HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. |  |
NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |VERSIONS |STANDARDS |HISTORY |CAVEATS |EXAMPLES |SEE ALSO |COLOPHON | |
ctime(3) Library Functions Manualctime(3)asctime, ctime, gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r, localtime_r - transform date and time to broken-down time or ASCII
Standard C library (libc,-lc)
#include <time.h>char *asctime(const struct tm *tm);char *asctime_r(const struct tm *restricttm,charbuf[restrict 26]);char *ctime(const time_t *timep);char *ctime_r(const time_t *restricttimep,charbuf[restrict 26]);struct tm *gmtime(const time_t *timep);struct tm *gmtime_r(const time_t *restricttimep,struct tm *restrictresult);struct tm *localtime(const time_t *timep);struct tm *localtime_r(const time_t *restricttimep,struct tm *restrictresult);time_t mktime(struct tm *tm); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):asctime_r(),ctime_r(),gmtime_r(),localtime_r(): _POSIX_C_SOURCE || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
Thectime(),gmtime(), andlocaltime() functions all take an argument of data typetime_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). Theasctime() andmktime() 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 structuretm, described intm(3type). The callctime(t)is equivalent toasctime(localtime(t)). It converts the calendar timet 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 variablestzname,timezone, anddaylight as if it calledtzset(3). The reentrant versionctime_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 settzname,timezone, anddaylight. Thegmtime() function converts the calendar timetimep 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. Thegmtime_r() function does the same, but stores the data in a user-supplied struct. Thelocaltime() function converts the calendar timetimep to broken-down time representation, expressed relative to the user's specified timezone. The function also sets the external variablestzname,timezone, anddaylight as if it calledtzset(3). The return value points to a statically allocated struct which might be overwritten by subsequent calls to any of the date and time functions. Thelocaltime_r() function does the same, but stores the data in a user-supplied struct. It need not settzname,timezone, anddaylight. Theasctime() function converts the broken-down time valuetm into a null-terminated string with the same format asctime(). The return value points to a statically allocated string which might be overwritten by subsequent calls to any of the date and time functions. Theasctime_r() function does the same, but stores the string in a user-supplied buffer which should have room for at least 26 bytes. Themktime() 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 thetm_wday andtm_yday fields. The value specified in thetm_isdst field informsmktime() whether or not daylight saving time (DST) is in effect for the time supplied in thetm structure: a positive value means DST is in effect; zero means that DST is not in effect; and a negative value means thatmktime() should (use timezone information and system databases to) attempt to determine whether DST is in effect at the specified time. Seetimegm(3) for a UTC equivalent of this function. Themktime() function modifies the fields of thetm structure as follows:tm_wday andtm_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 variablestzname,timezone, anddaylight as if it calledtzset(3). 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.
On success,gmtime() andlocaltime() return a pointer to astruct tm. On success,gmtime_r() andlocaltime_r() return the address of the structure pointed to byresult. On success,asctime() andctime() return a pointer to a string. On success,asctime_r() andctime_r() return a pointer to the string pointed to bybuf. On success,mktime() returns the calendar time (seconds since the Epoch), expressed as a value of typetime_t. On error,mktime() returns the value(time_t) -1, and leaves thetm->tm_wday member unmodified. The remaining functions return NULL on error. On error,errno is set to indicate the error.
EOVERFLOW The result cannot be represented.
For an explanation of the terms used in this section, seeattributes(7). ┌────────────────┬───────────────┬───────────────────────────────┐ │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(), │ Thread safety │ MT-Safe env locale │ │gmtime_r(), │ │ │ │localtime_r(), │ │ │ │mktime() │ │ │ ├────────────────┼───────────────┼───────────────────────────────┤ │gmtime(), │ Thread safety │ MT-Unsafe race:tmbuf env │ │localtime() │ │ locale │ └────────────────┴───────────────┴───────────────────────────────┘
POSIX doesn't specify the parameters ofctime_r() to berestrict; that is specific to glibc. In many implementations, including glibc, a 0 intm_mday is interpreted as meaning the last day of the preceding month. According to POSIX.1,localtime() is required to behave as thoughtzset(3) was called, whilelocaltime_r() does not have this requirement. For portable code,tzset(3) should be called beforelocaltime_r().
asctime()ctime()gmtime()localtime()mktime() C23, POSIX.1-2024.gmtime_r()localtime_r() POSIX.1-2024.asctime_r()ctime_r() None.
gmtime()localtime()mktime() C89, POSIX.1-1988.asctime()ctime() C89, POSIX.1-1988. Marked obsolescent in C23 and in POSIX.1-2008 (recommendingstrftime(3)).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 (recommendingstrftime(3)).
Thread safety The four functionsasctime(),ctime(),gmtime(), andlocaltime() return a pointer to static data and hence are not thread-safe. The thread-safe versions,asctime_r(),ctime_r(),gmtime_r(), andlocaltime_r(), are specified by SUSv2. POSIX.1 says: "Theasctime(),ctime(),gmtime(), andlocaltime() functions shall return values in one of two static objects: a broken-down time structure and an array of typechar. 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 whethermktime() failed, one must use thetm->tm_wday field. See the example program in EXAMPLES. The handling of a non-negativetm_isdst inmktime() is poorly specified, and passing a value that is incorrect for the time specified yields unspecified results. Sincemktime() 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 callmktime() twice, once withtm_isdst set to zero, and once withtm_isdst set to a positive value, and discarding the results from the call that changes it. If neither call changestm_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 beyondmktime'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.
The program below defines a wrapper that allows detecting invalid and ambiguous times, withEINVALandENOTUNIQ, 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 1677668400Program source: mktime.c #include <err.h> #include <errno.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <time.h> #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; }date(1),gettimeofday(2),time(2),utime(2),clock(3),difftime(3),strftime(3),strptime(3),timegm(3),tzset(3),time(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17ctime(3)Pages that refer to this page:find(1), pcp-iostat(1), pcp-ss(1), pcp-tapestat(1), pmie(1), utmpdump(1), clock_getres(2), gettimeofday(2), time(2), difftime(3), getdate(3), pmctime(3), pmlocaltime(3), __pmmktime(3), pmnotifyerr(3), __pmparsectime(3), strftime(3), timegm(3), time_t(3type), tm(3type), tzset(3), rtc(4), localtime(5), tzfile(5), environ(7), time(7), audispd-zos-remote(8), lsof(8), zdump(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |