clock(3) - f38

1clock(3)                   Library Functions Manual                   clock(3)
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NAME

6       clock - determine processor time
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <time.h>
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14       clock_t clock(void);
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DESCRIPTION

17       The clock() function returns an approximation of processor time used by
18       the program.
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RETURN VALUE

21       The value returned is the CPU time used so far as a clock_t; to get the
22       number  of  seconds  used,  divide by CLOCKS_PER_SEC.  If the processor
23       time used is not available or its  value  cannot  be  represented,  the
24       function returns the value (clock_t) -1.
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ATTRIBUTES

27       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
28       tributes(7).
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30       ┌────────────────────────────────────────────┬───────────────┬─────────┐
31       │Interface                                   │ Attribute     │ Value   │
32       ├────────────────────────────────────────────┼───────────────┼─────────┤
33       │clock()                                     │ Thread safety │ MT-Safe │
34       └────────────────────────────────────────────┴───────────────┴─────────┘
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VERSIONS

37       XSI requires that CLOCKS_PER_SEC equals 1000000 independent of the  ac‐
38       tual resolution.
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40       On  several  other  implementations, the value returned by clock() also
41       includes the times of any children whose status has been collected  via
42       wait(2)  (or another wait-type call).  Linux does not include the times
43       of waited-for children in the value returned by clock().  The  times(2)
44       function,  which  explicitly  returns  (separate) information about the
45       caller and its children, may be preferable.
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STANDARDS

48       C11, POSIX.1-2008.
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HISTORY

51       POSIX.1-2001, C89.
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53       In glibc 2.17 and earlier, clock() was implemented on top of  times(2).
54       For  improved  accuracy,  since glibc 2.18, it is implemented on top of
55       clock_gettime(2) (using the CLOCK_PROCESS_CPUTIME_ID clock).
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NOTES

58       The C standard allows for arbitrary values at the start of the program;
59       subtract  the value returned from a call to clock() at the start of the
60       program to get maximum portability.
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62       Note that  the  time  can  wrap  around.   On  a  32-bit  system  where
63       CLOCKS_PER_SEC  equals 1000000 this function will return the same value
64       approximately every 72 minutes.
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