about_nspr.rst - mozsearch

About NSPR

==========

NetScape Portable Runtime (NSPR) provides platform independence for

non-GUI operating system facilities. These facilities include threads,

thread synchronization, normal file and network I/O, interval timing and

calendar time, basic memory management (malloc and free) and shared

library linking.

History

~~~~~~~

A good portion of the library's purpose, and perhaps the primary purpose

in the Gromit environment, was to provide the underpinnings of the Java

VM, more or less mapping the *sys layer* that Sun defined for the

porting of the Java VM to various platforms. NSPR went beyond that

requirement in some areas and since it was also the platform independent

layer for most of the servers produced by Netscape. It was expected and

preferred that existing code be restructured and perhaps even rewritten

in order to use the NSPR API. It is not a goal to provide a platform for

the porting into Netscape of externally developed code.

At the time of writing the current generation of NSPR was known as

NSPR20. The first generation of NSPR was originally conceived just to

satisfy the requirements of porting Java to various host environments.

NSPR20, an effort started in 1996, built on that original idea, though

very little is left of the original code. (The "20" in "NSPR20" does not

mean "version 2.0" but rather "second generation".) Many of the concepts

have been reformed, expanded, and matured. Today NSPR may still be

appropriate as the platform dependent layer under Java, but its primary

application is supporting clients written entirely in C or C++.

.. _How_It_Works:

How It Works

~~~~~~~~~~~~

NSPR's goal is to provide uniform service over a wide range of operating

system environments. It strives to not export the *lowest common

denominator*, but to exploit the best features of each operating system

on which it runs, and still provide a uniform service across a wide

range of host offerings.

Threads

^^^^^^^

Threads are the major feature of NSPR. The industry's offering of

threads is quite sundry. NSPR, while far from perfect, does provide a

single API to which clients may program and expect reasonably consistent

behavior. The operating systems provide everything from no concept of

threading at all up to and including sophisticated, scalable and

efficient implementations. NSPR makes as much use of what the systems

offer as it can. It is a goal of NSPR that NSPR impose as little

overhead as possible in accessing those appropriate system features.

.. _Thread_synchronization:

Thread synchronization

^^^^^^^^^^^^^^^^^^^^^^

Thread synchronization is loosely based on Monitors as described by

C.A.R. Hoare in *Monitors: An operating system structuring concept* ,

Communications of the ACM, 17(10), October 1974 and then formalized by

Xerox' Mesa programming language ("Mesa Language Manual", J.G. Mitchell

et al, Xerox PARC, CSL-79-3 (Apr 1979)). This mechanism provides the

basic mutual exclusion (mutex) and thread notification facilities

(condition variables) implemented by NSPR. Additionally, NSPR provides

synchronization methods more suited for use by Java. The Java-like

facilities include monitor *reentrancy*, implicit and tightly bound

notification capabilities with the ability to associate the

synchronization objects dynamically.

.. _I.2FO:

I/O

^^^

NSPR's I/O is a slightly augmented BSD sockets model that allows

arbitrary layering. It was originally intended to export synchronous I/O

methods only, relying on threads to provide the concurrency needed for

complex applications. That method of operation is preferred though it is

possible to configure the network I/O channels as *non-blocking* in the

traditional sense.

.. _Network_addresses:

Network addresses

^^^^^^^^^^^^^^^^^

Part of NSPR deals with manipulation of network addresses. NSPR defines

a network address object that is Internet Protocol (IP) centric. While

the object is not declared as opaque, the API provides methods that

allow and encourage clients to treat the addresses as polymorphic items.

The goal in this area is to provide a migration path between IPv4 and

IPv6. To that end it is possible to perform translations of ASCII

strings (DNS names) into NSPR's network address structures, with no

regard to whether the addressing technology is IPv4 or IPv6.

Time

^^^^

Timing facilities are available in two forms: interval timing and

calendar functions.

Interval timers are based on a free running, 32-bit, platform dependent

resolution timer. Such timers are normally used to specify timeouts on

I/O, waiting on condition variables and other rudimentary thread

scheduling. Since these timers have finite namespace and are free

running, they can wrap at any time. NSPR does not provide an *epoch* ,

but expects clients to deal with that issue. The *granularity* of the

timers is guaranteed to be between 10 microseconds and 1 millisecond.

This allows a minimal timer *period* in of approximately 12 hours. But

in order to deal with the wrap-around issue, only half that namespace

may be utilized. Therefore, the minimal usable interval available from

the timers is slightly less than six hours.

Calendar times are 64-bit signed numbers with units of microseconds. The

*epoch* for calendar times is midnight, January 1, 1970, Greenwich Mean

Time. Negative times extend to times before 1970, and positive numbers

forward. Use of 64 bits allows a representation of times approximately

in the range of -30000 to the year 30000. There is a structural

representation (*i.e., exploded* view), routines to acquire the current

time from the host system, and convert them to and from the 64-bit and

structural representation. Additionally there are routines to convert to

and from most well-known forms of ASCII into the 64-bit NSPR

representation.

.. _Memory_management:

Memory management

^^^^^^^^^^^^^^^^^

NSPR provides API to perform the basic malloc, calloc, realloc and free

functions. Depending on the platform, the functions may be implemented

almost entirely in the NSPR runtime or simply shims that call

immediately into the host operating system's offerings.

Linking

^^^^^^^

Support for linking (shared library loading and unloading) is part of

NSPR's feature set. In most cases this is simply a smoothing over of the

facilities offered by the various platform providers.

Where It's Headed

~~~~~~~~~~~~~~~~~

NSPR is applicable as a platform on which to write threaded applications

that need to be ported to multiple platforms.

NSPR is functionally complete and has entered a mode of sustaining

engineering. As operating system vendors issue new releases of their

operating systems, NSPR will be moved forward to these new releases by

interested players.