time (original) (raw)

The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

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NAME

time - get time

SYNOPSIS

`#include <time.h>

time_t time(time_t *

tloc);
`

DESCRIPTION

[CX] 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 IEEE Std 1003.1-2001 defers to the ISO C standard.

The time() function shall return the value of time [CX] 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.

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

Extensions beyond the ISO C standard are marked.

The EXAMPLES, RATIONALE, and FUTURE DIRECTIONS sections are added.

End of informative text.

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