int pthread_setconcurrency(int new_level);
Unbound threads in a process may or may not be required to be simultaneously active. By default, the threads implementation ensures that a sufficient number of threads are active so that the process can continue to make progress. While this conserves system resources, it may not produce the most effective level of concurrency.
The pthread_setconcurrency() function allows an application to inform the threads implementation of its desired concurrency level, new_level. The actual level of concurrency provided by the implementation as a result of this function call is unspecified.
If new_level is zero, it causes the implementation to maintain the concurrency level at its discretion as if pthread_setconcurrency() had never been called.
The pthread_getconcurrency() function shall return the value set by a previous call to the pthread_setconcurrency() function. If the pthread_setconcurrency() function was not previously called, this function shall return zero to indicate that the implementation is maintaining the concurrency level.
A call to pthread_setconcurrency() shall inform the implementation of its desired concurrency level. The implementation shall use this as a hint, not a requirement.
If an implementation does not support multiplexing of user threads on top of several kernel-scheduled entities, the pthread_setconcurrency() and pthread_getconcurrency() functions are provided for source code compatibility but they shall have no effect when called. To maintain the function semantics, the new_level parameter is saved when pthread_setconcurrency() is called so that a subsequent call to pthread_getconcurrency() shall return the same value.
If successful, the pthread_setconcurrency() function shall return zero; otherwise, an error number shall be returned to indicate the error.
The pthread_getconcurrency() function shall always return the concurrency level set by a previous call to pthread_setconcurrency(). If the pthread_setconcurrency() function has never been called, pthread_getconcurrency() shall return zero.
The pthread_setconcurrency() function shall fail if:
These functions shall not return an error code of [EINTR].
The following sections are informative.
Use of these functions changes the state of the underlying concurrency upon which the application depends. Library developers are advised to not use the pthread_getconcurrency() and pthread_setconcurrency() functions since their use may conflict with an applications use of these functions.
The Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>