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.

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 specified by new_level would cause a system resource to be exceeded.

       The pthread_setconcurrency() function shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       Application developers should note that an implementation can always ignore any  calls  to
       pthread_setconcurrency()  and  return  a  constant for pthread_getconcurrency().  For this
       reason, it is not recommended that portable applications use this function.

RATIONALE

       None.

FUTURE DIRECTIONS

       These functions may be removed in a future version.

SEE ALSO

       The Base Definitions volume of POSIX.1‐2017, <pthread.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  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 .

       Any typographical or formatting errors that appear in this page are most  likely  to  have
       been  introduced  during  the conversion of the source files to man page format. To report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .