PTHREAD_GETCONCURRENCY(3)  POSIX Programmer's Manual PTHREAD_GETCONCURRENCY(3)



PPRROOLLOOGG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NNAAMMEE
       pthread_getconcurrency, pthread_setconcurrency -- get and set the level
       of concurrency

SSYYNNOOPPSSIISS
       #include <pthread.h>

       int pthread_getconcurrency(void);
       int pthread_setconcurrency(int _n_e_w___l_e_v_e_l);

DDEESSCCRRIIPPTTIIOONN
       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 _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() function allows an application to inform
       the threads implementation of its desired concurrency level, _n_e_w___l_e_v_e_l.
       The actual level of concurrency provided by the implementation as a
       result of this function call is unspecified.

       If _n_e_w___l_e_v_e_l is zero, it causes the implementation to maintain the
       concurrency level at its discretion as if _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() had
       never been called.

       The _p_t_h_r_e_a_d___g_e_t_c_o_n_c_u_r_r_e_n_c_y() function shall return the value set by a
       previous call to the _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() function. If the
       _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() function was not previously called, this
       function shall return zero to indicate that the implementation is
       maintaining the concurrency level.

       A call to _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() 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 _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y()
       and _p_t_h_r_e_a_d___g_e_t_c_o_n_c_u_r_r_e_n_c_y() functions are provided for source code
       compatibility but they shall have no effect when called. To maintain
       the function semantics, the _n_e_w___l_e_v_e_l parameter is saved when
       _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() is called so that a subsequent call to
       _p_t_h_r_e_a_d___g_e_t_c_o_n_c_u_r_r_e_n_c_y() shall return the same value.

RREETTUURRNN VVAALLUUEE
       If successful, the _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() function shall return zero;
       otherwise, an error number shall be returned to indicate the error.

       The _p_t_h_r_e_a_d___g_e_t_c_o_n_c_u_r_r_e_n_c_y() function shall always return the
       concurrency level set by a previous call to _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y().
       If the _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() function has never been called,
       _p_t_h_r_e_a_d___g_e_t_c_o_n_c_u_r_r_e_n_c_y() shall return zero.

EERRRROORRSS
       The _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() function shall fail if:

       EEIINNVVAALL The value specified by _n_e_w___l_e_v_e_l is negative.

       EEAAGGAAIINN The value specified by _n_e_w___l_e_v_e_l would cause a system resource
              to be exceeded.

       The _p_t_h_r_e_a_d___s_e_t_c_o_n_c_u_r_r_e_n_c_y() function shall not return an error code of
       [[EEIINNTTRR]].

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
       None.

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

RRAATTIIOONNAALLEE
       None.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       These functions may be removed in a future version.

SSEEEE AALLSSOO
       The Base Definitions volume of POSIX.1-2017, <<pptthhrreeaadd..hh>>

CCOOPPYYRRIIGGHHTT
       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(3)
