TIME(3) BSD Library Functions Manual TIME(3)NAME
time -- get time of day
Standard C Library (libc, -lc)
The time() function returns the value of time in seconds since 0 hours, 0 minutes, 0 seconds, January 1, 1970, Coordinated Universal Time,
without including leap seconds. If an error occurs, time() returns the value (time_t)-1.
The return value is also stored in *tloc, provided that tloc is non-null.
The time() function may fail for any of the reasons described in gettimeofday(2).
SEE ALSO gettimeofday(2), ctime(3)STANDARDS
The time function conforms to IEEE Std 1003.1-2001 (``POSIX.1'').
A time() function appeared in Version 6 AT&T UNIX.
Neither ISO/IEC 9899:1999 (``ISO C99'') nor IEEE Std 1003.1-2001 (``POSIX.1'') requires time() to set errno on failure; thus, it is impossi-
ble for an application to distinguish the valid time value -1 (representing the last UTC second of 1969) from the error return value.
Systems conforming to earlier versions of the C and POSIX standards (including older versions of FreeBSD) did not set *tloc in the error
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TIME(P) POSIX Programmer's Manual TIME(P)
time - get time
time_t time(time_t *tloc);
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.
Upon successful completion, time() shall return the value of time. Otherwise, (time_t)-1 shall be returned.
No errors are defined.
The following sections are informative.
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.
result = time(NULL);
printf("%s%ju secs since the Epoch
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.
minutes_to_event = ...;
printf("The time is ");
printf("There are %d minutes to the event.
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.
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 deter-
mine whether their lifetime will extend past 2038.
asctime() , clock() , ctime() , difftime() , gettimeofday() , gmtime() , localtime() , mktime() , strftime() , strptime() , utime() , the
Base Definitions volume of IEEE Std 1003.1-2001, <time.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
ogy -- 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(P)