man time () - get time

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(N);
    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 .