NAME
pthread_getconcurrency, pthread_setconcurrency - get and set the level
of concurrency
SYNOPSIS
#include <pthread.h>
int pthread_getconcurrency(void);
int pthread_setconcurrency(int new_level);
DESCRIPTION
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.
ERRORS
The pthread_setconcurrency() function shall fail if:
EINVAL The value specified by new_level is negative.
EAGAIN The value specific by new_level would cause a system resource to
be exceeded.
These functions shall not return an error code of [EINTR].
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
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.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
The Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>
COPYRIGHT
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 .