man3/pthread_setconcurrency.3

.\" Copyright (c) 2009 Michael Kerrisk, <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH PTHREAD_SETCONCURRENCY 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_setconcurrency, pthread_getconcurrency \- set/get
the concurrency level
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.PP
.BI "int pthread_setconcurrency(int " new_level );
.BI "int pthread_getconcurrency(" void );
.PP
Compile and link with \fI\-pthread\fP.
.fi
.SH DESCRIPTION
The
.BR pthread_setconcurrency ()
function informs the implementation of the application's
desired concurrency level, specified in
.IR new_level .
The implementation takes this only as a hint:
POSIX.1 does not specify the level of concurrency that
should be provided as a result of calling
.BR pthread_setconcurrency ().
.PP
Specifying
.I new_level
as 0 instructs the implementation to manage the concurrency level
as it deems appropriate.
.PP
.BR pthread_getconcurrency ()
returns the current value of the concurrency level for this process.
.SH RETURN VALUE
On success,
.BR pthread_setconcurrency ()
returns 0;
on error, it returns a nonzero error number.
.PP
.BR pthread_getconcurrency ()
always succeeds, returning the concurrency level set by a previous call to
.BR pthread_setconcurrency (),
or 0, if
.BR pthread_setconcurrency ()
has not previously been called.
.SH ERRORS
.BR pthread_setconcurrency ()
can fail with the following error:
.TP
.B EINVAL
.I new_level
is negative.
.PP
POSIX.1 also documents an
.BR EAGAIN
error ("the value specified by
.I new_level
would cause a system resource to be exceeded").
.SH VERSIONS
These functions are available in glibc since version 2.1.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR pthread_setconcurrency (),
.BR pthread_getconcurrency ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.SH NOTES
The default concurrency level is 0.
.PP
Concurrency levels are meaningful only for M:N threading implementations,
where at any moment a subset of a process's set of user-level threads
may be bound to a smaller number of kernel-scheduling entities.
Setting the concurrency level allows the application to
give the system a hint as to the number of kernel-scheduling entities
that should be provided for efficient execution of the application.
.PP
Both LinuxThreads and NPTL are 1:1 threading implementations,
so setting the concurrency level has no meaning.
In other words,
on Linux these functions merely exist for compatibility with other systems,
and they have no effect on the execution of a program.
.SH SEE ALSO
.BR pthread_attr_setscope (3),
.BR pthreads (7)