CLOCK(3)                   POSIX Programmer's Manual                  CLOCK(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
       clock -- report CPU time used

SSYYNNOOPPSSIISS
       #include <time.h>

       clock_t clock(void);

DDEESSCCRRIIPPTTIIOONN
       The functionality described on this reference page is aligned with the
       ISO C standard. Any conflict between the requirements described here
       and the ISO C standard is unintentional. This volume of POSIX.1-2017
       defers to the ISO C standard.

       The _c_l_o_c_k() function shall return the implementation's best
       approximation to the processor time used by the process since the
       beginning of an implementation-defined era related only to the process
       invocation.

RREETTUURRNN VVAALLUUEE
       To determine the time in seconds, the value returned by _c_l_o_c_k() should
       be divided by the value of the macro CLOCKS_PER_SEC.  CLOCKS_PER_SEC is
       defined to be one million in _<_t_i_m_e_._h_>.  If the processor time used is
       not available or its value cannot be represented, the function shall
       return the value (cclloocckk__tt)-1.

EERRRROORRSS
       No errors are defined.

       _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
       In programming environments where cclloocckk__tt is a 32-bit integer type and
       CLOCKS_PER_SEC is one million, _c_l_o_c_k() will start failing in less than
       36 minutes of processor time for signed cclloocckk__tt, or 72 minutes for
       unsigned cclloocckk__tt.  Applications intended to be portable to such
       environments should use _t_i_m_e_s() instead (or _c_l_o_c_k___g_e_t_t_i_m_e() with
       CLOCK_PROCESS_CPUTIME_ID, if supported).

       In order to measure the time spent in a program, _c_l_o_c_k() should be
       called at the start of the program and its return value subtracted from
       the value returned by subsequent calls. The value returned by _c_l_o_c_k()
       is defined for compatibility across systems that have clocks with
       different resolutions. The resolution on any particular system need not
       be to microsecond accuracy.

RRAATTIIOONNAALLEE
       None.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _a_s_c_t_i_m_e(), _c_l_o_c_k___g_e_t_r_e_s(), _c_t_i_m_e(), _d_i_f_f_t_i_m_e(), _g_m_t_i_m_e(), _l_o_c_a_l_t_i_m_e(),
       _m_k_t_i_m_e(), _s_t_r_f_t_i_m_e(), _s_t_r_p_t_i_m_e(), _t_i_m_e(), _t_i_m_e_s(), _u_t_i_m_e()

       The Base Definitions volume of POSIX.1-2017, <<ttiimmee..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                             CLOCK(3)
