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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       time -- get time

SYNOPSIS
       #include <time.h>

       time_t time(time_t *tloc);

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

ERRORS
       The time() function may fail if:

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

       The following sections are informative.

EXAMPLES
   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",
                   asctime(localtime(&result)),
                       (uintmax_t)result);
               return(0);
           }

       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
       timed.


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

APPLICATION USAGE
       None.

RATIONALE
       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.

FUTURE DIRECTIONS
       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 2038.

SEE ALSO
       asctime(), clock(), clock__getres(), ctime(), difftime(), futimens(),
       gettimeofday(), gmtime(), localtime(), mktime(), strftime(),
       strptime(), times(), utime()

       The Base Definitions volume of POSIX.1-2017, <time.h>

COPYRIGHT
       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(3)
