CLOCK(P) POSIX Programmer's Manual CLOCK(P)
clock - report CPU time used
The clock() 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.
To determine the time in seconds, the value returned by clock() should be divided by the
value of the macro CLOCKS_PER_SEC. CLOCKS_PER_SEC is defined to be one million in
<time.h>. If the processor time used is not available or its value cannot be represented,
the function shall return the value ( clock_t)-1.
No errors are defined.
The following sections are informative.
In order to measure the time spent in a program, clock() 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 clock() is defined for compatibility across systems that have clocks
with different resolutions. The resolution on any particular system need not be to
The value returned by clock() may wrap around on some implementations. For example, on a
machine with 32-bit values for clock_t, it wraps after 2147 seconds or 36 minutes.
asctime() , ctime() , difftime() , gmtime() , localtime() , mktime() , strftime() , strp-
time() , time() , utime() , the Base Definitions volume of IEEE Std 1003.1-2001, <time.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 CLOCK(P)