about_nspr.rst (6717B)
1 About NSPR 2 ========== 3 4 NetScape Portable Runtime (NSPR) provides platform independence for 5 non-GUI operating system facilities. These facilities include threads, 6 thread synchronization, normal file and network I/O, interval timing and 7 calendar time, basic memory management (malloc and free) and shared 8 library linking. 9 10 History 11 ~~~~~~~ 12 13 A good portion of the library's purpose, and perhaps the primary purpose 14 in the Gromit environment, was to provide the underpinnings of the Java 15 VM, more or less mapping the *sys layer* that Sun defined for the 16 porting of the Java VM to various platforms. NSPR went beyond that 17 requirement in some areas and since it was also the platform independent 18 layer for most of the servers produced by Netscape. It was expected and 19 preferred that existing code be restructured and perhaps even rewritten 20 in order to use the NSPR API. It is not a goal to provide a platform for 21 the porting into Netscape of externally developed code. 22 23 At the time of writing the current generation of NSPR was known as 24 NSPR20. The first generation of NSPR was originally conceived just to 25 satisfy the requirements of porting Java to various host environments. 26 NSPR20, an effort started in 1996, built on that original idea, though 27 very little is left of the original code. (The "20" in "NSPR20" does not 28 mean "version 2.0" but rather "second generation".) Many of the concepts 29 have been reformed, expanded, and matured. Today NSPR may still be 30 appropriate as the platform dependent layer under Java, but its primary 31 application is supporting clients written entirely in C or C++. 32 33 .. _How_It_Works: 34 35 How It Works 36 ~~~~~~~~~~~~ 37 38 NSPR's goal is to provide uniform service over a wide range of operating 39 system environments. It strives to not export the *lowest common 40 denominator*, but to exploit the best features of each operating system 41 on which it runs, and still provide a uniform service across a wide 42 range of host offerings. 43 44 Threads 45 ^^^^^^^ 46 47 Threads are the major feature of NSPR. The industry's offering of 48 threads is quite sundry. NSPR, while far from perfect, does provide a 49 single API to which clients may program and expect reasonably consistent 50 behavior. The operating systems provide everything from no concept of 51 threading at all up to and including sophisticated, scalable and 52 efficient implementations. NSPR makes as much use of what the systems 53 offer as it can. It is a goal of NSPR that NSPR impose as little 54 overhead as possible in accessing those appropriate system features. 55 56 .. _Thread_synchronization: 57 58 Thread synchronization 59 ^^^^^^^^^^^^^^^^^^^^^^ 60 61 Thread synchronization is loosely based on Monitors as described by 62 C.A.R. Hoare in *Monitors: An operating system structuring concept* , 63 Communications of the ACM, 17(10), October 1974 and then formalized by 64 Xerox' Mesa programming language ("Mesa Language Manual", J.G. Mitchell 65 et al, Xerox PARC, CSL-79-3 (Apr 1979)). This mechanism provides the 66 basic mutual exclusion (mutex) and thread notification facilities 67 (condition variables) implemented by NSPR. Additionally, NSPR provides 68 synchronization methods more suited for use by Java. The Java-like 69 facilities include monitor *reentrancy*, implicit and tightly bound 70 notification capabilities with the ability to associate the 71 synchronization objects dynamically. 72 73 .. _I.2FO: 74 75 I/O 76 ^^^ 77 78 NSPR's I/O is a slightly augmented BSD sockets model that allows 79 arbitrary layering. It was originally intended to export synchronous I/O 80 methods only, relying on threads to provide the concurrency needed for 81 complex applications. That method of operation is preferred though it is 82 possible to configure the network I/O channels as *non-blocking* in the 83 traditional sense. 84 85 .. _Network_addresses: 86 87 Network addresses 88 ^^^^^^^^^^^^^^^^^ 89 90 Part of NSPR deals with manipulation of network addresses. NSPR defines 91 a network address object that is Internet Protocol (IP) centric. While 92 the object is not declared as opaque, the API provides methods that 93 allow and encourage clients to treat the addresses as polymorphic items. 94 The goal in this area is to provide a migration path between IPv4 and 95 IPv6. To that end it is possible to perform translations of ASCII 96 strings (DNS names) into NSPR's network address structures, with no 97 regard to whether the addressing technology is IPv4 or IPv6. 98 99 Time 100 ^^^^ 101 102 Timing facilities are available in two forms: interval timing and 103 calendar functions. 104 105 Interval timers are based on a free running, 32-bit, platform dependent 106 resolution timer. Such timers are normally used to specify timeouts on 107 I/O, waiting on condition variables and other rudimentary thread 108 scheduling. Since these timers have finite namespace and are free 109 running, they can wrap at any time. NSPR does not provide an *epoch* , 110 but expects clients to deal with that issue. The *granularity* of the 111 timers is guaranteed to be between 10 microseconds and 1 millisecond. 112 This allows a minimal timer *period* in of approximately 12 hours. But 113 in order to deal with the wrap-around issue, only half that namespace 114 may be utilized. Therefore, the minimal usable interval available from 115 the timers is slightly less than six hours. 116 117 Calendar times are 64-bit signed numbers with units of microseconds. The 118 *epoch* for calendar times is midnight, January 1, 1970, Greenwich Mean 119 Time. Negative times extend to times before 1970, and positive numbers 120 forward. Use of 64 bits allows a representation of times approximately 121 in the range of -30000 to the year 30000. There is a structural 122 representation (*i.e., exploded* view), routines to acquire the current 123 time from the host system, and convert them to and from the 64-bit and 124 structural representation. Additionally there are routines to convert to 125 and from most well-known forms of ASCII into the 64-bit NSPR 126 representation. 127 128 .. _Memory_management: 129 130 Memory management 131 ^^^^^^^^^^^^^^^^^ 132 133 NSPR provides API to perform the basic malloc, calloc, realloc and free 134 functions. Depending on the platform, the functions may be implemented 135 almost entirely in the NSPR runtime or simply shims that call 136 immediately into the host operating system's offerings. 137 138 Linking 139 ^^^^^^^ 140 141 Support for linking (shared library loading and unloading) is part of 142 NSPR's feature set. In most cases this is simply a smoothing over of the 143 facilities offered by the various platform providers. 144 145 Where It's Headed 146 ~~~~~~~~~~~~~~~~~ 147 148 NSPR is applicable as a platform on which to write threaded applications 149 that need to be ported to multiple platforms. 150 151 NSPR is functionally complete and has entered a mode of sustaining 152 engineering. As operating system vendors issue new releases of their 153 operating systems, NSPR will be moved forward to these new releases by 154 interested players.