clock(3) - Linux manual page (original) (raw)

clock(3) Library Functions Manual clock(3)

NAME top

   clock - determine processor time

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <time.h>**

   **clock_t clock(void);**

DESCRIPTION top

   The **clock**() function returns an approximation of processor time
   used by the program.

RETURN VALUE top

   The value returned is the CPU time used so far as a _clockt_; to
   get the number of seconds used, divide by **CLOCKS_PER_SEC**.  If the
   processor time used is not available or its value cannot be
   represented, the function returns the value _(clockt) -1_.

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **clock**()                              │ Thread safety │ MT-Safe │
   └──────────────────────────────────────┴───────────────┴─────────┘

VERSIONS top

   XSI requires that **CLOCKS_PER_SEC** equals 1000000 independent of the
   actual resolution.

   On several other implementations, the value returned by **clock**()
   also includes the times of any children whose status has been
   collected via [wait(2)](../man2/wait.2.html) (or another wait-type call).  Linux does not
   include the times of waited-for children in the value returned by
   **clock**().  The [times(2)](../man2/times.2.html) function, which explicitly returns
   (separate) information about the caller and its children, may be
   preferable.

STANDARDS top

   C11, POSIX.1-2008.

HISTORY top

   POSIX.1-2001, C89.

   In glibc 2.17 and earlier, **clock**() was implemented on top of
   [times(2)](../man2/times.2.html).  For improved accuracy, since glibc 2.18, it is
   implemented on top of [clock_gettime(2)](../man2/clock%5Fgettime.2.html) (using the
   **CLOCK_PROCESS_CPUTIME_ID** clock).

NOTES top

   The C standard allows for arbitrary values at the start of the
   program; subtract the value returned from a call to **clock**() at the
   start of the program to get maximum portability.

   Note that the time can wrap around.  On a 32-bit system where
   **CLOCKS_PER_SEC** equals 1000000 this function will return the same
   value approximately every 72 minutes.

SEE ALSO top

   [clock_gettime(2)](../man2/clock%5Fgettime.2.html), [getrusage(2)](../man2/getrusage.2.html), [times(2)](../man2/times.2.html)

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Linux man-pages 6.10 2024-07-23 clock(3)

Pages that refer to this page:getrusage(2), times(2), clock_t(3type), ctime(3), time(7)