clock (original) (raw)

The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

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NAME

clock - report CPU time used

SYNOPSIS

`#include <time.h>

clock_t clock(void);

`

DESCRIPTION

[CX] [Option Start] 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 IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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.

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. [XSI] [Option Start] CLOCKS_PER_SEC is defined to be one million in <time.h>. [Option End] 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 different 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 forclock_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 volume of IEEE Std 1003.1-2001, <time.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

End of informative text.

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