TIME(3P) POSIX Programmer's Manual TIME(3P)PROLOG
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.
NAMEtime — get timeSYNOPSIS
#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.1‐2008
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
Upon successful completion, time() shall return the value of time. Oth‐
erwise, (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 his‐
torical 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 reli‐
ably detected.
The use of the <time.h> header instead of <sys/types.h> allows compati‐
bility 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
standard.
FUTURE DIRECTIONS
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 ALSOasctime(), clock(), clock_getres(), ctime(), difftime(), futimens(),
gettimeofday(), gmtime(), localtime(), mktime(), strftime(), strp‐
time(), times(), utime()
The Base Definitions volume of POSIX.1‐2008, <time.h>
COPYRIGHT
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 Electri‐
cal 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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 TIME(3P)
