LOCALTIME(3P) POSIX Programmer's Manual LOCALTIME(3P)
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
localtime, localtime_r -- convert a time value to a broken-down local
time
SYNOPSIS
#include <time.h>
struct tm *localtime(const time_t *timer);
struct tm *localtime_r(const time_t *restrict timer,
struct tm *restrict result);
DESCRIPTION
For localtime(): 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.1-2008 defers to the ISO C standard.
The localtime() function shall convert the time in seconds since the
Epoch pointed to by timer into a broken-down time, expressed as a local
time. The function corrects for the timezone and any seasonal time
adjustments. Local timezone information is used as though localtime()
calls tzset().
The relationship between a time in seconds since the Epoch used as an
argument to localtime() and the tm structure (defined in the <time.h>
header) is that the result shall be as specified in the expression
given in the definition of seconds since the Epoch (see the Base Defi-
nitions volume of POSIX.1-2008, Section 4.15, Seconds Since the Epoch)
corrected for timezone and any seasonal time adjustments, where the
names in the structure and in the expression correspond.
The same relationship shall apply for localtime_r().
The localtime() function need not be thread-safe.
The asctime(), ctime(), gmtime(), and localtime() functions shall
return values in one of two static objects: a broken-down time struc-
ture and an array of type char. Execution of any of the functions may
overwrite the information returned in either of these objects by any of
the other functions.
The localtime_r() function shall convert the time in seconds since the
Epoch pointed to by timer into a broken-down time stored in the struc-
ture to which result points. The localtime_r() function shall also
return a pointer to that same structure.
Unlike localtime(), the localtime_r() function is not required to set
tzname. If localtime_r() does not set tzname, it shall not set day-
light and shall not set timezone.
RETURN VALUE
Upon successful completion, the localtime() function shall return a
pointer to the broken-down time structure. If an error is detected,
localtime() shall return a null pointer and set errno to indicate the
error.
Upon successful completion, localtime_r() shall return a pointer to the
structure pointed to by the argument result. If an error is detected,
localtime_r() shall return a null pointer and set errno to indicate the
error.
ERRORS
The localtime() and localtime_r() functions shall fail if:
EOVERFLOW
The result cannot be represented.
The following sections are informative.
EXAMPLES
Getting the Local Date and Time
The following example uses the time() function to calculate the time
elapsed, in seconds, since January 1, 1970 0:00 UTC (the Epoch), local-
time() 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
Getting the Modification Time for a File
The following example prints the last data modification timestamp in
the local timezone for a given file.
#include <stdio.h>
#include <time.h>
#include <sys/stat.h>
int
print_file_time(const char *pathname)
{
struct stat statbuf;
struct tm *tm;
char timestr[BUFSIZ];
if(stat(pathname, &statbuf) == -1)
return -1;
if((tm = localtime(&statbuf.st_mtime)) == NULL)
return -1;
if(strftime(timestr, sizeof(timestr), "%Y-%m-%d %H:%M:%S", tm) == 0)
return -1;
printf("%s: %s.%09ld\n", pathname, timestr, statbuf.st_mtim.tv_nsec);
return 0;
}
Timing an Event
The following example gets the current time, converts it to a string
using localtime() and asctime(), and prints it to standard output using
fputs(). It then 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);
printf("The time is ");
fputs(asctime(localtime(&now)), stdout);
printf("There are still %d minutes to the event.\n",
minutes_to_event);
...
APPLICATION USAGE
The localtime_r() function is thread-safe and returns values in a user-
supplied buffer instead of possibly using a static data area that may
be overwritten by each call.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
asctime(), clock(), ctime(), difftime(), getdate(), gmtime(), mktime(),
strftime(), strptime(), time(), tzset(), utime()
The Base Definitions volume of POSIX.1-2008, Section 4.15, Seconds
Since the Epoch, <time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 LOCALTIME(3P)
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