CLOCK(P) POSIX Programmer s Manual CLOCK(P)

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 clock - report CPU time used

SYNOPSIS #include <time.h>

clock_t clock(void);

DESCRIPTION The clock() function shall return the implementations best approxima- tion to the processor time used by the process since the beginning of an implementation-defined era related only to the process invocation.

RETURN VALUE 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.

ERRORS No errors are defined.

The following sections are informative.

EXAMPLES None.

APPLICATION USAGE 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 dif- ferent resolutions. The resolution on any particular system need not be to microsecond accuracy.

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.

RATIONALE None.

FUTURE DIRECTIONS None.

SEE ALSO asctime() , ctime() , difftime() , gmtime() , localtime() , mktime() , strftime() , strptime() , time() , utime() , the Base Definitions vol- ume of IEEE Std 1003.1-2001, <time.h>

COPYRIGHT 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 original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group 2003 CLOCK(P)