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All Rights Reserved. .\" Copyright (c) 2005, Sun Microsystems, Inc. All Rights Reserved. .\" .TH CTIME 3C "May 27, 2005" .SH NAME ctime, ctime_r, localtime, localtime_r, gmtime, gmtime_r, asctime, asctime_r, tzset \- convert date and time to string .SH SYNOPSIS .LP .nf #include \fBchar *\fR\fBctime\fR(\fBconst time_t *\fR\fIclock\fR); .fi .LP .nf \fBstruct tm *\fR\fBlocaltime\fR(\fBconst time_t *\fR\fIclock\fR); .fi .LP .nf \fBstruct tm *\fR\fBgmtime\fR(\fBconst time_t *\fR\fIclock\fR); .fi .LP .nf \fBchar *\fR\fBasctime\fR(\fBconst struct tm *\fR\fItm\fR); .fi .LP .nf extern time_t timezone, altzone; extern int daylight; extern char *tzname[2]; \fBvoid\fR \fBtzset\fR(\fBvoid\fR); .fi .LP .nf \fBchar *\fR\fBctime_r\fR(\fBconst time_t *\fR\fIclock\fR, \fBchar *\fR\fIbuf\fR, \fBint\fR \fIbuflen\fR); .fi .LP .nf \fBstruct tm *\fR\fBlocaltime_r\fR(\fBconst time_t *restrict\fR \fIclock\fR, \fBstruct tm *restrict\fR \fIres\fR); .fi .LP .nf \fBstruct tm *\fR\fBgmtime_r\fR(\fBconst time_t *restrict\fR \fIclock\fR, \fBstruct tm *restrict\fR \fIres\fR); .fi .LP .nf \fBchar *\fR\fBasctime_r\fR(\fBconst struct tm *restrict\fR \fItm\fR, \fBchar *restrict\fR \fIbuf\fR, \fBint\fR \fIbuflen\fR); .fi .SS "Standard conforming" .LP .nf cc [ \fIflag\fR... ] \fIfile\fR... \fB-D_POSIX_PTHREAD_SEMANTICS\fR [ \fIlibrary\fR... ] \fBchar *\fR\fBctime_r\fR(\fBconst time_t *\fR\fIclock\fR, \fBchar *\fR\fIbuf\fR); .fi .LP .nf \fBchar *\fR\fBasctime_r\fR(\fBconst struct tm *\fR\fItm\fR, \fBchar *\fR\fIbuf\fR); .fi .SH DESCRIPTION .sp .LP The \fBctime()\fR function converts the time pointed to by \fIclock\fR, representing the time in seconds since the Epoch (00:00:00 UTC, January 1, 1970), to local time in the form of a 26-character string, as shown below. Time zone and daylight savings corrections are made before string generation. The fields are in constant width: .sp .LP Fri Sep 13 00:00:00 1986\en\e0 .sp .LP The \fBctime()\fR function is equivalent to: .sp .LP \fBasctime(localtime(\fR\fIclock\fR\fB\fR\fB))\fR .sp .LP The \fBctime()\fR, \fBasctime()\fR, \fBgmtime()\fR, and \fBlocaltime()\fR functions return values in one of two thread-specific data objects: a broken-down time structure and an array of \fBchar\fR. Execution of any of the functions can overwrite the information returned in either of these objects by any of the other functions executed by the same thread. .sp .LP The \fBctime_r()\fR function has the same functionality as \fBctime()\fR except that the caller must supply a buffer \fIbuf\fR with length \fIbuflen\fR to store the result; \fIbuf\fR must be at least 26 bytes. The standard-conforming \fBctime_r()\fR function does not take a \fIbuflen\fR parameter. .sp .LP The \fBlocaltime()\fR and \fBgmtime()\fR functions return pointers to \fBtm\fR structures (see below). The \fBlocaltime()\fR function corrects for the main time zone and possible alternate ("daylight savings") time zone; the \fBgmtime()\fR function converts directly to Coordinated Universal Time (UTC), which is what the UNIX system uses internally. .sp .LP The \fBlocaltime_r()\fR and \fBgmtime_r()\fR functions have the same functionality as \fBlocaltime()\fR and \fBgmtime()\fR respectively, except that the caller must supply a buffer \fIres\fR to store the result. .sp .LP The \fBasctime()\fR function converts a \fBtm\fR structure to a 26-character string, as shown in the previous example, and returns a pointer to the string. .sp .LP The \fBasctime_r()\fR function has the same functionality as \fBasctime()\fR except that the caller must supply a buffer \fIbuf\fR with length \fIbuflen\fR for the result to be stored. The \fIbuf\fR argument must be at least 26 bytes. The standard-conforming \fBasctime_r()\fR function does not take a \fIbuflen\fR parameter. The \fBasctime_r()\fR function returns a pointer to \fIbuf\fR upon success. In case of failure, \fINULL\fR is returned and \fBerrno\fR is set. .sp .LP Declarations of all the functions and externals, and the \fBtm\fR structure, are in the <\fBtime.h\fR> header. The members of the \fBtm\fR structure are: .sp .in +2 .nf int tm_sec; /* seconds after the minute \(em [0, 60] */ /* for leap seconds */ int tm_min; /* minutes after the hour \(em [0, 59] */ int tm_hour; /* hour since midnight \(em [0, 23] */ int tm_mday; /* day of the month \(em [1, 31] */ int tm_mon; /* months since January \(em [0, 11] */ int tm_year; /* years since 1900 */ int tm_wday; /* days since Sunday \(em [0, 6] */ int tm_yday; /* days since January 1 \(em [0, 365] */ int tm_isdst; /* flag for alternate daylight savings time */ .fi .in -2 .sp .LP The value of \fBtm_isdst\fR is positive if daylight savings time is in effect, zero if daylight savings time is not in effect, and negative if the information is not available. Previously, the value of \fBtm_isdst\fR was defined as non-zero if daylight savings was in effect. .sp .LP The external \fBtime_t\fR variable \fBaltzone\fR contains the difference, in seconds, between Coordinated Universal Time and the alternate time zone. The external variable \fBtimezone\fR contains the difference, in seconds, between UTC and local standard time. The external variable \fBdaylight\fR indicates whether time should reflect daylight savings time. Both \fBtimezone\fR and \fBaltzone\fR default to 0 (UTC). The external variable \fBdaylight\fR is non-zero if an alternate time zone exists. The time zone names are contained in the external variable \fBtzname\fR, which by default is set to: .sp .LP char *tzname[2] = { "GMT", "\|" }; .sp .LP These functions know about the peculiarities of this conversion for various time periods for the \fBU.S.\fR (specifically, the years 1974, 1975, and 1987). They start handling the new daylight savings time starting with the first Sunday in April, 1987. .sp .LP The \fBtzset()\fR function uses the contents of the environment variable \fBTZ\fR to override the value of the different external variables. It is called by \fBasctime()\fR and can also be called by the user. If \fBTZ\fR is not specified or has an invalid setting, \fBtzset()\fR uses \fBGMT0\fR. See \fBenviron\fR(7) for a description of the \fBTZ\fR environment variable. .sp .LP Starting and ending times are relative to the current local time zone. If the alternate time zone start and end dates and the time are not provided, the days for the United States that year will be used and the time will be 2 AM. If the start and end dates are provided but the time is not provided, the time will be 2 AM. The effects of \fBtzset()\fR change the values of the external variables \fBtimezone\fR, \fBaltzone\fR, \fBdaylight\fR, and \fBtzname\fR. .sp .LP Note that in most installations, \fBTZ\fR is set to the correct value by default when the user logs on, using the local \fB/etc/default/init\fR file (see \fBTIMEZONE\fR(5)). .SH RETURN VALUES .sp .LP Upon successful completion, the \fBgmtime()\fR and \fBlocaltime()\fR functions return a pointer to a \fBstruct tm\fR. If an error is detected, \fBgmtime()\fR and \fBlocaltime()\fR return a null pointer. .sp .LP Upon successful completion, the \fBgmtime_r()\fR and \fBlocaltime_r()\fR functions return the address of the structure pointed to by the \fIres\fR argument. If an error is detected, \fBgmtime_r()\fR and \fBlocaltime_r()\fR return a null pointer and set \fBerrno\fR to indicate the error. .SH ERRORS .sp .LP The \fBctime_r()\fR and \fBasctime_r()\fR functions will fail if: .sp .ne 2 .na \fB\fBERANGE\fR\fR .ad .RS 10n The length of the buffer supplied by the caller is not large enough to store the result. .RE .sp .LP The \fBgmtime()\fR, \fBgmtime_r()\fR, \fBlocaltime()\fR, and \fBlocaltime_r()\fR functions will fail if: .sp .ne 2 .na \fB\fBEOVERFLOW\fR\fR .ad .RS 13n The result cannot be represented. .RE .SH USAGE .sp .LP These functions do not support localized date and time formats. The \fBstrftime\fR(3C) function can be used when localization is required. .sp .LP The \fBlocaltime()\fR, \fBlocaltime_r()\fR, \fBgmtime()\fR, \fBgmtime_r()\fR, \fBctime()\fR, and \fBctime_r()\fR functions assume Gregorian dates. Times before the adoption of the Gregorian calendar will not match historial records. .SH EXAMPLES .LP \fBExample 1 \fRExamples of the \fBtzset()\fR function. .sp .LP The \fBtzset()\fR function scans the contents of the environment variable and assigns the different fields to the respective variable. For example, the most complete setting for New Jersey in 1986 could be: .sp .in +2 .nf EST5EDT4,116/2:00:00,298/2:00:00 .fi .in -2 .sp .LP or simply .sp .in +2 .nf EST5EDT .fi .in -2 .sp .LP An example of a southern hemisphere setting such as the Cook Islands could be .sp .in +2 .nf KDT9:30KST10:00,63/5:00,302/20:00 .fi .in -2 .sp .LP In the longer version of the New Jersey example of \fBTZ,\fR \fBtzname\fR[\fB0\fR] is EST, \fBtimezone\fR is set to 5*60*60, \fBtzname\fR[\fB1\fR] is EDT, \fBaltzone\fR is set to 4*60*60, the starting date of the alternate time zone is the 117th day at 2 AM, the ending date of the alternate time zone is the 299th day at 2 AM (using zero-based Julian days), and \fBdaylight\fR is set positive. Starting and ending times are relative to the current local time zone. If the alternate time zone start and end dates and the time are not provided, the days for the United States that year will be used and the time will be 2 AM. If the start and end dates are provided but the time is not provided, the time will be 2 AM. The effects of \fBtzset()\fR are thus to change the values of the external variables \fBtimezone\fR, \fBaltzone\fR, \fBdaylight\fR, and \fBtzname\fR. The \fBctime()\fR, \fBlocaltime()\fR, \fBmktime()\fR, and \fBstrftime()\fR functions also update these external variables as if they had called \fBtzset()\fR at the time specified by the \fBtime_t\fR or \fBstruct tm\fR value that they are converting. .SH BUGS .sp .LP The \fBzoneinfo\fR timezone data files do not transition past Tue Jan 19 03:14:07 2038 UTC. Therefore for 64-bit applications using \fBzoneinfo\fR timezones, calculations beyond this date might not use the correct offset from standard time, and could return incorrect values. This affects the 64-bit version of \fBlocaltime()\fR, \fBlocaltime_r()\fR, \fBctime()\fR, and \fBctime_r()\fR. .SH ATTRIBUTES .sp .LP See \fBattributes\fR(7) for descriptions of the following attributes: .sp .sp .TS box; c | c l | l . ATTRIBUTE TYPE ATTRIBUTE VALUE _ CSI Enabled _ Interface Stability Standard _ MT-Level MT-Safe with exceptions .TE .sp .LP The \fBasctime()\fR, \fBctime()\fR, \fBgmtime()\fR, and \fBlocaltime()\fR functions are safe to use in multithread applications because they employ thread-specific data. However, their use is discouraged because standards do not require them to be thread-safe. The \fBasctime_r()\fR and \fBgmtime_r()\fR functions are MT-Safe. The \fBctime_r()\fR, \fBlocaltime_r()\fR, and \fBtzset()\fR functions are MT-Safe in multithread applications, as long as no user-defined function directly modifies one of the following variables: \fBtimezone\fR, \fBaltzone\fR, \fBdaylight\fR, and \fBtzname\fR. These four variables are not MT-Safe to access. They are modified by the \fBtzset()\fR function in an MT-Safe manner. The \fBmktime()\fR, \fBlocaltime_r()\fR, and \fBctime_r()\fR functions call \fBtzset()\fR. .SH SEE ALSO .sp .LP .BR time (2), .BR Intro (3), .BR getenv (3C), .BR mktime (3C), .BR printf (3C), .BR putenv (3C), .BR setlocale (3C), .BR strftime (3C), .BR TIMEZONE (5), .BR attributes (7), .BR environ (7), .BR standards (7) .SH NOTES .sp .LP When compiling multithreaded programs, see \fBIntro\fR(3). .sp .LP The return values for \fBasctime()\fR, \fBctime()\fR, \fBgmtime()\fR, and \fBlocaltime()\fR point to thread-specific data whose content is overwritten by each call by the same thread. .sp .LP Setting the time during the interval of change from \fBtimezone\fR to \fBaltzone\fR or vice versa can produce unpredictable results. The system administrator must change the Julian start and end days annually. .sp .LP If \fBtzset()\fR has previously evaluated the timezone identified by the value of the \fBTZ\fR environment variable, \fBtzset()\fR can reuse the previous settings of the external variables \fBaltzone\fR, \fBdaylight\fR, \fBtimezone\fR, and \fBtzname\fR[] associated with that timezone. .sp .LP Solaris 2.4 and earlier releases provided definitions of the \fBctime_r()\fR, \fBlocaltime_r()\fR, \fBgmtime_r()\fR, and \fBasctime_r()\fR functions as specified in POSIX.1c Draft 6. The final POSIX.1c standard changed the interface for \fBctime_r()\fR and \fBasctime_r()\fR. Support for the Draft 6 interface is provided for compatibility only and might not be supported in future releases. New applications and libraries should use the standard-conforming interface. .sp .LP For POSIX.1c-conforming applications, the \fB_POSIX_PTHREAD_SEMANTICS\fR and \fB_REENTRANT\fR flags are automatically turned on by defining the \fB_POSIX_C_SOURCE\fR flag with a value >= 199506L. .sp .LP In Solaris 10, \fBgmtime()\fR, \fBgmtime_r()\fR, \fBlocaltime()\fR, and \fBlocaltime_r()\fR were updated to return a null pointer if an error is detected. This change was based on the SUSv3 specification. See \fBstandards\fR(7).