Calendar time is represented by the usual GNU C library functions as an elapsed time since a fixed base calendar time. This is convenient for computation, but has no relation to the way people normally think of calendar time. By contrast, broken-down time is a binary representation of calendar time separated into year, month, day, and so on. Broken-down time values are not useful for calculations, but they are useful for printing human readable time information.
A broken-down time value is always relative to a choice of time zone, and it also indicates which time zone that is.
The symbols in this section are declared in the header file `time.h'.
int tm_sec
0
through 59
, but the actual upper limit is
60
, to allow for leap seconds if leap second support is
available).
int tm_min
0
through 59
).
int tm_hour
0
through
23
).
int tm_mday
1
through 31
).
Watch out for this one! As the only ordinal number in the structure, it is
inconsistent with the rest of the structure.
int tm_mon
0
through 11
). Watch out for this one!
People usually use ordinal numbers for month-of-year (where January = 1).
int tm_year
int tm_wday
0
through
6
).
int tm_yday
0
through 365
).
int tm_isdst
long int tm_gmtoff
-5*60*60
.
The tm_gmtoff
field is derived from BSD and is a GNU library
extension; it is not visible in a strict ISO C environment.
const char *tm_zone
tm_gmtoff
, this field is a BSD and
GNU extension, and is not visible in a strict ISO C environment.
localtime
function converts the simple time pointed to by
time to broken-down time representation, expressed relative to the
user's specified time zone.
The return value is a pointer to a static broken-down time structure, which
might be overwritten by subsequent calls to ctime
, gmtime
,
or localtime
. (But no other library function overwrites the contents
of this object.)
The return value is the null pointer if time cannot be represented
as a broken-down time; typically this is because the year cannot fit into
an int
.
Calling localtime
has one other effect: it sets the variable
tzname
with information about the current time zone. See section Functions and Variables for Time Zones.
Using the localtime
function is a big problem in multi-threaded
programs. The result is returned in a static buffer and this is used in
all threads. POSIX.1c introduced a variant of this function.
localtime_r
function works just like the localtime
function. It takes a pointer to a variable containing a simple time
and converts it to the broken-down time format.
But the result is not placed in a static buffer. Instead it is placed
in the object of type struct tm
to which the parameter
resultp points.
If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.
localtime
, except that the broken-down
time is expressed as Coordinated Universal Time (UTC) (formerly called
Greenwich Mean Time (GMT)) rather than relative to a local time zone.
As for the localtime
function we have the problem that the result
is placed in a static variable. POSIX.1c also provides a replacement for
gmtime
.
localtime_r
, except that it converts
just like gmtime
the given time as Coordinated Universal Time.
If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.
mktime
function is used to convert a broken-down time structure
to a simple time representation. It also "normalizes" the contents of
the broken-down time structure, by filling in the day of week and day of
year based on the other date and time components.
The mktime
function ignores the specified contents of the
tm_wday
and tm_yday
members of the broken-down time
structure. It uses the values of the other components to determine the
calendar time; it's permissible for these components to have
unnormalized values outside their normal ranges. The last thing that
mktime
does is adjust the components of the brokentime
structure (including the tm_wday
and tm_yday
).
If the specified broken-down time cannot be represented as a simple time,
mktime
returns a value of (time_t)(-1)
and does not modify
the contents of brokentime.
Calling mktime
also sets the variable tzname
with
information about the current time zone. See section Functions and Variables for Time Zones.
timelocal
is functionally identical to mktime
, but more
mnemonically named. Note that it is the inverse of the localtime
function.
Portability note: mktime
is essentially universally
available. timelocal
is rather rare.
timegm
is functionally identical to mktime
except it
always takes the input values to be Coordinated Universal Time (UTC)
regardless of any local time zone setting.
Note that timegm
is the inverse of gmtime
.
Portability note: mktime
is essentially universally
available. timegm
is rather rare. For the most portable
conversion from a UTC broken-down time to a simple time, set
the TZ
environment variable to UTC, call mktime
, then set
TZ
back.
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