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.1‐2008 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 this:

           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.1‐2008 uses the latter type in order to agree with the ISO C


       In a future version of this volume of POSIX.1‐2008, 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.1‐2008 should be evaluated to determine whether
       their lifetime will extend past 2038.

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), utime(3p)

       The Base Definitions volume of POSIX.1‐2008, time.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                            TIME(3P)

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