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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. Otherwise, (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 historical
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 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 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 flexible 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 ALSO
asctime() , clock() , ctime() , difftime() , gettimeofday() , gmtime() , localtime() ,
mktime() , strftime() , strptime() , utime() , the Base Definitions 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 .