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.
NAME
time - get time
SYNOPSIS
#include <time.h>
time_t time(time_t *tloc);
DESCRIPTION
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
No errors are defined.
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 (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
adopted.
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.
FUTURE DIRECTIONS
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.
SEE ALSOasctime(), clock(), ctime(), difftime(), gettimeofday(), gmtime(),
localtime(), mktime(), strftime(), strptime(), utime(), the Base Defi‐
nitions volume of IEEE Std 1003.1-2001, <time.h>
COPYRIGHT
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
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 .
IEEE/The Open Group 2003 TIME(3P)
