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.TH PTHREAD_GETCONCURRENCY "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH PROLOG
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.

.SH NAME
pthread_getconcurrency,
pthread_setconcurrency
\(em get and set the level of concurrency
.SH SYNOPSIS
.LP
.nf
#include <pthread.h>
.P
int pthread_getconcurrency(void);
int pthread_setconcurrency(int \fInew_level\fP);
.fi
.SH 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.
.P
The
\fIpthread_setconcurrency\fR()
function allows an application to inform the threads implementation of
its desired concurrency level,
.IR new_level .
The actual level of concurrency provided by the implementation as a
result of this function call is unspecified.
.P
If
.IR new_level
is zero, it causes the implementation to maintain the concurrency level
at its discretion as if
\fIpthread_setconcurrency\fR()
had never been called.
.P
The
\fIpthread_getconcurrency\fR()
function shall return the value set by a previous call to the
\fIpthread_setconcurrency\fR()
function. If the
\fIpthread_setconcurrency\fR()
function was not previously called, this function shall return zero to
indicate that the implementation is maintaining the concurrency level.
.P
A call to
\fIpthread_setconcurrency\fR()
shall inform the implementation of its desired concurrency level.
The implementation shall use this as a hint, not a requirement.
.P
If an implementation does not support multiplexing of user threads on
top of several kernel-scheduled entities, the
\fIpthread_setconcurrency\fR()
and
\fIpthread_getconcurrency\fR()
functions are provided for source code compatibility but they shall
have no effect when called. To maintain the function semantics, the
.IR new_level
parameter is saved when
\fIpthread_setconcurrency\fR()
is called so that a subsequent call to
\fIpthread_getconcurrency\fR()
shall return the same value.
.SH "RETURN VALUE"
If successful, the
\fIpthread_setconcurrency\fR()
function shall return zero; otherwise, an error number shall be
returned to indicate the error.
.P
The
\fIpthread_getconcurrency\fR()
function shall always return the concurrency level set by a previous
call to
\fIpthread_setconcurrency\fR().
If the
\fIpthread_setconcurrency\fR()
function has never been called,
\fIpthread_getconcurrency\fR()
shall return zero.
.SH ERRORS
The
\fIpthread_setconcurrency\fR()
function shall fail if:
.TP
.BR EINVAL
The value specified by
.IR new_level
is negative.
.TP
.BR EAGAIN
The value specified by
.IR new_level
would cause a system resource to be exceeded.
.P
The
\fIpthread_setconcurrency\fR()
function shall not return an error code of
.BR [EINTR] .
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
Application developers should note that an implementation can always
ignore any calls to
\fIpthread_setconcurrency\fR()
and return a constant for
\fIpthread_getconcurrency\fR().
For this reason, it is not recommended that portable applications
use this function.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
These functions may be removed in a future version.
.SH "SEE ALSO"
The Base Definitions volume of POSIX.1\(hy2008,
.IR "\fB<pthread.h>\fP"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

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