Pthreads is intended to address a wide variety of audiences. High-performance computational programs can use it to support parallel decomposition of loops. Realtime programs can use it to support concurrent realtime I/O. Database and network servers can use it to easily support concurrent clients. Business or software development programs can use it to take advantage of parallel and concurrent operations on time-sharing systems.

The Pthreads standard allows you to determine which optional capabilities are provided by the system, by defining a set of feature-test macros, which are shown in Table 9.1. Any implementation of Pthreads must inform you whether each option is supported, by three means:

• By making a formal statement of support in the POSIX Conformance Document. You can use this information to help design your application to work on specific systems.

• By defining compile-time symbolic constants in the header file. You can test for these symbolic constants using #ifdef or #ifndef preprocessor conditionals to support a variety of Pthreads systems.

• By returning a positive nonzero value when the sysconf function is called with the associated sysconf symbol. (This is not usually useful for the "feature-test" macros that specify whether options are present — if they are not, the associated interfaces usually are not supplied, and your code will not link, and may not even compile.)

You might, for example, choose to avoid relying on priority scheduling because after reading the conformance documents you discovered that three out of the four systems you wish to support do not provide the feature. Or you might prefer to use priority inheritance for your mutexes on systems that provide the feature, but write the code so that it will not try to access the mutex protocol attribute on systems that do not provide that option.

TABLE 9.1POSIX 1003.1c-1995 options

9.2 POSIX1003.1c-1995 limits

The Pthreads standard allows you to determine the run-time limits of the system that may affect your application, for example, how many threads you can create, by defining a set of macros, which are shown in Table 9.2. Any implementation of Pthreads must inform you of its limits, by three means:

• By making a formal statement in the POSIX Conformance Document. You can use this information to help design your application to work on specific systems.