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TIME(3P) POSIX Programmer's Manual TIME(3P)
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.
time — get time
#include <time.h>
time_t time(time_t *tloc);
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.
Upon successful completion, time() shall return the value of time.
Otherwise, (time_t)−1 shall be returned.
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.
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);
...
None.
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
standard.
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.
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)
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)
Pages that refer to this page: time.h(0p), touch(1p), asctime(3p), clock(3p), clock_getres(3p), ctime(3p), difftime(3p), fstatvfs(3p), getrusage(3p), gmtime(3p), localtime(3p), mktime(3p), mq_receive(3p), mq_send(3p), posix_trace_create(3p), pthread_mutex_timedlock(3p), sem_timedwait(3p), strftime(3p), strptime(3p), times(3p)