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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017 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.
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.1‐2017 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
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.1‐2017, time.h(0p)
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)
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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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