LOCALTIME(3P)

🕰️ LOCALTIME(3P) — bestie, it's giving timezone glow-up ✨

okay so real talk: this page lives in the POSIX Programmer's Manual, and it's the whole saga of localtime and localtime_r — the two functions that take your crunchy raw seconds-since-the-Epoch and turn them into a cute little broken-down local time. 🎀

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💖 PROLOG 💖

This manual page is part of the POSIX Programmer's Manual. Heads up though: the Linux implementation of this interface may differ (so go check the corresponding Linux manual page for the tea on Linux behavior), or honestly the interface may not even be implemented on Linux. 🙈

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✨ NAME ✨

localtime, localtime_r — convert a time value to a broken-down local time. that's it, that's the job. 💅

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🌸 SYNOPSIS 🌸

#include <time.h>

struct tm *localtime(const time_t *timer);
struct tm *localtime_r(const time_t *restrict timer,
    struct tm *restrict result);

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🦋 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 — nobody's trying to start drama. This volume of POSIX.1‐2017 defers to the ISO C standard. periodt.

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. iykyk 💫

The same relationship shall apply for localtime_r().

⚠️ The localtime() function need not be thread-safe. no cap, that's just how it is.

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. so like... they're all sharing one communal closet and they WILL steal each other's outfits. 💀

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. it's an all-or-nothing kinda girlie. 👑

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🌈 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.

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🔥 ERRORS 🔥

The localtime() and localtime_r() functions shall fail if:

• EOVERFLOW — The result cannot be represented. that's it, that's the one. 💀

The following sections are informative. (translation: the rest is bonus content, not rules 💖)

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🎀 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. iconic behavior fr fr ⭐

#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);
...

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💜 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. main character energy, no shared closet, no drama. 👑

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🧚 RATIONALE 🧚

None. (she said nothing and left. queen. 💅)

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⭐ FUTURE DIRECTIONS ⭐

None.

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🌸 SEE ALSO 🌸

asctime(), clock(), ctime(), difftime(), getdate(), gmtime(), mktime(), strftime(), strptime(), time(), tzset(), utime()

The Base Definitions volume of POSIX.1‐2017, Section 4.16, Seconds Since the Epoch, <time.h>

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👑 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 .

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 .

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and that's the whole page bestie, ily, go forth and convert your timestamps responsibly 🫶💖

IEEE/The Open Group · 2017 · LOCALTIME(3P) ✨