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.TH PTHREAD_GETCONCURRENCY 3C "Mar 23, 2005"
.SH NAME
pthread_getconcurrency, pthread_setconcurrency \- get or set level of
concurrency
.SH SYNOPSIS
.LP
.nf
cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ]
#include <pthread.h>

\fBint\fR \fBpthread_getconcurrency\fR(\fBvoid\fR);
.fi

.LP
.nf
\fBint\fR \fBpthread_setconcurrency\fR(\fBint\fR \fInew_level\fR);
.fi

.SH DESCRIPTION
.sp
.LP
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.
.sp
.LP
The  \fBpthread_setconcurrency()\fR function allows an application to inform
the threads implementation of its desired concurrency level, \fInew_level\fR.
The actual level of concurrency provided by the implementation as a result of
this function call is unspecified.
.sp
.LP
If \fInew_level\fR is  \fB0\fR, it causes the implementation to maintain the
concurrency level at its discretion as if \fBpthread_setconcurrency()\fR was
never called.
.sp
.LP
The  \fBpthread_getconcurrency()\fR function returns the  value set by a
previous call to the  \fBpthread_setconcurrency()\fR function. If the
\fBpthread_setconcurrency()\fR function was not previously called, this
function returns  \fB0\fR to indicate that the implementation is maintaining
the concurrency level.
.sp
.LP
When an application calls  \fBpthread_setconcurrency()\fR it is informing the
implementation of its desired concurrency level. The implementation uses this
as a hint, not a requirement.
.sp
.LP
If an implementation does not support multiplexing of user threads on top of
several kernel scheduled entities, the  \fBpthread_setconcurrency()\fR and
\fBpthread_getconcurrency()\fR functions will be provided for source code
compatibility but they will have no effect when called. To maintain the
function semantics, the \fInew_level\fR parameter will be saved when
\fBpthread_setconcurrency()\fR is called so that a subsequent call to
\fBpthread_getconcurrency()\fR returns the same value.
.SH RETURN VALUES
.sp
.LP
If successful, the  \fBpthread_setconcurrency()\fR function returns  \fB0\fR.
Otherwise, an error number is returned to indicate the error.
.sp
.LP
The  \fBpthread_getconcurrency()\fR function always returns the concurrency
level set by a previous call to  \fBpthread_setconcurrency()\fR. If the
\fBpthread_setconcurrency()\fR function has never been called,
\fBpthread_getconcurrency()\fR returns  \fB0\fR.
.SH ERRORS
.sp
.LP
The \fBpthread_setconcurrency()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The value specified by \fInew_level\fR is negative.
.RE

.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
The value specific by \fInew_level\fR would cause a system resource to be
exceeded.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.sp
.LP
\fBpthread_create\fR(3C), \fBpthread_attr_init\fR(3C), \fBattributes\fR(5),
\fBstandards\fR(5)