time(3p) — Linux manual page


TIME(3P)                POSIX Programmer's Manual               TIME(3P)

PROLOG         top

       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         top

       time — get time

SYNOPSIS         top

       #include <time.h>

       time_t time(time_t *tloc);

DESCRIPTION         top

       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 POSIX.1‐2017 defers to the ISO C standard.

       The time() function shall return the value of time in seconds
       since the Epoch.

       The tloc argument points to an area where the return value is
       also stored. If tloc is a null pointer, no value is stored.

RETURN VALUE         top

       Upon successful completion, time() shall return the value of
       time. Otherwise, (time_t)-1 shall be returned.

ERRORS         top

       The time() function may fail if:

              The number of seconds since the Epoch will not fit in an
              object of type time_t.

       The following sections are informative.

EXAMPLES         top

   Getting the Current Time
       The following example uses the time() function to calculate the
       time elapsed, in seconds, since the Epoch, localtime() to convert
       that value to a broken-down time, and asctime() to convert the
       broken-down time values into a printable string.

           #include <stdio.h>
           #include <time.h>

           int main(void)
           time_t result;

               result = time(NULL);
               printf("%s%ju secs since the Epoch\n",

       This example writes the current time to stdout in a form like

           Wed Jun 26 10:32:15 1996
           835810335 secs since the Epoch

   Timing an Event
       The following example gets the current time, prints it out in the
       user's format, and prints the number of minutes to an event being

           #include <time.h>
           #include <stdio.h>
           time_t now;
           int minutes_to_event;
           minutes_to_event = ...;
           printf("The time is ");
           printf("There are %d minutes to the event.\n",



RATIONALE         top

       The time() function returns a value in seconds while
       clock_gettime() and gettimeofday() return a struct timespec
       (seconds and nanoseconds) and struct timeval (seconds and
       microseconds), respectively, and are therefore capable of
       returning more precise times. The times() function is also
       capable of more precision than time() as it returns a value in
       clock ticks, although it returns the elapsed time since an
       arbitrary point such as system boot time, not since the epoch.

       Implementations in which time_t is a 32-bit signed integer (many
       historical implementations) fail in the year 2038. POSIX.1‐2008
       does not address this problem. However, the use of the time_t
       type is mandated in order to ease the eventual fix.

       On some systems the time() function is implemented using a system
       call that does not return an error condition in addition to the
       return value. On these systems it is impossible to differentiate
       between valid and invalid return values and hence overflow
       conditions cannot be reliably detected.

       The use of the <time.h> header instead of <sys/types.h> allows
       compatibility with the ISO C standard.

       Many historical implementations (including Version 7) and the
       1984 /usr/group standard use long instead of time_t.  This volume
       of POSIX.1‐2017 uses the latter type in order to agree with the
       ISO C standard.


       In a future version of this volume of POSIX.1‐2017, time_t is
       likely to be required to be capable of representing times far in
       the future. Whether this will be mandated as a 64-bit type or a
       requirement that a specific date in the future be representable
       (for example, 10000 AD) is not yet determined. Systems purchased
       after the approval of this volume of POSIX.1‐2017 should be
       evaluated to determine whether their lifetime will extend past

SEE ALSO         top

       asctime(3p), clock(3p), clock_getres(3p), ctime(3p),
       difftime(3p), futimens(3p), gettimeofday(3p), gmtime(3p),
       localtime(3p), mktime(3p), strftime(3p), strptime(3p), times(3p),

       The Base Definitions volume of POSIX.1‐2017, time.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                          TIME(3P)

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