2.11BSD/man/cat3/asctime.0
CTIME(3) UNIX Programmer's Manual CTIME(3)
NAME
ctime, localtime, gmtime, asctime, timezone, tzset - con-
vert date and time to ASCII
SYNOPSIS
void tzset()
char *ctime(clock)
time_t *clock;
#include <time.h>
char *asctime(tm)
struct tm *tm;
struct tm *localtime(clock)
time_t *clock;
struct tm *gmtime(clock)
time_t *clock;
char *timezone(zone, dst)
DESCRIPTION
_T_z_s_e_t uses the value of the environment variable TZ to set
up the time conversion information used by _l_o_c_a_l_t_i_m_e.
If TZ does not appear in the environment, the TZDEFAULT file
(as defined in _t_z_f_i_l_e._h) is used by _l_o_c_a_l_t_i_m_e. If this file
fails for any reason, the GMT offset as provided by the ker-
nel is used. In this case, DST is ignored, resulting in the
time being incorrect by some amount if DST is currently in
effect. If this fails for any reason, GMT is used.
If TZ appears in the environment but its value is a null
string, Greenwich Mean Time is used; if TZ appears and
begins with a slash, it is used as the absolute pathname of
the _t_z_f_i_l_e(5)-format file from which to read the time
conversion information; if TZ appears and begins with a
character other than a slash, it's used as a pathname rela-
tive to the system time conversion information directory,
defined as TZDIR in the include file _t_z_f_i_l_e._h. If this file
fails for any reason, GMT is used.
Programs that always wish to use local wall clock time
should explicitly remove the environmental variable TZ with
_u_n_s_e_t_e_n_v(3).
_C_t_i_m_e converts a long integer, pointed to by _c_l_o_c_k, such as
returned by _t_i_m_e(2) into ASCII and returns a pointer to a
26-character string in the following form. All the fields
have constant width.
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CTIME(3) UNIX Programmer's Manual CTIME(3)
Sun Sep 16 01:03:52 1973\n
_L_o_c_a_l_t_i_m_e and _g_m_t_i_m_e return pointers to structures contain-
ing the broken-down time. _L_o_c_a_l_t_i_m_e corrects for the time
zone and possible daylight savings time; _g_m_t_i_m_e converts
directly to GMT, which is the time UNIX uses. _A_s_c_t_i_m_e con-
verts a broken-down time to ASCII and returns a pointer to a
26-character string.
The structure declaration from the include file is:
struct tm {
int tm_sec; /* 0-59 seconds */
int tm_min; /* 0-59 minutes */
int tm_hour; /* 0-23 hour */
int tm_mday; /* 1-31 day of month */
int tm_mon; /* 0-11 month */
int tm_year; /* 0- year - 1900 */
int tm_wday; /* 0-6 day of week (Sunday = 0) */
int tm_yday; /* 0-365 day of year */
int tm_isdst; /* flag: daylight savings time in effect */
char **tm_zone; /* abbreviation of timezone name */
long tm_gmtoff; /* offset from GMT in seconds */
};
_T_m__i_s_d_s_t is non-zero if a time zone adjustment such as Day-
light Savings time is in effect.
_T_m__g_m_t_o_f_f is the offset (in seconds) of the time represented
from GMT, with positive values indicating East of Greenwich.
_T_i_m_e_z_o_n_e remains for compatibility reasons only; it's impos-
sible to reliably map timezone's arguments (_z_o_n_e, a "minutes
west of GMT" value and _d_s_t, a "daylight saving time in
effect" flag) to a time zone abbreviation.
If the environmental string _T_Z_N_A_M_E exists, _t_i_m_e_z_o_n_e returns
its value, unless it consists of two comma separated
strings, in which case the second string is returned if _d_s_t
is non-zero, else the first string. If _T_Z_N_A_M_E doesn't
exist, _z_o_n_e is checked for equality with a built-in table of
values, in which case _t_i_m_e_z_o_n_e returns the time zone or day-
light time zone abbreviation associated with that value. If
the requested _z_o_n_e does not appear in the table, the differ-
ence from GMT is returned; e.g. in Afghanistan, _t_i_m_e_z_o_n_e(-
(_6_0*_4+_3_0), _0) is appropriate because it is 4:30 ahead of
GMT, and the string GMT+4:30 is returned. Programs that in
the past used the _t_i_m_e_z_o_n_e function should return the zone
name as set by _l_o_c_a_l_t_i_m_e to assure correctness.
FILES
/usr/share/zoneinfotime zone information directory
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CTIME(3) UNIX Programmer's Manual CTIME(3)
/etc/localtime local time zone file
SEE ALSO
gettimeofday(2), getenv(3), time(3), tzfile(5), environ(7)
NOTE
The return values point to static data whose content is
overwritten by each call. The tm_zone field of a returned
struct tm points to a static array of characters, which will
also be overwritten at the next call (and by calls to
_t_z_s_e_t).
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