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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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LOCALTIME(3P) POSIX Programmer's Manual LOCALTIME(3P)
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.
localtime, localtime_r — convert a time value to a broken-down
local time
#include <time.h>
struct tm *localtime(const time_t *timer);
struct tm *localtime_r(const time_t *restrict timer,
struct tm *restrict result);
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.1‐2017 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 Definitions volume of POSIX.1‐2017, Section 4.16,
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
structure 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 structure 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() sets tzname, it shall also set
daylight and timezone. If localtime_r() does not set tzname, it
shall not set daylight and shall not set timezone.
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.
The localtime() and localtime_r() functions shall fail if:
EOVERFLOW
The result cannot be represented.
The following sections are informative.
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), 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
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);
...
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.
None.
None.
asctime(3p), clock(3p), ctime(3p), difftime(3p), getdate(3p),
gmtime(3p), mktime(3p), strftime(3p), strptime(3p), time(3p),
tzset(3p), utime(3p)
The Base Definitions volume of POSIX.1‐2017, Section 4.16,
Seconds Since the Epoch, time.h(0p)
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 .
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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 LOCALTIME(3P)
Pages that refer to this page: time.h(0p), asctime(3p), clock(3p), ctime(3p), difftime(3p), getdate(3p), gmtime(3p), mktime(3p), strftime(3p), time(3p), tzset(3p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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