pthread_getconcurrency man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

pthread_getconcurrency, pthread_setconcurrency — get and set the level of concurrency


#include <pthread.h>

int pthread_getconcurrency(void);
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.

Return 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:

The value specified by new_level is negative.
The value specified by new_level would cause a system resource to be exceeded.

The pthread_setconcurrency() function shall not return an error code of [EINTR].

The following sections are informative.

Application Usage

Application developers should note that an implementation can always ignore any calls to pthread_setconcurrency() and return a constant for pthread_getconcurrency(). For this reason, it is not recommended that portable applications use this function.

Future Directions

These functions may be removed in a future version.

See Also

The Base Definitions volume of POSIX.1‐2008, <pthread.h>