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
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.
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.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html