */
#  ifndef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
#     ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED
  keep = png_chunk_unknown_handling(png_ptr, png_ptr->chunk_name);
#     endif
#  endif

  /* One of the following methods will read the chunk or skip it (at least one
   * of these is always defined because this is the only way to switch on
   * PNG_READ_UNKNOWN_CHUNKS_SUPPORTED)
   */
#  ifdef PNG_READ_USER_CHUNKS_SUPPORTED
  /* The user callback takes precedence over the chunk keep value, but the
   * keep value is still required to validate a save of a critical chunk.
   */
  if (png_ptr->read_user_chunk_fn != NULL)
  {
     if (png_cache_unknown_chunk(png_ptr, length) != 0)
     {
        /* Callback to user unknown chunk handler */
        int ret = (*(png_ptr->read_user_chunk_fn))(png_ptr,
            &png_ptr->unknown_chunk);

        /* ret is:
         * negative: An error occurred; png_chunk_error will be called.
         *     zero: The chunk was not handled, the chunk will be discarded
         *           unless png_set_keep_unknown_chunks has been used to set
         *           a 'keep' behavior for this particular chunk, in which
         *           case that will be used.  A critical chunk will cause an
         *           error at this point unless it is to be saved.
         * positive: The chunk was handled, libpng will ignore/discard it.
         */
        if (ret < 0) /* handled_error */
           png_chunk_error(png_ptr, "error in user chunk");

        else if (ret == 0)
        {
           /* If the keep value is 'default' or 'never' override it, but
            * still error out on critical chunks unless the keep value is
            * 'always'  While this is weird it is the behavior in 1.4.12.
            * A possible improvement would be to obey the value set for the
            * chunk, but this would be an API change that would probably
            * damage some applications.
            *
            * The png_app_warning below catches the case that matters, where
            * the application has not set specific save or ignore for this
            * chunk or global save or ignore.
            */
           if (keep < PNG_HANDLE_CHUNK_IF_SAFE)
           {
#              ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED
              if (png_ptr->unknown_default < PNG_HANDLE_CHUNK_IF_SAFE)
              {
                 png_chunk_warning(png_ptr, "Saving unknown chunk:");
                 png_app_warning(png_ptr,
                     "forcing save of an unhandled chunk;"
                     " please call png_set_keep_unknown_chunks");
                     /* with keep = PNG_HANDLE_CHUNK_IF_SAFE */
              }
#              endif
              keep = PNG_HANDLE_CHUNK_IF_SAFE;
           }
        }

        else /* chunk was handled */
        {
           handled = handled_ok;
           /* Critical chunks can be safely discarded at this point. */
           keep = PNG_HANDLE_CHUNK_NEVER;
        }
     }

     else
        keep = PNG_HANDLE_CHUNK_NEVER; /* insufficient memory */
  }

  else
  /* Use the SAVE_UNKNOWN_CHUNKS code or skip the chunk */
#  endif /* READ_USER_CHUNKS */

#  ifdef PNG_SAVE_UNKNOWN_CHUNKS_SUPPORTED
  {
     /* keep is currently just the per-chunk setting, if there was no
      * setting change it to the global default now (not that this may
      * still be AS_DEFAULT) then obtain the cache of the chunk if required,
      * if not simply skip the chunk.
      */
     if (keep == PNG_HANDLE_CHUNK_AS_DEFAULT)
        keep = png_ptr->unknown_default;

     if (keep == PNG_HANDLE_CHUNK_ALWAYS ||
        (keep == PNG_HANDLE_CHUNK_IF_SAFE &&
         PNG_CHUNK_ANCILLARY(png_ptr->chunk_name)))
     {
        if (png_cache_unknown_chunk(png_ptr, length) == 0)
           keep = PNG_HANDLE_CHUNK_NEVER;
     }

     else
        png_crc_finish(png_ptr, length);
  }
#  else
#     ifndef PNG_READ_USER_CHUNKS_SUPPORTED
#        error no method to support READ_UNKNOWN_CHUNKS
#     endif

  {
     /* If here there is no read callback pointer set and no support is
      * compiled in to just save the unknown chunks, so simply skip this
      * chunk.  If 'keep' is something other than AS_DEFAULT or NEVER then
      * the app has erroneously asked for unknown chunk saving when there
      * is no support.
      */
     if (keep > PNG_HANDLE_CHUNK_NEVER)
        png_app_error(png_ptr, "no unknown chunk support available");

     png_crc_finish(png_ptr, length);
  }
#  endif

#  ifdef PNG_STORE_UNKNOWN_CHUNKS_SUPPORTED
  /* Now store the chunk in the chunk list if appropriate, and if the limits
   * permit it.
   */
  if (keep == PNG_HANDLE_CHUNK_ALWAYS ||
     (keep == PNG_HANDLE_CHUNK_IF_SAFE &&
      PNG_CHUNK_ANCILLARY(png_ptr->chunk_name)))
  {
#     ifdef PNG_USER_LIMITS_SUPPORTED
     switch (png_ptr->user_chunk_cache_max)
     {
        case 2:
           png_ptr->user_chunk_cache_max = 1;
           png_chunk_benign_error(png_ptr, "no space in chunk cache");
           /* FALLTHROUGH */
        case 1:
           /* NOTE: prior to 1.6.0 this case resulted in an unknown critical
            * chunk being skipped, now there will be a hard error below.
            */
           break;

        default: /* not at limit */
           --(png_ptr->user_chunk_cache_max);
           /* FALLTHROUGH */
        case 0: /* no limit */
#  endif /* USER_LIMITS */
           /* Here when the limit isn't reached or when limits are compiled
            * out; store the chunk.
            */
           png_set_unknown_chunks(png_ptr, info_ptr,
               &png_ptr->unknown_chunk, 1);
           handled = handled_saved;
#  ifdef PNG_USER_LIMITS_SUPPORTED
           break;
     }
#  endif
  }
#  else /* no store support: the chunk must be handled by the user callback */
  PNG_UNUSED(info_ptr)
#  endif

  /* Regardless of the error handling below the cached data (if any) can be
   * freed now.  Notice that the data is not freed if there is a png_error, but
   * it will be freed by destroy_read_struct.
   */
  if (png_ptr->unknown_chunk.data != NULL)
     png_free(png_ptr, png_ptr->unknown_chunk.data);
  png_ptr->unknown_chunk.data = NULL;

#else /* !PNG_READ_UNKNOWN_CHUNKS_SUPPORTED */
  /* There is no support to read an unknown chunk, so just skip it. */
  png_crc_finish(png_ptr, length);
  PNG_UNUSED(info_ptr)
  PNG_UNUSED(keep)
#endif /* !READ_UNKNOWN_CHUNKS */

  /* Check for unhandled critical chunks */
  if (handled < handled_saved && PNG_CHUNK_CRITICAL(png_ptr->chunk_name))
     png_chunk_error(png_ptr, "unhandled critical chunk");

  return handled;
}

/* APNG handling: the minimal implementation of APNG handling in libpng 1.6
* requires that those significant applications which already handle APNG not
* get hosed.  To do this ensure the code here will have to ensure than APNG
* data by default (at least in 1.6) gets stored in the unknown chunk list.
* Maybe this can be relaxed in a few years but at present it's just the only
* safe way.
*
* ATM just cause unknown handling for all three chunks:
*/
#define png_handle_acTL NULL
#define png_handle_fcTL NULL
#define png_handle_fdAT NULL

/*
* 1.6.47: This is the new table driven interface to all the chunk handling.
*
* The table describes the PNG standard rules for **reading** known chunks -
* every chunk which has an entry in PNG_KNOWN_CHUNKS.  The table contains an
* entry for each PNG_INDEX_cHNK describing the rules.
*
* In this initial version the only information in the entry is the
* png_handle_cHNK function for the chunk in question.  When chunk support is
* compiled out the entry will be NULL.
*/
static const struct
{
  png_handle_result_code (*handler)(
        png_structrp, png_inforp, png_uint_32 length);
     /* A chunk-specific 'handler', NULL if the chunk is not supported in this
      * build.
      */

  /* Crushing these values helps on modern 32-bit architectures because the
   * pointer and the following bit fields both end up requiring 32 bits.
   * Typically this will halve the table size.  On 64-bit architectures the
   * table entries will typically be 8 bytes.
   */
  png_uint_32 max_length :12; /* Length min, max in bytes */
  png_uint_32 min_length :8;
     /* Length errors on critical chunks have special handling to preserve the
      * existing behaviour in libpng 1.6.  Anciallary chunks are checked below
      * and produce a 'benign' error.
      */
  png_uint_32 pos_before :4; /* PNG_HAVE_ values chunk must precede */
  png_uint_32 pos_after  :4; /* PNG_HAVE_ values chunk must follow */
     /* NOTE: PLTE, tRNS and bKGD require special handling which depends on
      * the colour type of the base image.
      */
  png_uint_32 multiple   :1; /* Multiple occurences permitted */
     /* This is enabled for PLTE because PLTE may, in practice, be optional */
}
read_chunks[PNG_INDEX_unknown] =
{
  /* Definitions as above but done indirectly by #define so that
   * PNG_KNOWN_CHUNKS can be used safely to build the table in order.
   *
   * Each CDcHNK definition lists the values for the parameters **after**
   * the first, 'handler', function.  'handler' is NULL when the chunk has no
   * compiled in support.
   */
#  define NoCheck 0x801U      /* Do not check the maximum length */
#  define Limit   0x802U      /* Limit to png_chunk_max bytes */
#  define LKMin   3U+LZ77Min  /* Minimum length of keyword+LZ77 */

#define hIHDR PNG_HAVE_IHDR
#define hPLTE PNG_HAVE_PLTE
#define hIDAT PNG_HAVE_IDAT
  /* For the two chunks, tRNS and bKGD which can occur in PNGs without a PLTE
   * but must occur after the PLTE use this and put the check in the handler
   * routine for colour mapped images were PLTE is required.  Also put a check
   * in PLTE for other image types to drop the PLTE if tRNS or bKGD have been
   * seen.
   */
#define hCOL  (PNG_HAVE_PLTE|PNG_HAVE_IDAT)
  /* Used for the decoding chunks which must be before PLTE. */
#define aIDAT PNG_AFTER_IDAT

  /* Chunks from W3C PNG v3: */
  /*       cHNK  max_len,   min, before, after, multiple */
#  define CDIHDR      13U,   13U,  hIHDR,     0,        0
#  define CDPLTE  NoCheck,    0U,      0, hIHDR,        1
     /* PLTE errors are only critical for colour-map images, consequently the
      * hander does all the checks.
      */
#  define CDIDAT  NoCheck,    0U,  aIDAT, hIHDR,        1
#  define CDIEND  NoCheck,    0U,      0, aIDAT,        0
     /* Historically data was allowed in IEND */
#  define CDtRNS     256U,    0U,  hIDAT, hIHDR,        0
#  define CDcHRM      32U,   32U,   hCOL, hIHDR,        0
#  define CDgAMA       4U,    4U,   hCOL, hIHDR,        0
#  define CDiCCP  NoCheck, LKMin,   hCOL, hIHDR,        0
#  define CDsBIT       4U,    1U,   hCOL, hIHDR,        0
#  define CDsRGB       1U,    1U,   hCOL, hIHDR,        0
#  define CDcICP       4U,    4U,   hCOL, hIHDR,        0
#  define CDmDCV      24U,   24U,   hCOL, hIHDR,        0
#  define CDeXIf    Limit,    4U,      0, hIHDR,        0
#  define CDcLLI       8U,    8U,   hCOL, hIHDR,        0
#  define CDtEXt  NoCheck,    2U,      0, hIHDR,        1
     /* Allocates 'length+1'; checked in the handler */
#  define CDzTXt    Limit, LKMin,      0, hIHDR,        1
#  define CDiTXt  NoCheck,    6U,      0, hIHDR,        1
     /* Allocates 'length+1'; checked in the handler */
#  define CDbKGD       6U,    1U,  hIDAT, hIHDR,        0
#  define CDhIST    1024U,    0U,  hPLTE, hIHDR,        0
#  define CDpHYs       9U,    9U,  hIDAT, hIHDR,        0
#  define CDsPLT  NoCheck,    3U,  hIDAT, hIHDR,        1
     /* Allocates 'length+1'; checked in the handler */
#  define CDtIME       7U,    7U,      0, hIHDR,        0
#  define CDacTL       8U,    8U,  hIDAT, hIHDR,        0
#  define CDfcTL      25U,   26U,      0, hIHDR,        1
#  define CDfdAT    Limit,    4U,  hIDAT, hIHDR,        1
  /* Supported chunks from PNG extensions 1.5.0, NYI so limit */
#  define CDoFFs       9U,    9U,  hIDAT, hIHDR,        0
#  define CDpCAL  NoCheck,   14U,  hIDAT, hIHDR,        0
     /* Allocates 'length+1'; checked in the handler */
#  define CDsCAL    Limit,    4U,  hIDAT, hIHDR,        0
     /* Allocates 'length+1'; checked in the handler */

#  define PNG_CHUNK(cHNK, index) { png_handle_ ## cHNK, CD ## cHNK },
  PNG_KNOWN_CHUNKS
#  undef PNG_CHUNK
};


static png_index
png_chunk_index_from_name(png_uint_32 chunk_name)
{
  /* For chunk png_cHNK return PNG_INDEX_cHNK.  Return PNG_INDEX_unknown if
   * chunk_name is not known.  Notice that in a particular build "known" does
   * not necessarily mean "supported", although the inverse applies.
   */
  switch (chunk_name)
  {
#     define PNG_CHUNK(cHNK, index)\
        case png_ ## cHNK: return PNG_INDEX_ ## cHNK; /* == index */

     PNG_KNOWN_CHUNKS

#     undef PNG_CHUNK

     default: return PNG_INDEX_unknown;
  }
}

png_handle_result_code /*PRIVATE*/
png_handle_chunk(png_structrp png_ptr, png_inforp info_ptr, png_uint_32 length)
{
  /* CSE: these things don't change, these autos are just to save typing and
   * make the code more clear.
   */
  const png_uint_32 chunk_name = png_ptr->chunk_name;
  const png_index chunk_index = png_chunk_index_from_name(chunk_name);

  png_handle_result_code handled = handled_error;
  png_const_charp errmsg = NULL;

  /* Is this a known chunk?  If not there are no checks performed here;
   * png_handle_unknown does the correct checks.  This means that the values
   * for known but unsupported chunks in the above table are not used here
   * however the chunks_seen fields in png_struct are still set.
   */
  if (chunk_index == PNG_INDEX_unknown ||
      read_chunks[chunk_index].handler == NULL)
  {
     handled = png_handle_unknown(
           png_ptr, info_ptr, length, PNG_HANDLE_CHUNK_AS_DEFAULT);
  }

  /* First check the position.   The first check is historical; the stream must
   * start with IHDR and anything else causes libpng to give up immediately.
   */
  else if (chunk_index != PNG_INDEX_IHDR &&
           (png_ptr->mode & PNG_HAVE_IHDR) == 0)
     png_chunk_error(png_ptr, "missing IHDR"); /* NORETURN */

  /* Before all the pos_before chunks, after all the pos_after chunks. */
  else if (((png_ptr->mode & read_chunks[chunk_index].pos_before) != 0) ||
           ((png_ptr->mode & read_chunks[chunk_index].pos_after) !=
            read_chunks[chunk_index].pos_after))
  {
     errmsg = "out of place";
  }

  /* Now check for duplicates: duplicated critical chunks also produce a
   * full error.
   */
  else if (read_chunks[chunk_index].multiple == 0 &&
           png_file_has_chunk(png_ptr, chunk_index))
  {
     errmsg = "duplicate";
  }

  else if (length < read_chunks[chunk_index].min_length)
     errmsg = "too short";
  else
  {
     /* NOTE: apart from IHDR the critical chunks (PLTE, IDAT and IEND) are set
      * up above not to do any length checks.
      *
      * The png_chunk_max check ensures that the variable length chunks are
      * always checked at this point for being within the system allocation
      * limits.
      */
     unsigned max_length = read_chunks[chunk_index].max_length;

     switch (max_length)
     {
        case Limit:
           /* png_read_chunk_header has already png_error'ed chunks with a
            * length exceeding the 31-bit PNG limit, so just check the memory
            * limit:
            */
           if (length <= png_chunk_max(png_ptr))
              goto MeetsLimit;

           errmsg = "length exceeds libpng limit";
           break;

        default:
           if (length <= max_length)
              goto MeetsLimit;

           errmsg = "too long";
           break;

        case NoCheck:
        MeetsLimit:
           handled = read_chunks[chunk_index].handler(
                 png_ptr, info_ptr, length);
           break;
     }
  }

  /* If there was an error or the chunk was simply skipped it is not counted as
   * 'seen'.
   */
  if (errmsg != NULL)
  {
     if (PNG_CHUNK_CRITICAL(chunk_name)) /* stop immediately */
        png_chunk_error(png_ptr, errmsg);
     else /* ancillary chunk */
     {
        /* The chunk data is skipped: */
        png_crc_finish(png_ptr, length);
        png_chunk_benign_error(png_ptr, errmsg);
     }
  }

  else if (handled >= handled_saved)
  {
     if (chunk_index != PNG_INDEX_unknown)
        png_file_add_chunk(png_ptr, chunk_index);
  }

  return handled;
}

/* Combines the row recently read in with the existing pixels in the row.  This
* routine takes care of alpha and transparency if requested.  This routine also
* handles the two methods of progressive display of interlaced images,
* depending on the 'display' value; if 'display' is true then the whole row
* (dp) is filled from the start by replicating the available pixels.  If
* 'display' is false only those pixels present in the pass are filled in.
*/
void /* PRIVATE */
png_combine_row(png_const_structrp png_ptr, png_bytep dp, int display)
{
  unsigned int pixel_depth = png_ptr->transformed_pixel_depth;
  png_const_bytep sp = png_ptr->row_buf + 1;
  png_alloc_size_t row_width = png_ptr->width;
  unsigned int pass = png_ptr->pass;
  png_bytep end_ptr = 0;
  png_byte end_byte = 0;
  unsigned int end_mask;

  png_debug(1, "in png_combine_row");

  /* Added in 1.5.6: it should not be possible to enter this routine until at
   * least one row has been read from the PNG data and transformed.
   */
  if (pixel_depth == 0)
     png_error(png_ptr, "internal row logic error");

  /* Added in 1.5.4: the pixel depth should match the information returned by
   * any call to png_read_update_info at this point.  Do not continue if we got
   * this wrong.
   */
  if (png_ptr->info_rowbytes != 0 && png_ptr->info_rowbytes !=
         PNG_ROWBYTES(pixel_depth, row_width))
     png_error(png_ptr, "internal row size calculation error");

  /* Don't expect this to ever happen: */
  if (row_width == 0)
     png_error(png_ptr, "internal row width error");

  /* Preserve the last byte in cases where only part of it will be overwritten,
   * the multiply below may overflow, we don't care because ANSI-C guarantees
   * we get the low bits.
   */
  end_mask = (pixel_depth * row_width) & 7;
  if (end_mask != 0)
  {
     /* end_ptr == NULL is a flag to say do nothing */
     end_ptr = dp + PNG_ROWBYTES(pixel_depth, row_width) - 1;
     end_byte = *end_ptr;
#     ifdef PNG_READ_PACKSWAP_SUPPORTED
     if ((png_ptr->transformations & PNG_PACKSWAP) != 0)
        /* little-endian byte */
        end_mask = (unsigned int)(0xff << end_mask);

     else /* big-endian byte */
#     endif
     end_mask = 0xff >> end_mask;
     /* end_mask is now the bits to *keep* from the destination row */
  }

  /* For non-interlaced images this reduces to a memcpy(). A memcpy()
   * will also happen if interlacing isn't supported or if the application
   * does not call png_set_interlace_handling().  In the latter cases the
   * caller just gets a sequence of the unexpanded rows from each interlace
   * pass.
   */
#ifdef PNG_READ_INTERLACING_SUPPORTED
  if (png_ptr->interlaced != 0 &&
      (png_ptr->transformations & PNG_INTERLACE) != 0 &&
      pass < 6 && (display == 0 ||
      /* The following copies everything for 'display' on passes 0, 2 and 4. */
      (display == 1 && (pass & 1) != 0)))
  {
     /* Narrow images may have no bits in a pass; the caller should handle
      * this, but this test is cheap:
      */
     if (row_width <= PNG_PASS_START_COL(pass))
        return;

     if (pixel_depth < 8)
     {
        /* For pixel depths up to 4 bpp the 8-pixel mask can be expanded to fit
         * into 32 bits, then a single loop over the bytes using the four byte
         * values in the 32-bit mask can be used.  For the 'display' option the
         * expanded mask may also not require any masking within a byte.  To
         * make this work the PACKSWAP option must be taken into account - it
         * simply requires the pixels to be reversed in each byte.
         *
         * The 'regular' case requires a mask for each of the first 6 passes,
         * the 'display' case does a copy for the even passes in the range
         * 0..6.  This has already been handled in the test above.
         *
         * The masks are arranged as four bytes with the first byte to use in
         * the lowest bits (little-endian) regardless of the order (PACKSWAP or
         * not) of the pixels in each byte.
         *
         * NOTE: the whole of this logic depends on the caller of this function
         * only calling it on rows appropriate to the pass.  This function only
         * understands the 'x' logic; the 'y' logic is handled by the caller.
         *
         * The following defines allow generation of compile time constant bit
         * masks for each pixel depth and each possibility of swapped or not
         * swapped bytes.  Pass 'p' is in the range 0..6; 'x', a pixel index,
         * is in the range 0..7; and the result is 1 if the pixel is to be
         * copied in the pass, 0 if not.  'S' is for the sparkle method, 'B'
         * for the block method.
         *
         * With some compilers a compile time expression of the general form:
         *
         *    (shift >= 32) ? (a >> (shift-32)) : (b >> shift)
         *
         * Produces warnings with values of 'shift' in the range 33 to 63
         * because the right hand side of the ?: expression is evaluated by
         * the compiler even though it isn't used.  Microsoft Visual C (various
         * versions) and the Intel C compiler are known to do this.  To avoid
         * this the following macros are used in 1.5.6.  This is a temporary
         * solution to avoid destabilizing the code during the release process.
         */
#        if PNG_USE_COMPILE_TIME_MASKS
#           define PNG_LSR(x,s) ((x)>>((s) & 0x1f))
#           define PNG_LSL(x,s) ((x)<<((s) & 0x1f))
#        else
#           define PNG_LSR(x,s) ((x)>>(s))
#           define PNG_LSL(x,s) ((x)<<(s))
#        endif
#        define S_COPY(p,x) (((p)<4 ? PNG_LSR(0x80088822,(3-(p))*8+(7-(x))) :\
          PNG_LSR(0xaa55ff00,(7-(p))*8+(7-(x)))) & 1)
#        define B_COPY(p,x) (((p)<4 ? PNG_LSR(0xff0fff33,(3-(p))*8+(7-(x))) :\
          PNG_LSR(0xff55ff00,(7-(p))*8+(7-(x)))) & 1)

        /* Return a mask for pass 'p' pixel 'x' at depth 'd'.  The mask is
         * little endian - the first pixel is at bit 0 - however the extra
         * parameter 's' can be set to cause the mask position to be swapped
         * within each byte, to match the PNG format.  This is done by XOR of
         * the shift with 7, 6 or 4 for bit depths 1, 2 and 4.
         */
#        define PIXEL_MASK(p,x,d,s) \
           (PNG_LSL(((PNG_LSL(1U,(d)))-1),(((x)*(d))^((s)?8-(d):0))))

        /* Hence generate the appropriate 'block' or 'sparkle' pixel copy mask.
         */
#        define S_MASKx(p,x,d,s) (S_COPY(p,x)?PIXEL_MASK(p,x,d,s):0)
#        define B_MASKx(p,x,d,s) (B_COPY(p,x)?PIXEL_MASK(p,x,d,s):0)

        /* Combine 8 of these to get the full mask.  For the 1-bpp and 2-bpp
         * cases the result needs replicating, for the 4-bpp case the above
         * generates a full 32 bits.
         */
#        define MASK_EXPAND(m,d) ((m)*((d)==1?0x01010101:((d)==2?0x00010001:1)))

#        define S_MASK(p,d,s) MASK_EXPAND(S_MASKx(p,0,d,s) + S_MASKx(p,1,d,s) +\
           S_MASKx(p,2,d,s) + S_MASKx(p,3,d,s) + S_MASKx(p,4,d,s) +\
           S_MASKx(p,5,d,s) + S_MASKx(p,6,d,s) + S_MASKx(p,7,d,s), d)

#        define B_MASK(p,d,s) MASK_EXPAND(B_MASKx(p,0,d,s) + B_MASKx(p,1,d,s) +\
           B_MASKx(p,2,d,s) + B_MASKx(p,3,d,s) + B_MASKx(p,4,d,s) +\
           B_MASKx(p,5,d,s) + B_MASKx(p,6,d,s) + B_MASKx(p,7,d,s), d)

#if PNG_USE_COMPILE_TIME_MASKS
        /* Utility macros to construct all the masks for a depth/swap
         * combination.  The 's' parameter says whether the format is PNG
         * (big endian bytes) or not.  Only the three odd-numbered passes are
         * required for the display/block algorithm.
         */
#        define S_MASKS(d,s) { S_MASK(0,d,s), S_MASK(1,d,s), S_MASK(2,d,s),\
           S_MASK(3,d,s), S_MASK(4,d,s), S_MASK(5,d,s) }

#        define B_MASKS(d,s) { B_MASK(1,d,s), B_MASK(3,d,s), B_MASK(5,d,s) }

#        define DEPTH_INDEX(d) ((d)==1?0:((d)==2?1:2))

        /* Hence the pre-compiled masks indexed by PACKSWAP (or not), depth and
         * then pass:
         */
        static const png_uint_32 row_mask[2/*PACKSWAP*/][3/*depth*/][6] =
        {
           /* Little-endian byte masks for PACKSWAP */
           { S_MASKS(1,0), S_MASKS(2,0), S_MASKS(4,0) },
           /* Normal (big-endian byte) masks - PNG format */
           { S_MASKS(1,1), S_MASKS(2,1), S_MASKS(4,1) }
        };

        /* display_mask has only three entries for the odd passes, so index by
         * pass>>1.
         */
        static const png_uint_32 display_mask[2][3][3] =
        {
           /* Little-endian byte masks for PACKSWAP */
           { B_MASKS(1,0), B_MASKS(2,0), B_MASKS(4,0) },
           /* Normal (big-endian byte) masks - PNG format */
           { B_MASKS(1,1), B_MASKS(2,1), B_MASKS(4,1) }
        };

#        define MASK(pass,depth,display,png)\
           ((display)?display_mask[png][DEPTH_INDEX(depth)][pass>>1]:\
              row_mask[png][DEPTH_INDEX(depth)][pass])

#else /* !PNG_USE_COMPILE_TIME_MASKS */
        /* This is the runtime alternative: it seems unlikely that this will
         * ever be either smaller or faster than the compile time approach.
         */
#        define MASK(pass,depth,display,png)\
           ((display)?B_MASK(pass,depth,png):S_MASK(pass,depth,png))
#endif /* !USE_COMPILE_TIME_MASKS */

        /* Use the appropriate mask to copy the required bits.  In some cases
         * the byte mask will be 0 or 0xff; optimize these cases.  row_width is
         * the number of pixels, but the code copies bytes, so it is necessary
         * to special case the end.
         */
        png_uint_32 pixels_per_byte = 8 / pixel_depth;
        png_uint_32 mask;

#        ifdef PNG_READ_PACKSWAP_SUPPORTED
        if ((png_ptr->transformations & PNG_PACKSWAP) != 0)
           mask = MASK(pass, pixel_depth, display, 0);

        else
#        endif
        mask = MASK(pass, pixel_depth, display, 1);

        for (;;)
        {
           png_uint_32 m;

           /* It doesn't matter in the following if png_uint_32 has more than
            * 32 bits because the high bits always match those in m<<24; it is,
            * however, essential to use OR here, not +, because of this.
            */
           m = mask;
           mask = (m >> 8) | (m << 24); /* rotate right to good compilers */
           m &= 0xff;

           if (m != 0) /* something to copy */
           {
              if (m != 0xff)
                 *dp = (png_byte)((*dp & ~m) | (*sp & m));
              else
                 *dp = *sp;
           }

           /* NOTE: this may overwrite the last byte with garbage if the image
            * is not an exact number of bytes wide; libpng has always done
            * this.
            */
           if (row_width <= pixels_per_byte)
              break; /* May need to restore part of the last byte */

           row_width -= pixels_per_byte;
           ++dp;
           ++sp;
        }
     }

     else /* pixel_depth >= 8 */
     {
        unsigned int bytes_to_copy, bytes_to_jump;

        /* Validate the depth - it must be a multiple of 8 */
        if (pixel_depth & 7)
           png_error(png_ptr, "invalid user transform pixel depth");

        pixel_depth >>= 3; /* now in bytes */
        row_width *= pixel_depth;

        /* Regardless of pass number the Adam 7 interlace always results in a
         * fixed number of pixels to copy then to skip.  There may be a
         * different number of pixels to skip at the start though.
         */
        {
           unsigned int offset = PNG_PASS_START_COL(pass) * pixel_depth;

           row_width -= offset;
           dp += offset;
           sp += offset;
        }

        /* Work out the bytes to copy. */
        if (display != 0)
        {
           /* When doing the 'block' algorithm the pixel in the pass gets
            * replicated to adjacent pixels.  This is why the even (0,2,4,6)
            * passes are skipped above - the entire expanded row is copied.
            */
           bytes_to_copy = (1<<((6-pass)>>1)) * pixel_depth;

           /* But don't allow this number to exceed the actual row width. */
           if (bytes_to_copy > row_width)
              bytes_to_copy = (unsigned int)/*SAFE*/row_width;
        }

        else /* normal row; Adam7 only ever gives us one pixel to copy. */
           bytes_to_copy = pixel_depth;

        /* In Adam7 there is a constant offset between where the pixels go. */
        bytes_to_jump = PNG_PASS_COL_OFFSET(pass) * pixel_depth;

        /* And simply copy these bytes.  Some optimization is possible here,
         * depending on the value of 'bytes_to_copy'.  Special case the low
         * byte counts, which we know to be frequent.
         *
         * Notice that these cases all 'return' rather than 'break' - this
         * avoids an unnecessary test on whether to restore the last byte
         * below.
         */
        switch (bytes_to_copy)
        {
           case 1:
              for (;;)
              {
                 *dp = *sp;

                 if (row_width <= bytes_to_jump)
                    return;

                 dp += bytes_to_jump;
                 sp += bytes_to_jump;
                 row_width -= bytes_to_jump;
              }

           case 2:
              /* There is a possibility of a partial copy at the end here; this
               * slows the code down somewhat.
               */
              do
              {
                 dp[0] = sp[0]; dp[1] = sp[1];

                 if (row_width <= bytes_to_jump)
                    return;

                 sp += bytes_to_jump;
                 dp += bytes_to_jump;
                 row_width -= bytes_to_jump;
              }
              while (row_width > 1);

              /* And there can only be one byte left at this point: */
              *dp = *sp;
              return;

           case 3:
              /* This can only be the RGB case, so each copy is exactly one
               * pixel and it is not necessary to check for a partial copy.
               */
              for (;;)
              {
                 dp[0] = sp[0]; dp[1] = sp[1]; dp[2] = sp[2];

                 if (row_width <= bytes_to_jump)
                    return;

                 sp += bytes_to_jump;
                 dp += bytes_to_jump;
                 row_width -= bytes_to_jump;
              }

           default:
#if PNG_ALIGN_TYPE != PNG_ALIGN_NONE
              /* Check for double byte alignment and, if possible, use a
               * 16-bit copy.  Don't attempt this for narrow images - ones that
               * are less than an interlace panel wide.  Don't attempt it for
               * wide bytes_to_copy either - use the memcpy there.
               */
              if (bytes_to_copy < 16 /*else use memcpy*/ &&
                  png_isaligned(dp, png_uint_16) &&
                  png_isaligned(sp, png_uint_16) &&
                  bytes_to_copy % (sizeof (png_uint_16)) == 0 &&
                  bytes_to_jump % (sizeof (png_uint_16)) == 0)
              {
                 /* Everything is aligned for png_uint_16 copies, but try for
                  * png_uint_32 first.
                  */
                 if (png_isaligned(dp, png_uint_32) &&
                     png_isaligned(sp, png_uint_32) &&
                     bytes_to_copy % (sizeof (png_uint_32)) == 0 &&
                     bytes_to_jump % (sizeof (png_uint_32)) == 0)
                 {
                    png_uint_32p dp32 = png_aligncast(png_uint_32p,dp);
                    png_const_uint_32p sp32 = png_aligncastconst(
                        png_const_uint_32p, sp);
                    size_t skip = (bytes_to_jump-bytes_to_copy) /
                        (sizeof (png_uint_32));

                    do
                    {
                       size_t c = bytes_to_copy;
                       do
                       {
                          *dp32++ = *sp32++;
                          c -= (sizeof (png_uint_32));
                       }
                       while (c > 0);

                       if (row_width <= bytes_to_jump)
                          return;

                       dp32 += skip;
                       sp32 += skip;
                       row_width -= bytes_to_jump;
                    }
                    while (bytes_to_copy <= row_width);

                    /* Get to here when the row_width truncates the final copy.
                     * There will be 1-3 bytes left to copy, so don't try the
                     * 16-bit loop below.
                     */
                    dp = (png_bytep)dp32;
                    sp = (png_const_bytep)sp32;
                    do
                       *dp++ = *sp++;
                    while (--row_width > 0);
                    return;
                 }

                 /* Else do it in 16-bit quantities, but only if the size is
                  * not too large.
                  */
                 else
                 {
                    png_uint_16p dp16 = png_aligncast(png_uint_16p, dp);
                    png_const_uint_16p sp16 = png_aligncastconst(
                       png_const_uint_16p, sp);
                    size_t skip = (bytes_to_jump-bytes_to_copy) /
                       (sizeof (png_uint_16));

                    do
                    {
                       size_t c = bytes_to_copy;
                       do
                       {
                          *dp16++ = *sp16++;
                          c -= (sizeof (png_uint_16));
                       }
                       while (c > 0);

                       if (row_width <= bytes_to_jump)
                          return;

                       dp16 += skip;
                       sp16 += skip;
                       row_width -= bytes_to_jump;
                    }
                    while (bytes_to_copy <= row_width);

                    /* End of row - 1 byte left, bytes_to_copy > row_width: */
                    dp = (png_bytep)dp16;
                    sp = (png_const_bytep)sp16;
                    do
                       *dp++ = *sp++;
                    while (--row_width > 0);
                    return;
                 }
              }
#endif /* ALIGN_TYPE code */

              /* The true default - use a memcpy: */
              for (;;)
              {
                 memcpy(dp, sp, bytes_to_copy);

                 if (row_width <= bytes_to_jump)
                    return;

                 sp += bytes_to_jump;
                 dp += bytes_to_jump;
                 row_width -= bytes_to_jump;
                 if (bytes_to_copy > row_width)
                    bytes_to_copy = (unsigned int)/*SAFE*/row_width;
              }
        }

        /* NOT REACHED*/
     } /* pixel_depth >= 8 */

     /* Here if pixel_depth < 8 to check 'end_ptr' below. */
  }
  else
#endif /* READ_INTERLACING */

  /* If here then the switch above wasn't used so just memcpy the whole row
   * from the temporary row buffer (notice that this overwrites the end of the
   * destination row if it is a partial byte.)
   */
  memcpy(dp, sp, PNG_ROWBYTES(pixel_depth, row_width));

  /* Restore the overwritten bits from the last byte if necessary. */
  if (end_ptr != NULL)
     *end_ptr = (png_byte)((end_byte & end_mask) | (*end_ptr & ~end_mask));
}

#ifdef PNG_READ_INTERLACING_SUPPORTED
void /* PRIVATE */
png_do_read_interlace(png_row_infop row_info, png_bytep row, int pass,
   png_uint_32 transformations /* Because these may affect the byte layout */)
{
  png_debug(1, "in png_do_read_interlace");
  if (row != NULL && row_info != NULL)
  {
     png_uint_32 final_width;

     final_width = row_info->width * png_pass_inc[pass];

     switch (row_info->pixel_depth)
     {
        case 1:
        {
           png_bytep sp = row + (size_t)((row_info->width - 1) >> 3);
           png_bytep dp = row + (size_t)((final_width - 1) >> 3);
           unsigned int sshift, dshift;
           unsigned int s_start, s_end;
           int s_inc;
           int jstop = (int)png_pass_inc[pass];
           png_byte v;
           png_uint_32 i;
           int j;

#ifdef PNG_READ_PACKSWAP_SUPPORTED
           if ((transformations & PNG_PACKSWAP) != 0)
           {
               sshift = ((row_info->width + 7) & 0x07);
               dshift = ((final_width + 7) & 0x07);
               s_start = 7;
               s_end = 0;
               s_inc = -1;
           }

           else
#endif
           {
               sshift = 7 - ((row_info->width + 7) & 0x07);
               dshift = 7 - ((final_width + 7) & 0x07);
               s_start = 0;
               s_end = 7;
               s_inc = 1;
           }

           for (i = 0; i < row_info->width; i++)
           {
              v = (png_byte)((*sp >> sshift) & 0x01);
              for (j = 0; j < jstop; j++)
              {
                 unsigned int tmp = *dp & (0x7f7f >> (7 - dshift));
                 tmp |= (unsigned int)(v << dshift);
                 *dp = (png_byte)(tmp & 0xff);

                 if (dshift == s_end)
                 {
                    dshift = s_start;
                    dp--;
                 }

                 else
                    dshift = (unsigned int)((int)dshift + s_inc);
              }

              if (sshift == s_end)
              {
                 sshift = s_start;
                 sp--;
              }

              else
                 sshift = (unsigned int)((int)sshift + s_inc);
           }
           break;
        }

        case 2:
        {
           png_bytep sp = row + (png_uint_32)((row_info->width - 1) >> 2);
           png_bytep dp = row + (png_uint_32)((final_width - 1) >> 2);
           unsigned int sshift, dshift;
           unsigned int s_start, s_end;
           int s_inc;
           int jstop = (int)png_pass_inc[pass];
           png_uint_32 i;

#ifdef PNG_READ_PACKSWAP_SUPPORTED
           if ((transformations & PNG_PACKSWAP) != 0)
           {
              sshift = (((row_info->width + 3) & 0x03) << 1);
              dshift = (((final_width + 3) & 0x03) << 1);
              s_start = 6;
              s_end = 0;
              s_inc = -2;
           }

           else
#endif
           {
              sshift = ((3 - ((row_info->width + 3) & 0x03)) << 1);
              dshift = ((3 - ((final_width + 3) & 0x03)) << 1);
              s_start = 0;
              s_end = 6;
              s_inc = 2;
           }

           for (i = 0; i < row_info->width; i++)
           {
              png_byte v;
              int j;

              v = (png_byte)((*sp >> sshift) & 0x03);
              for (j = 0; j < jstop; j++)
              {
                 unsigned int tmp = *dp & (0x3f3f >> (6 - dshift));
                 tmp |= (unsigned int)(v << dshift);
                 *dp = (png_byte)(tmp & 0xff);

                 if (dshift == s_end)
                 {
                    dshift = s_start;
                    dp--;
                 }

                 else
                    dshift = (unsigned int)((int)dshift + s_inc);
              }

              if (sshift == s_end)
              {
                 sshift = s_start;
                 sp--;
              }

              else
                 sshift = (unsigned int)((int)sshift + s_inc);
           }
           break;
        }

        case 4:
        {
           png_bytep sp = row + (size_t)((row_info->width - 1) >> 1);
           png_bytep dp = row + (size_t)((final_width - 1) >> 1);
           unsigned int sshift, dshift;
           unsigned int s_start, s_end;
           int s_inc;
           png_uint_32 i;
           int jstop = (int)png_pass_inc[pass];

#ifdef PNG_READ_PACKSWAP_SUPPORTED
           if ((transformations & PNG_PACKSWAP) != 0)
           {
              sshift = (((row_info->width + 1) & 0x01) << 2);
              dshift = (((final_width + 1) & 0x01) << 2);
              s_start = 4;
              s_end = 0;
              s_inc = -4;
           }

           else
#endif
           {
              sshift = ((1 - ((row_info->width + 1) & 0x01)) << 2);
              dshift = ((1 - ((final_width + 1) & 0x01)) << 2);
              s_start = 0;
              s_end = 4;
              s_inc = 4;
           }

           for (i = 0; i < row_info->width; i++)
           {
              png_byte v = (png_byte)((*sp >> sshift) & 0x0f);
              int j;

              for (j = 0; j < jstop; j++)
              {
                 unsigned int tmp = *dp & (0xf0f >> (4 - dshift));
                 tmp |= (unsigned int)(v << dshift);
                 *dp = (png_byte)(tmp & 0xff);

                 if (dshift == s_end)
                 {
                    dshift = s_start;
                    dp--;
                 }

                 else
                    dshift = (unsigned int)((int)dshift + s_inc);
              }

              if (sshift == s_end)
              {
                 sshift = s_start;
                 sp--;
              }

              else
                 sshift = (unsigned int)((int)sshift + s_inc);
           }
           break;
        }

        default:
        {
           size_t pixel_bytes = (row_info->pixel_depth >> 3);

           png_bytep sp = row + (size_t)(row_info->width - 1)
               * pixel_bytes;

           png_bytep dp = row + (size_t)(final_width - 1) * pixel_bytes;

           int jstop = (int)png_pass_inc[pass];
           png_uint_32 i;

           for (i = 0; i < row_info->width; i++)
           {
              png_byte v[8]; /* SAFE; pixel_depth does not exceed 64 */
              int j;

              memcpy(v, sp, pixel_bytes);

              for (j = 0; j < jstop; j++)
              {
                 memcpy(dp, v, pixel_bytes);
                 dp -= pixel_bytes;
              }

              sp -= pixel_bytes;
           }
           break;
        }
     }

     row_info->width = final_width;
     row_info->rowbytes = PNG_ROWBYTES(row_info->pixel_depth, final_width);
  }
#ifndef PNG_READ_PACKSWAP_SUPPORTED
  PNG_UNUSED(transformations)  /* Silence compiler warning */
#endif
}
#endif /* READ_INTERLACING */

static void
png_read_filter_row_sub(png_row_infop row_info, png_bytep row,
   png_const_bytep prev_row)
{
  size_t i;
  size_t istop = row_info->rowbytes;
  unsigned int bpp = (row_info->pixel_depth + 7) >> 3;
  png_bytep rp = row + bpp;

  PNG_UNUSED(prev_row)

  for (i = bpp; i < istop; i++)
  {
     *rp = (png_byte)(((int)(*rp) + (int)(*(rp-bpp))) & 0xff);
     rp++;
  }
}

static void
png_read_filter_row_up(png_row_infop row_info, png_bytep row,
   png_const_bytep prev_row)
{
  size_t i;
  size_t istop = row_info->rowbytes;
  png_bytep rp = row;
  png_const_bytep pp = prev_row;

  for (i = 0; i < istop; i++)
  {
     *rp = (png_byte)(((int)(*rp) + (int)(*pp++)) & 0xff);
     rp++;
  }
}

static void
png_read_filter_row_avg(png_row_infop row_info, png_bytep row,
   png_const_bytep prev_row)
{
  size_t i;
  png_bytep rp = row;
  png_const_bytep pp = prev_row;
  unsigned int bpp = (row_info->pixel_depth + 7) >> 3;
  size_t istop = row_info->rowbytes - bpp;

  for (i = 0; i < bpp; i++)
  {
     *rp = (png_byte)(((int)(*rp) +
        ((int)(*pp++) / 2 )) & 0xff);

     rp++;
  }

  for (i = 0; i < istop; i++)
  {
     *rp = (png_byte)(((int)(*rp) +
        (int)(*pp++ + *(rp-bpp)) / 2 ) & 0xff);

     rp++;
  }
}

static void
png_read_filter_row_paeth_1byte_pixel(png_row_infop row_info, png_bytep row,
   png_const_bytep prev_row)
{
  png_bytep rp_end = row + row_info->rowbytes;
  int a, c;

  /* First pixel/byte */
  c = *prev_row++;
  a = *row + c;
  *row++ = (png_byte)a;

  /* Remainder */
  while (row < rp_end)
  {
     int b, pa, pb, pc, p;

     a &= 0xff; /* From previous iteration or start */
     b = *prev_row++;

     p = b - c;
     pc = a - c;

#ifdef PNG_USE_ABS
     pa = abs(p);
     pb = abs(pc);
     pc = abs(p + pc);
#else
     pa = p < 0 ? -p : p;
     pb = pc < 0 ? -pc : pc;
     pc = (p + pc) < 0 ? -(p + pc) : p + pc;
#endif

     /* Find the best predictor, the least of pa, pb, pc favoring the earlier
      * ones in the case of a tie.
      */
     if (pb < pa)
     {
        pa = pb; a = b;
     }
     if (pc < pa) a = c;

     /* Calculate the current pixel in a, and move the previous row pixel to c
      * for the next time round the loop
      */
     c = b;
     a += *row;
     *row++ = (png_byte)a;
  }
}

static void
png_read_filter_row_paeth_multibyte_pixel(png_row_infop row_info, png_bytep row,
   png_const_bytep prev_row)
{
  unsigned int bpp = (row_info->pixel_depth + 7) >> 3;
  png_bytep rp_end = row + bpp;

  /* Process the first pixel in the row completely (this is the same as 'up'
   * because there is only one candidate predictor for the first row).
   */
  while (row < rp_end)
  {
     int a = *row + *prev_row++;
     *row++ = (png_byte)a;
  }

  /* Remainder */
  rp_end = rp_end + (row_info->rowbytes - bpp);

  while (row < rp_end)
  {
     int a, b, c, pa, pb, pc, p;

     c = *(prev_row - bpp);
     a = *(row - bpp);
     b = *prev_row++;

     p = b - c;
     pc = a - c;

#ifdef PNG_USE_ABS
     pa = abs(p);
     pb = abs(pc);
     pc = abs(p + pc);
#else
     pa = p < 0 ? -p : p;
     pb = pc < 0 ? -pc : pc;
     pc = (p + pc) < 0 ? -(p + pc) : p + pc;
#endif

     if (pb < pa)
     {
        pa = pb; a = b;
     }
     if (pc < pa) a = c;

     a += *row;
     *row++ = (png_byte)a;
  }
}

static void
png_init_filter_functions(png_structrp pp)
  /* This function is called once for every PNG image (except for PNG images
   * that only use PNG_FILTER_VALUE_NONE for all rows) to set the
   * implementations required to reverse the filtering of PNG rows.  Reversing
   * the filter is the first transformation performed on the row data.  It is
   * performed in place, therefore an implementation can be selected based on
   * the image pixel format.  If the implementation depends on image width then
   * take care to ensure that it works correctly if the image is interlaced -
   * interlacing causes the actual row width to vary.
   */
{
  unsigned int bpp = (pp->pixel_depth + 7) >> 3;

  pp->read_filter[PNG_FILTER_VALUE_SUB-1] = png_read_filter_row_sub;
  pp->read_filter[PNG_FILTER_VALUE_UP-1] = png_read_filter_row_up;
  pp->read_filter[PNG_FILTER_VALUE_AVG-1] = png_read_filter_row_avg;
  if (bpp == 1)
     pp->read_filter[PNG_FILTER_VALUE_PAETH-1] =
        png_read_filter_row_paeth_1byte_pixel;
  else
     pp->read_filter[PNG_FILTER_VALUE_PAETH-1] =
        png_read_filter_row_paeth_multibyte_pixel;

#ifdef PNG_FILTER_OPTIMIZATIONS
  /* To use this define PNG_FILTER_OPTIMIZATIONS as the name of a function to
   * call to install hardware optimizations for the above functions; simply
   * replace whatever elements of the pp->read_filter[] array with a hardware
   * specific (or, for that matter, generic) optimization.
   *
   * To see an example of this examine what configure.ac does when
   * --enable-arm-neon is specified on the command line.
   */
  PNG_FILTER_OPTIMIZATIONS(pp, bpp);
#endif
}

void /* PRIVATE */
png_read_filter_row(png_structrp pp, png_row_infop row_info, png_bytep row,
   png_const_bytep prev_row, int filter)
{
  /* OPTIMIZATION: DO NOT MODIFY THIS FUNCTION, instead #define
   * PNG_FILTER_OPTIMIZATIONS to a function that overrides the generic
   * implementations.  See png_init_filter_functions above.
   */
  if (filter > PNG_FILTER_VALUE_NONE && filter < PNG_FILTER_VALUE_LAST)
  {
     if (pp->read_filter[0] == NULL)
        png_init_filter_functions(pp);

     pp->read_filter[filter-1](row_info, row, prev_row);
  }
}

#ifdef PNG_SEQUENTIAL_READ_SUPPORTED
void /* PRIVATE */
png_read_IDAT_data(png_structrp png_ptr, png_bytep output,
   png_alloc_size_t avail_out)
{
  /* Loop reading IDATs and decompressing the result into output[avail_out] */
  png_ptr->zstream.next_out = output;
  png_ptr->zstream.avail_out = 0; /* safety: set below */

  if (output == NULL)
     avail_out = 0;

  do
  {
     int ret;
     png_byte tmpbuf[PNG_INFLATE_BUF_SIZE];

     if (png_ptr->zstream.avail_in == 0)
     {
        uInt avail_in;
        png_bytep buffer;

#ifdef PNG_READ_APNG_SUPPORTED
        png_uint_32 bytes_to_skip = 0;

        while (png_ptr->idat_size == 0 || bytes_to_skip != 0)
        {
           png_crc_finish(png_ptr, bytes_to_skip);
           bytes_to_skip = 0;

           png_ptr->idat_size = png_read_chunk_header(png_ptr);
           if (png_ptr->num_frames_read == 0)
           {
              if (png_ptr->chunk_name != png_IDAT)
                 png_error(png_ptr, "Not enough image data");
           }
           else
           {
              if (png_ptr->chunk_name == png_IEND)
                 png_error(png_ptr, "Not enough image data");
              if (png_ptr->chunk_name != png_fdAT)
              {
                 png_warning(png_ptr, "Skipped (ignored) a chunk "
                                      "between APNG chunks");
                 bytes_to_skip = png_ptr->idat_size;
                 continue;
              }

              png_ensure_sequence_number(png_ptr, png_ptr->idat_size);

              png_ptr->idat_size -= 4;
           }
        }
#else
        while (png_ptr->idat_size == 0)
        {
           png_crc_finish(png_ptr, 0);

           png_ptr->idat_size = png_read_chunk_header(png_ptr);
           /* This is an error even in the 'check' case because the code just
            * consumed a non-IDAT header.
            */
           if (png_ptr->chunk_name != png_IDAT)
              png_error(png_ptr, "Not enough image data");
        }
#endif /* READ_APNG */

        avail_in = png_ptr->IDAT_read_size;

        if (avail_in > png_chunk_max(png_ptr))
           avail_in = (uInt)/*SAFE*/png_chunk_max(png_ptr);

        if (avail_in > png_ptr->idat_size)
           avail_in = (uInt)png_ptr->idat_size;

        /* A PNG with a gradually increasing IDAT size will defeat this attempt
         * to minimize memory usage by causing lots of re-allocs, but
         * realistically doing IDAT_read_size re-allocs is not likely to be a
         * big problem.
         *
         * An error here corresponds to the system being out of memory.
         */
        buffer = png_read_buffer(png_ptr, avail_in);

        if (buffer == NULL)
           png_chunk_error(png_ptr, "out of memory");

        png_crc_read(png_ptr, buffer, avail_in);
        png_ptr->idat_size -= avail_in;

        png_ptr->zstream.next_in = buffer;
        png_ptr->zstream.avail_in = avail_in;
     }

     /* And set up the output side. */
     if (output != NULL) /* standard read */
     {
        uInt out = ZLIB_IO_MAX;

        if (out > avail_out)
           out = (uInt)avail_out;

        avail_out -= out;
        png_ptr->zstream.avail_out = out;
     }

     else /* after last row, checking for end */
     {
        png_ptr->zstream.next_out = tmpbuf;
        png_ptr->zstream.avail_out = (sizeof tmpbuf);
     }

     /* Use NO_FLUSH; this gives zlib the maximum opportunity to optimize the
      * process.  If the LZ stream is truncated the sequential reader will
      * terminally damage the stream, above, by reading the chunk header of the
      * following chunk (it then exits with png_error).
      *
      * TODO: deal more elegantly with truncated IDAT lists.
      */
     ret = PNG_INFLATE(png_ptr, Z_NO_FLUSH);

     /* Take the unconsumed output back. */
     if (output != NULL)
        avail_out += png_ptr->zstream.avail_out;

     else /* avail_out counts the extra bytes */
        avail_out += (sizeof tmpbuf) - png_ptr->zstream.avail_out;

     png_ptr->zstream.avail_out = 0;

     if (ret == Z_STREAM_END)
     {
        /* Do this for safety; we won't read any more into this row. */
        png_ptr->zstream.next_out = NULL;

        png_ptr->mode |= PNG_AFTER_IDAT;
        png_ptr->flags |= PNG_FLAG_ZSTREAM_ENDED;
#ifdef PNG_READ_APNG_SUPPORTED
        png_ptr->num_frames_read++;
#endif

        if (png_ptr->zstream.avail_in > 0 || png_ptr->idat_size > 0)
           png_chunk_benign_error(png_ptr, "Extra compressed data");
        break;
     }

     if (ret != Z_OK)
     {
        png_zstream_error(png_ptr, ret);

        if (output != NULL)
           png_chunk_error(png_ptr, png_ptr->zstream.msg);

        else /* checking */
        {
           png_chunk_benign_error(png_ptr, png_ptr->zstream.msg);
           return;
        }
     }
  } while (avail_out > 0);

  if (avail_out > 0)
  {
     /* The stream ended before the image; this is the same as too few IDATs so
      * should be handled the same way.
      */
     if (output != NULL)
        png_error(png_ptr, "Not enough image data");

     else /* the deflate stream contained extra data */
        png_chunk_benign_error(png_ptr, "Too much image data");
  }
}

void /* PRIVATE */
png_read_finish_IDAT(png_structrp png_ptr)
{
  /* We don't need any more data and the stream should have ended, however the
   * LZ end code may actually not have been processed.  In this case we must
   * read it otherwise stray unread IDAT data or, more likely, an IDAT chunk
   * may still remain to be consumed.
   */
  if ((png_ptr->flags & PNG_FLAG_ZSTREAM_ENDED) == 0)
  {
     /* The NULL causes png_read_IDAT_data to swallow any remaining bytes in
      * the compressed stream, but the stream may be damaged too, so even after
      * this call we may need to terminate the zstream ownership.
      */
     png_read_IDAT_data(png_ptr, NULL, 0);
     png_ptr->zstream.next_out = NULL; /* safety */

     /* Now clear everything out for safety; the following may not have been
      * done.
      */
     if ((png_ptr->flags & PNG_FLAG_ZSTREAM_ENDED) == 0)
     {
        png_ptr->mode |= PNG_AFTER_IDAT;
        png_ptr->flags |= PNG_FLAG_ZSTREAM_ENDED;
     }
  }

  /* If the zstream has not been released do it now *and* terminate the reading
   * of the final IDAT chunk.
   */
  if (png_ptr->zowner == png_IDAT)
  {
     /* Always do this; the pointers otherwise point into the read buffer. */
     png_ptr->zstream.next_in = NULL;
     png_ptr->zstream.avail_in = 0;

     /* Now we no longer own the zstream. */
     png_ptr->zowner = 0;

     /* The slightly weird semantics of the sequential IDAT reading is that we
      * are always in or at the end of an IDAT chunk, so we always need to do a
      * crc_finish here.  If idat_size is non-zero we also need to read the
      * spurious bytes at the end of the chunk now.
      */
     (void)png_crc_finish(png_ptr, png_ptr->idat_size);
  }
}

void /* PRIVATE */
png_read_finish_row(png_structrp png_ptr)
{
  png_debug(1, "in png_read_finish_row");
  png_ptr->row_number++;
  if (png_ptr->row_number < png_ptr->num_rows)
     return;

  if (png_ptr->interlaced != 0)
  {
     png_ptr->row_number = 0;

     /* TO DO: don't do this if prev_row isn't needed (requires
      * read-ahead of the next row's filter byte.
      */
     memset(png_ptr->prev_row, 0, png_ptr->rowbytes + 1);

     do
     {
        png_ptr->pass++;

        if (png_ptr->pass >= 7)
           break;

        png_ptr->iwidth = (png_ptr->width +
           png_pass_inc[png_ptr->pass] - 1 -
           png_pass_start[png_ptr->pass]) /
           png_pass_inc[png_ptr->pass];

        if ((png_ptr->transformations & PNG_INTERLACE) == 0)
        {
           png_ptr->num_rows = (png_ptr->height +
               png_pass_yinc[png_ptr->pass] - 1 -
               png_pass_ystart[png_ptr->pass]) /
               png_pass_yinc[png_ptr->pass];
        }

        else  /* if (png_ptr->transformations & PNG_INTERLACE) */
           break; /* libpng deinterlacing sees every row */

     } while (png_ptr->num_rows == 0 || png_ptr->iwidth == 0);

     if (png_ptr->pass < 7)
        return;
  }

  /* Here after at the end of the last row of the last pass. */
  png_read_finish_IDAT(png_ptr);
}
#endif /* SEQUENTIAL_READ */

void /* PRIVATE */
png_read_start_row(png_structrp png_ptr)
{
  unsigned int max_pixel_depth;
  size_t row_bytes;

  png_debug(1, "in png_read_start_row");

#ifdef PNG_READ_TRANSFORMS_SUPPORTED
  png_init_read_transformations(png_ptr);
#endif
  if (png_ptr->interlaced != 0)
  {
     if ((png_ptr->transformations & PNG_INTERLACE) == 0)
        png_ptr->num_rows = (png_ptr->height + png_pass_yinc[0] - 1 -
            png_pass_ystart[0]) / png_pass_yinc[0];

     else
        png_ptr->num_rows = png_ptr->height;

     png_ptr->iwidth = (png_ptr->width +
         png_pass_inc[png_ptr->pass] - 1 -
         png_pass_start[png_ptr->pass]) /
         png_pass_inc[png_ptr->pass];
  }

  else
  {
     png_ptr->num_rows = png_ptr->height;
     png_ptr->iwidth = png_ptr->width;
  }

  max_pixel_depth = (unsigned int)png_ptr->pixel_depth;

  /* WARNING: * png_read_transform_info (pngrtran.c) performs a simpler set of
   * calculations to calculate the final pixel depth, then
   * png_do_read_transforms actually does the transforms.  This means that the
   * code which effectively calculates this value is actually repeated in three
   * separate places.  They must all match.  Innocent changes to the order of
   * transformations can and will break libpng in a way that causes memory
   * overwrites.
   *
   * TODO: fix this.
   */
#ifdef PNG_READ_PACK_SUPPORTED
  if ((png_ptr->transformations & PNG_PACK) != 0 && png_ptr->bit_depth < 8)
     max_pixel_depth = 8;
#endif

#ifdef PNG_READ_EXPAND_SUPPORTED
  if ((png_ptr->transformations & PNG_EXPAND) != 0)
  {
     if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
     {
        if (png_ptr->num_trans != 0)
           max_pixel_depth = 32;

        else
           max_pixel_depth = 24;
     }

     else if (png_ptr->color_type == PNG_COLOR_TYPE_GRAY)
     {
        if (max_pixel_depth < 8)
           max_pixel_depth = 8;

        if (png_ptr->num_trans != 0)
           max_pixel_depth *= 2;
     }

     else if (png_ptr->color_type == PNG_COLOR_TYPE_RGB)
     {
        if (png_ptr->num_trans != 0)
        {
           max_pixel_depth *= 4;
           max_pixel_depth /= 3;
        }
     }
  }
#endif

#ifdef PNG_READ_EXPAND_16_SUPPORTED
  if ((png_ptr->transformations & PNG_EXPAND_16) != 0)
  {
#  ifdef PNG_READ_EXPAND_SUPPORTED
     /* In fact it is an error if it isn't supported, but checking is
      * the safe way.
      */
     if ((png_ptr->transformations & PNG_EXPAND) != 0)
     {
        if (png_ptr->bit_depth < 16)
           max_pixel_depth *= 2;
     }
     else
#  endif
     png_ptr->transformations &= ~PNG_EXPAND_16;
  }
#endif

#ifdef PNG_READ_FILLER_SUPPORTED
  if ((png_ptr->transformations & (PNG_FILLER)) != 0)
  {
     if (png_ptr->color_type == PNG_COLOR_TYPE_GRAY)
     {
        if (max_pixel_depth <= 8)
           max_pixel_depth = 16;

        else
           max_pixel_depth = 32;
     }

     else if (png_ptr->color_type == PNG_COLOR_TYPE_RGB ||
        png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
     {
        if (max_pixel_depth <= 32)
           max_pixel_depth = 32;

        else
           max_pixel_depth = 64;
     }
  }
#endif

#ifdef PNG_READ_GRAY_TO_RGB_SUPPORTED
  if ((png_ptr->transformations & PNG_GRAY_TO_RGB) != 0)
  {
     if (
#ifdef PNG_READ_EXPAND_SUPPORTED
         (png_ptr->num_trans != 0 &&
         (png_ptr->transformations & PNG_EXPAND) != 0) ||
#endif
#ifdef PNG_READ_FILLER_SUPPORTED
         (png_ptr->transformations & (PNG_FILLER)) != 0 ||
#endif
         png_ptr->color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
     {
        if (max_pixel_depth <= 16)
           max_pixel_depth = 32;

        else
           max_pixel_depth = 64;
     }

     else
     {
        if (max_pixel_depth <= 8)
        {
           if (png_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              max_pixel_depth = 32;

           else
              max_pixel_depth = 24;
        }

        else if (png_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA)
           max_pixel_depth = 64;

        else
           max_pixel_depth = 48;
     }
  }
#endif

#if defined(PNG_READ_USER_TRANSFORM_SUPPORTED) && \
defined(PNG_USER_TRANSFORM_PTR_SUPPORTED)
  if ((png_ptr->transformations & PNG_USER_TRANSFORM) != 0)
  {
     unsigned int user_pixel_depth = png_ptr->user_transform_depth *
        png_ptr->user_transform_channels;

     if (user_pixel_depth > max_pixel_depth)
        max_pixel_depth = user_pixel_depth;
  }
#endif

  /* This value is stored in png_struct and double checked in the row read
   * code.
   */
  png_ptr->maximum_pixel_depth = (png_byte)max_pixel_depth;
  png_ptr->transformed_pixel_depth = 0; /* calculated on demand */

  /* Align the width on the next larger 8 pixels.  Mainly used
   * for interlacing
   */
  row_bytes = ((png_ptr->width + 7) & ~((png_uint_32)7));
  /* Calculate the maximum bytes needed, adding a byte and a pixel
   * for safety's sake
   */
  row_bytes = PNG_ROWBYTES(max_pixel_depth, row_bytes) +
      1 + ((max_pixel_depth + 7) >> 3U);

#ifdef PNG_MAX_MALLOC_64K
  if (row_bytes > (png_uint_32)65536L)
     png_error(png_ptr, "This image requires a row greater than 64KB");
#endif

  if (row_bytes + 48 > png_ptr->old_big_row_buf_size)
  {
     png_free(png_ptr, png_ptr->big_row_buf);
     png_free(png_ptr, png_ptr->big_prev_row);

     if (png_ptr->interlaced != 0)
        png_ptr->big_row_buf = (png_bytep)png_calloc(png_ptr,
            row_bytes + 48);

     else
        png_ptr->big_row_buf = (png_bytep)png_malloc(png_ptr, row_bytes + 48);

     png_ptr->big_prev_row = (png_bytep)png_malloc(png_ptr, row_bytes + 48);

#ifdef PNG_ALIGNED_MEMORY_SUPPORTED
     /* Use 16-byte aligned memory for row_buf with at least 16 bytes
      * of padding before and after row_buf; treat prev_row similarly.
      * NOTE: the alignment is to the start of the pixels, one beyond the start
      * of the buffer, because of the filter byte.  Prior to libpng 1.5.6 this
      * was incorrect; the filter byte was aligned, which had the exact
      * opposite effect of that intended.
      */
     {
        png_bytep temp = png_ptr->big_row_buf + 32;
        size_t extra = (size_t)temp & 0x0f;
        png_ptr->row_buf = temp - extra - 1/*filter byte*/;

        temp = png_ptr->big_prev_row + 32;
        extra = (size_t)temp & 0x0f;
        png_ptr->prev_row = temp - extra - 1/*filter byte*/;
     }
#else
     /* Use 31 bytes of padding before and 17 bytes after row_buf. */
     png_ptr->row_buf = png_ptr->big_row_buf + 31;
     png_ptr->prev_row = png_ptr->big_prev_row + 31;
#endif
     png_ptr->old_big_row_buf_size = row_bytes + 48;
  }

#ifdef PNG_MAX_MALLOC_64K
  if (png_ptr->rowbytes > 65535)
     png_error(png_ptr, "This image requires a row greater than 64KB");

#endif
  if (png_ptr->rowbytes > (PNG_SIZE_MAX - 1))
     png_error(png_ptr, "Row has too many bytes to allocate in memory");

  memset(png_ptr->prev_row, 0, png_ptr->rowbytes + 1);

  png_debug1(3, "width = %u,", png_ptr->width);
  png_debug1(3, "height = %u,", png_ptr->height);
  png_debug1(3, "iwidth = %u,", png_ptr->iwidth);
  png_debug1(3, "num_rows = %u,", png_ptr->num_rows);
  png_debug1(3, "rowbytes = %lu,", (unsigned long)png_ptr->rowbytes);
  png_debug1(3, "irowbytes = %lu",
      (unsigned long)PNG_ROWBYTES(png_ptr->pixel_depth, png_ptr->iwidth) + 1);

  /* The sequential reader needs a buffer for IDAT, but the progressive reader
   * does not, so free the read buffer now regardless; the sequential reader
   * reallocates it on demand.
   */
  if (png_ptr->read_buffer != NULL)
  {
     png_bytep buffer = png_ptr->read_buffer;

     png_ptr->read_buffer_size = 0;
     png_ptr->read_buffer = NULL;
     png_free(png_ptr, buffer);
  }

  /* Finally claim the zstream for the inflate of the IDAT data, use the bits
   * value from the stream (note that this will result in a fatal error if the
   * IDAT stream has a bogus deflate header window_bits value, but this should
   * not be happening any longer!)
   */
  if (png_inflate_claim(png_ptr, png_IDAT) != Z_OK)
     png_error(png_ptr, png_ptr->zstream.msg);

  png_ptr->flags |= PNG_FLAG_ROW_INIT;
}

#ifdef PNG_READ_APNG_SUPPORTED
/* This function is to be called after the main IDAT set has been read and
* before a new IDAT is read. It resets some parts of png_ptr
* to make them usable by the read functions again */
void /* PRIVATE */
png_read_reset(png_structp png_ptr)
{
   png_ptr->mode &= ~PNG_HAVE_IDAT;
   png_ptr->mode &= ~PNG_AFTER_IDAT;
   png_ptr->row_number = 0;
   png_ptr->pass = 0;
}

void /* PRIVATE */
png_read_reinit(png_structp png_ptr, png_infop info_ptr)
{
   png_ptr->width = info_ptr->next_frame_width;
   png_ptr->height = info_ptr->next_frame_height;
   png_ptr->rowbytes = PNG_ROWBYTES(png_ptr->pixel_depth,png_ptr->width);
   png_ptr->info_rowbytes = PNG_ROWBYTES(info_ptr->pixel_depth,
       png_ptr->width);
   if (png_ptr->prev_row != NULL)
       memset(png_ptr->prev_row, 0, png_ptr->rowbytes + 1);
}

#ifdef PNG_PROGRESSIVE_READ_SUPPORTED
/* same as png_read_reset() but for the progressive reader */
void /* PRIVATE */
png_progressive_read_reset(png_structp png_ptr)
{
#ifdef PNG_READ_INTERLACING_SUPPORTED
   /* Arrays to facilitate easy interlacing - use pass (0 - 6) as index */

   /* Start of interlace block */
   static PNG_CONST png_byte png_pass_start[] = {0, 4, 0, 2, 0, 1, 0};

   /* Offset to next interlace block */
   static PNG_CONST png_byte png_pass_inc[] = {8, 8, 4, 4, 2, 2, 1};

   /* Start of interlace block in the y direction */
   static PNG_CONST png_byte png_pass_ystart[] = {0, 0, 4, 0, 2, 0, 1};

   /* Offset to next interlace block in the y direction */
   static PNG_CONST png_byte png_pass_yinc[] = {8, 8, 8, 4, 4, 2, 2};

   if (png_ptr->interlaced != 0)
   {
       if ((png_ptr->transformations & PNG_INTERLACE) == 0)
           png_ptr->num_rows = (png_ptr->height + png_pass_yinc[0] - 1 -
                               png_pass_ystart[0]) / png_pass_yinc[0];
       else
           png_ptr->num_rows = png_ptr->height;

       png_ptr->iwidth = (png_ptr->width +
                          png_pass_inc[png_ptr->pass] - 1 -
                          png_pass_start[png_ptr->pass]) /
                          png_pass_inc[png_ptr->pass];
   }
   else
#endif /* READ_INTERLACING */
   {
       png_ptr->num_rows = png_ptr->height;
       png_ptr->iwidth = png_ptr->width;
   }
   png_ptr->flags &= ~PNG_FLAG_ZSTREAM_ENDED;
   if (inflateReset(&(png_ptr->zstream)) != Z_OK)
       png_error(png_ptr, "inflateReset failed");
   png_ptr->zstream.avail_in = 0;
   png_ptr->zstream.next_in = 0;
   png_ptr->zstream.next_out = png_ptr->row_buf;
   png_ptr->zstream.avail_out = (uInt)PNG_ROWBYTES(png_ptr->pixel_depth,
       png_ptr->iwidth) + 1;
}
#endif /* PROGRESSIVE_READ */
#endif /* READ_APNG */
#endif /* READ */