Manpages - time.3p
Table of Contents
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 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 POSIX.1‐2017 defers to the ISO C standard.
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
The /time/() function may fail if:
- EOVERFLOW
- The number of seconds since the Epoch will not fit in an object of type time_t.
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 while /clock_gettime/() and /gettimeofday/() return a struct timespec (seconds and nanoseconds) and struct timeval (seconds and microseconds), respectively, and are therefore capable of returning more precise times. The /times/() function is also capable of more precision than /time/() as it returns a value in clock ticks, although it returns the elapsed time since an arbitrary point such as system boot time, not since the epoch.
Implementations in which time_t is a 32-bit signed integer (many historical implementations) fail in the year 2038. POSIX.1‐2008 does not address this problem. However, the use of the time_t type is mandated in order to ease the eventual fix.
On some systems the /time/() function is implemented using a system call that does not return an error condition in addition to the return value. On these systems it is impossible to differentiate between valid and invalid return values and hence overflow conditions cannot be reliably detected.
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 POSIX.1‐2017 uses the latter type in order to agree with the ISO C standard.
FUTURE DIRECTIONS
In a future version of this volume of POSIX.1‐2017, 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 POSIX.1‐2017 should be evaluated to determine whether their lifetime will extend past 2038.
SEE ALSO
/asctime ( )/, /clock ( )/, /clock_getres ( )/, /ctime ( )/, /difftime ( )/, /futimens ( )/, /gettimeofday ( )/, /gmtime ( )/, /localtime ( )/, /mktime ( )/, /strftime ( )/, /strptime ( )/, /times ( )/, /utime ( )/
The Base Definitions volume of POSIX.1‐2017, <time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology – Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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