This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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.

       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 .

IEEE/The Open Group                                   2017                        PTHREAD_GETCONCURRENCY(3POSIX)

       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.

       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].

       Thefollowingsectionsareinformative.

       None.

       These functions may be removed in a future version.

       pthread_getconcurrency, pthread_setconcurrency — get and set the level of concurrency

       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.

       None.

       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.

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

       #include <pthread.h>

       int pthread_getconcurrency(void);
       int pthread_setconcurrency(int new_level);