time_t time(time_t *tloc);
The time() function shall return the value of time in seconds since
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. Oth-
erwise, (time_t)-1 shall be returned.
No errors are defined.
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.
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
minutes_to_event = ...;
printf("The time is ");
printf("There are %d minutes to the event.\n",
The time() function returns a value in seconds (type time_t) while
times() returns a set of values in clock ticks (type clock_t). Some
historical implementations, such as 4.3 BSD, have mechanisms capable of
returning more precise times (see below). A generalized timing scheme
to unify these various timing mechanisms has been proposed but not
Implementations in which time_t is a 32-bit signed integer (many his-
torical implementations) fail in the year 2038. IEEE Std 1003.1-2001
does not address this problem. However, the use of the time_t type is
mandated in order to ease the eventual fix.
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
IEEE Std 1003.1-2001 uses the latter type in order to agree with the
ISO C standard.
4.3 BSD includes time() only as an alternate function to the more flex-
ible gettimeofday() function.
In a future version of this volume of IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001 should be evaluated to determine
whether their lifetime will extend past 2038.
asctime() , clock() , ctime() , difftime() , gettimeofday() , gmtime()
, localtime() , mktime() , strftime() , strptime() , utime() , the Base
Definitions volume of IEEE Std 1003.1-2001, <time.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
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