The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### 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`);
> ``

> ``[CX] struct tm *localtime_r(const time_t *restrict` timer`,
> struct tm *restrict` result`);``
> `
> `

#### DESCRIPTION

> For localtime(): [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 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. [CX] 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 XBD 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.

#### 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 [CX] 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() [CX] and localtime_r() functions shall fail if:

> [EOVERFLOW]
>   [CX] 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), 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);
>     ...

#### 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

> XBD Seconds Since the Epoch, <time.h>

#### CHANGE HISTORY

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

#### Issue 5

> A note indicating that the localtime() function need not be reentrant is added to the DESCRIPTION.

> The localtime_r() function is included for alignment with the POSIX Threads Extension.

#### Issue 6

> The localtime_r() function is marked as part of the Thread-Safe Functions option.

> Extensions beyond the ISO C standard are marked.

> The APPLICATION USAGE section is updated to include a note on the thread-safe function and its avoidance of possibly using a static data area.

> The restrict keyword is added to the localtime_r() prototype for alignment with the ISO/IEC 9899:1999 standard.

> Examples are added.

> IEEE Std 1003.1-2001/Cor 1-2002, itemm XSH/TC1/D6/32 is applied, adding the [EOVERFLOW] error.

> IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/55 is applied, updating the error handling for localtime_r().

> IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/56 is applied, adding a requirement that if localtime_r() does not set the tzname variable, it shall not set the daylight or timezone variables. On systems supporting XSI, the daylight, timezone, and tzname variables should all be set to provide information for the same timezone. This updates the description of localtime_r() to mention daylight and timezone as well as tzname. The SEE ALSO section is updated.

#### Issue 7

> Austin Group Interpretation 1003.1-2001 #156 is applied.

> The localtime_r() function is moved from the Thread-Safe Functions option to the Base.

> Changes are made to the EXAMPLES section related to support for finegrained timestamps.

> POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0363 [291] is applied.

> POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0201 [664] is applied.

End of informative text.

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