--- name: "asctime" section: "3" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/ctime/3" --- # asctime(3) transform date and time to broken-down time or ASCII Part of [ctime(3)](/ctime/3.md). ## NAME asctime, ctime, gmtime, localtime, mktime, asctime\_r, ctime\_r, gmtime\_r, localtime\_r - transform date and time to broken-down time or ASCII ## LIBRARY Standard C library (*libc*, *-lc*) ## SYNOPSIS ``` #include ``` **char \*asctime(const struct tm \****tm***);** **char \*asctime\_r(const struct tm \*restrict** *tm***,** **char** *buf***\[restrict 26\]);** **char \*ctime(const time\_t \****timep***);** **char \*ctime\_r(const time\_t \*restrict** *timep***,** **char** *buf***\[restrict 26\]);** **struct tm \*gmtime(const time\_t \****timep***);** **struct tm \*gmtime\_r(const time\_t \*restrict** *timep***,** **struct tm \*restrict** *result***);** **struct tm \*localtime(const time\_t \****timep***);** **struct tm \*localtime\_r(const time\_t \*restrict** *timep***,** **struct tm \*restrict** *result***);** **time\_t mktime(struct tm \****tm***);** Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)): **asctime\_r**(), **ctime\_r**(), **gmtime\_r**(), **localtime\_r**(): ``` _POSIX_C_SOURCE || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE ``` ## DESCRIPTION The **ctime**(), **gmtime**(), and **localtime**() functions all take an argument of data type *time\_t*, which represents calendar time. When interpreted as an absolute time value, it represents the number of seconds elapsed since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). The **asctime**() and **mktime**() functions both take an argument representing broken-down time, which is a representation separated into year, month, day, and so on. Broken-down time is stored in the structure *tm*, described in [**tm**(3type)](/tm/3type.md). The call **ctime(***t***)** is equivalent to *asctime(localtime(t))*. It converts the calendar time *t* into a null-terminated string of the form ``` "Wed Jun 30 21:49:08 1993\n" ``` The abbreviations for the days of the week are "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", and "Sat". The abbreviations for the months are "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", and "Dec". The return value points to a statically allocated string which might be overwritten by subsequent calls to any of the date and time functions. The function also sets the external variables *tzname*, *timezone*, and *daylight* as if it called [**tzset**(3)](/tzset/3.md). The reentrant version **ctime\_r**() does the same, but stores the string in a user-supplied buffer which should have room for at least 26 bytes. It need not set *tzname*, *timezone*, and *daylight*. The **gmtime**() function converts the calendar time *timep* to broken-down time representation, expressed in Coordinated Universal Time (UTC). It may return NULL when the year does not fit into an integer. The return value points to a statically allocated struct which might be overwritten by subsequent calls to any of the date and time functions. The **gmtime\_r**() function does the same, but stores the data in a user-supplied struct. The **localtime**() function converts the calendar time *timep* to broken-down time representation, expressed relative to the user's specified timezone. The function also sets the external variables *tzname*, *timezone*, and *daylight* as if it called [**tzset**(3)](/tzset/3.md). The return value points to a statically allocated struct which might be overwritten by subsequent calls to any of the date and time functions. The **localtime\_r**() function does the same, but stores the data in a user-supplied struct. It need not set *tzname*, *timezone*, and *daylight*. The **asctime**() function converts the broken-down time value *tm* into a null-terminated string with the same format as **ctime**(). The return value points to a statically allocated string which might be overwritten by subsequent calls to any of the date and time functions. The **asctime\_r**() function does the same, but stores the string in a user-supplied buffer which should have room for at least 26 bytes. The **mktime**() function converts a broken-down time structure, expressed as local time, to calendar time representation. The function ignores the values supplied by the caller in the *tm\_wday* and *tm\_yday* fields. The value specified in the *tm\_isdst* field informs **mktime**() whether or not daylight saving time (DST) is in effect for the time supplied in the *tm* structure: a positive value means DST is in effect; zero means that DST is not in effect; and a negative value means that **mktime**() should (use timezone information and system databases to) attempt to determine whether DST is in effect at the specified time. See [**timegm**(3)](/timegm/3.md) for a UTC equivalent of this function. The **mktime**() function modifies the fields of the *tm* structure as follows: *tm\_wday* and *tm\_yday* are set to values determined from the contents of the other fields; if structure members are outside their valid interval, they will be normalized (so that, for example, 40 October is changed into 9 November); *tm\_isdst* is set (regardless of its initial value) to a positive value or to 0, respectively, to indicate whether DST is or is not in effect at the specified time. The function also sets the external variables *tzname*, *timezone*, and *daylight* as if it called [**tzset**(3)](/tzset/3.md). If the specified broken-down time cannot be represented as calendar time (seconds since the Epoch), **mktime**() returns *(time\_t) -1* and does not alter the members of the broken-down time structure. ## RETURN VALUE On success, **gmtime**() and **localtime**() return a pointer to a *struct tm*. On success, **gmtime\_r**() and **localtime\_r**() return the address of the structure pointed to by *result*. On success, **asctime**() and **ctime**() return a pointer to a string. On success, **asctime\_r**() and **ctime\_r**() return a pointer to the string pointed to by *buf*. On success, **mktime**() returns the calendar time (seconds since the Epoch), expressed as a value of type *time\_t*. On error, **mktime**() returns the value *(time\_t) -1*, and leaves the *tm-\>tm\_wday* member unmodified. The remaining functions return NULL on error. On error, *errno* is set to indicate the error. ## ERRORS - **EOVERFLOW** The result cannot be represented. ## ATTRIBUTES For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md). | | | | | --- | --- | --- | | **Interface** | **Attribute** | **Value** | | **asctime**() | Thread safety | MT-Unsafe race:asctime locale | | **asctime\_r**() | Thread safety | MT-Safe locale | | **ctime**() | Thread safety | MT-Unsafe race:tmbuf race:asctime env locale | | **ctime\_r**(), **gmtime\_r**(), **localtime\_r**(), **mktime**() | Thread safety | MT-Safe env locale | | **gmtime**(), **localtime**() | Thread safety | MT-Unsafe race:tmbuf env locale | ## VERSIONS POSIX doesn't specify the parameters of **ctime\_r**() to be *restrict*; that is specific to glibc. In many implementations, including glibc, a 0 in *tm\_mday* is interpreted as meaning the last day of the preceding month. According to POSIX.1, **localtime**() is required to behave as though [**tzset**(3)](/tzset/3.md) was called, while **localtime\_r**() does not have this requirement. For portable code, [**tzset**(3)](/tzset/3.md) should be called before **localtime\_r**(). ## STANDARDS - **asctime**() **ctime**() **gmtime**() **localtime**() **mktime**() C23, POSIX.1-2024. - **gmtime\_r**() **localtime\_r**() POSIX.1-2024. - **asctime\_r**() **ctime\_r**() None. ## HISTORY - **gmtime**() **localtime**() **mktime**() C89, POSIX.1-1988. - **asctime**() **ctime**() C89, POSIX.1-1988. Marked obsolescent in C23 and in POSIX.1-2008 (recommending [**strftime**(3)](/strftime/3.md)). - **gmtime\_r**() **localtime\_r**() POSIX.1-1996. - **asctime\_r**() **ctime\_r**() POSIX.1-1996. Marked obsolescent in POSIX.1-2008. Removed in POSIX.1-2024 (recommending [**strftime**(3)](/strftime/3.md)). ## CAVEATS ### Thread safety The four functions **asctime**(), **ctime**(), **gmtime**(), and **localtime**() return a pointer to static data and hence are not thread-safe. The thread-safe versions, **asctime\_r**(), **ctime\_r**(), **gmtime\_r**(), and **localtime\_r**(), are specified by SUSv2. POSIX.1 says: "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 that return a pointer to one of these object types may overwrite the information in any object of the same type pointed to by the value returned from any previous call to any of them." This can occur in the glibc implementation. ### mktime() *(time\_t) -1* can represent a valid time (one second before the Epoch). To determine whether **mktime**() failed, one must use the *tm-\>tm\_wday* field. See the example program in EXAMPLES. The handling of a non-negative *tm\_isdst* in **mktime**() is poorly specified, and passing a value that is incorrect for the time specified yields unspecified results. Since **mktime**() is one of the few functions that knows when DST is in effect, providing a correct value may be difficult. One workaround for this is to call **mktime**() twice, once with *tm\_isdst* set to zero, and once with *tm\_isdst* set to a positive value, and discarding the results from the call that changes it. If neither call changes *tm\_isdst* then the time specified probably happens during a fall-back period where DST begins or ends, and both results are valid but represent two different times. If both calls change it, that could indicate a fall-forward transition, or some other reason why the time specified does not exist. The specification of time zones and daylight saving time are up to regional governments, change often, and may include discontinuities beyond *mktime*'s ability to document a result. For example, a change in the timezone definition may cause a clock time to be repeated or skipped without a corresponding DST change. ## EXAMPLES The program below defines a wrapper that allows detecting invalid and ambiguous times, with **EINVAL** and **ENOTUNIQ**, respectively. The following shell session shows sample runs of the program: ``` $ TZ=UTC ./a.out 1969 12 31 23 59 59 0; -1 $ $ export TZ=Europe/Madrid; $ $ ./a.out 2147483647 2147483647 00 00 00 00 -1; a.out: mktime: Value too large for defined data type $ $ ./a.out 2024 08 23 00 17 53 -1; 1724365073 $ ./a.out 2024 08 23 00 17 53 0; a.out: my_mktime: Invalid argument 1724368673 $ ./a.out 2024 08 23 00 17 53 1; 1724365073 $ $ ./a.out 2024 02 23 00 17 53 -1; 1708643873 $ ./a.out 2024 02 23 00 17 53 0; 1708643873 $ ./a.out 2024 02 23 00 17 53 1; a.out: my_mktime: Invalid argument 1708640273 $ $ ./a.out 2023 03 26 02 17 53 -1; a.out: my_mktime: Invalid argument 1679793473 $ $ ./a.out 2023 10 29 02 17 53 -1; a.out: my_mktime: Name not unique on network 1698542273 $ ./a.out 2023 10 29 02 17 53 0; 1698542273 $ ./a.out 2023 10 29 02 17 53 1; 1698538673 $ $ ./a.out 2023 02 29 12 00 00 -1; a.out: my_mktime: Invalid argument 1677668400 ``` ### Program source: mktime.c ``` #include #include #include #include #include #include #include #define is_signed(T) ((T) -1 < 1) static time_t my_mktime(struct tm *tp); int main(int argc, char *argv[]) { char **p; time_t t; struct tm tm; if (argc != 8) { fprintf(stderr, "Usage: %s yyyy mm dd HH MM SS isdst\n", argv[0]); exit(EXIT_FAILURE); } p = &argv[1]; tm.tm_year = atoi(*p++) - 1900; tm.tm_mon = atoi(*p++) - 1; tm.tm_mday = atoi(*p++); tm.tm_hour = atoi(*p++); tm.tm_min = atoi(*p++); tm.tm_sec = atoi(*p++); tm.tm_isdst = atoi(*p++); errno = 0; tm.tm_wday = -1; t = my_mktime(&tm); if (tm.tm_wday == -1) err(EXIT_FAILURE, "mktime"); if (errno == EINVAL || errno == ENOTUNIQ) warn("my_mktime"); if (is_signed(time_t)) printf("%jd\n", (intmax_t) t); else printf("%ju\n", (uintmax_t) t); exit(EXIT_SUCCESS); } static time_t my_mktime(struct tm *tp) { int e, isdst; time_t t; struct tm tm; unsigned char wday[sizeof(tp->tm_wday)]; e = errno; tm = *tp; isdst = tp->tm_isdst; memcpy(wday, &tp->tm_wday, sizeof(wday)); tp->tm_wday = -1; t = mktime(tp); if (tp->tm_wday == -1) { memcpy(&tp->tm_wday, wday, sizeof(wday)); return -1; } if (isdst == -1) tm.tm_isdst = tp->tm_isdst; if ( tm.tm_sec != tp->tm_sec || tm.tm_min != tp->tm_min || tm.tm_hour != tp->tm_hour || tm.tm_mday != tp->tm_mday || tm.tm_mon != tp->tm_mon || tm.tm_year != tp->tm_year || tm.tm_isdst != tp->tm_isdst) { errno = EINVAL; return t; } if (isdst != -1) goto out; tm = *tp; tm.tm_isdst = !tm.tm_isdst; tm.tm_wday = -1; mktime(&tm); if (tm.tm_wday == -1) goto out; if (tm.tm_isdst != tp->tm_isdst) { errno = ENOTUNIQ; return t; } out: errno = e; return t; } ``` ## SEE ALSO [**date**(1)](/date/1.md), [**gettimeofday**(2)](/gettimeofday/2.md), [**time**(2)](/time/2.md), [**utime**(2)](/utime/2.md), [**clock**(3)](/clock/3.md), [**difftime**(3)](/difftime/3.md), [**strftime**(3)](/strftime/3.md), [**strptime**(3)](/strptime/3.md), [**timegm**(3)](/timegm/3.md), [**tzset**(3)](/tzset/3.md), [**time**(7)](/time/7.md)