This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding Linux
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implemented on Linux.
time.h — time types
Some of the functionality described on this reference page extends the
ISO C standard. Applications shall define the appropriate feature test
macro (see the System Interfaces volume of POSIX.1‐2008, Section
, The Compilation Environment
) to enable the visibility of these
symbols in this header.
header shall define the clock_t
, types as described in <sys/types.h>
header shall define the clockid_t
types as described in <sys/types.h>
header shall define the locale_t
described in <locale.h>
header shall define the pid_t
type as described
The tag sigevent
shall be declared as naming an incomplete structure
type, the contents of which are described in the <signal.h>
header shall declare the tm
shall include at least the following members:
int tm_sec Seconds [0,60].
int tm_min Minutes [0,59].
int tm_hour Hour [0,23].
int tm_mday Day of month [1,31].
int tm_mon Month of year [0,11].
int tm_year Years since 1900.
int tm_wday Day of week [0,6] (Sunday =0).
int tm_yday Day of year [0,365].
int tm_isdst Daylight Savings flag.
The value of tm_isdst
shall be positive if Daylight Savings Time is in
effect, 0 if Daylight Savings Time is not in effect, and negative if the
information is not available.
header shall declare the timespec
which shall include at least the following members:
time_t tv_sec Seconds.
long tv_nsec Nanoseconds.
header shall also declare the itimerspec
structure, which shall include at least the following members:
struct timespec it_interval Timer period.
struct timespec it_value Timer expiration.
header shall define the following macros:
- As described in <stddef.h>.
- A number used to convert the value returned by the
clock() function into seconds. The value shall be an expression
with type clock_t. The value of CLOCKS_PER_SEC shall be 1 million
on XSI-conformant systems. However, it may be variable on other systems,
and it should not be assumed that CLOCKS_PER_SEC is a compile-time
header shall define the following symbolic constants.
The values shall have a type that is assignment-compatible with
The identifier for the system-wide monotonic clock, which is defined as a
clock measuring real time, whose value cannot be set via
clock_settime() and which cannot have negative clock jumps. The
maximum possible clock jump shall be implementation-defined.
The identifier of the CPU-time clock associated with the process making a
clock() or timer*() function call.
- The identifier of the system-wide clock measuring real
The identifier of the CPU-time clock associated with the thread making a
clock() or timer*() function call.
header shall define the following symbolic constant:
- Flag indicating time is absolute. For functions taking
timer objects, this refers to the clock associated with the timer.
header shall provide a declaration or definition for
. The getdate_err
symbol shall expand to an
expression of type int
. It is unspecified whether getdate_err
a macro or an identifier declared with external linkage, and whether or not it
is a modifiable lvalue. If a macro definition is suppressed in order to access
an actual object, or a program defines an identifier with the name
, the behavior is undefined.
The following shall be declared as functions and may also be defined as macros.
Function prototypes shall be provided.
char *asctime(const struct tm *);
char *asctime_r(const struct tm *restrict, char *restrict);
int clock_getcpuclockid(pid_t, clockid_t *);
int clock_getres(clockid_t, struct timespec *);
int clock_gettime(clockid_t, struct timespec *);
int clock_nanosleep(clockid_t, int, const struct timespec *,
struct timespec *);
int clock_settime(clockid_t, const struct timespec *);
char *ctime(const time_t *);
char *ctime_r(const time_t *, char *);
double difftime(time_t, time_t);
struct tm *getdate(const char *);
struct tm *gmtime(const time_t *);
struct tm *gmtime_r(const time_t *restrict, struct tm *restrict);
struct tm *localtime(const time_t *);
struct tm *localtime_r(const time_t *restrict, struct tm *restrict);
time_t mktime(struct tm *);
int nanosleep(const struct timespec *, struct timespec *);
size_t strftime(char *restrict, size_t, const char *restrict,
const struct tm *restrict);
size_t strftime_l(char *restrict, size_t, const char *restrict,
const struct tm *restrict, locale_t);
char *strptime(const char *restrict, const char *restrict,
struct tm *restrict);
time_t time(time_t *);
int timer_create(clockid_t, struct sigevent *restrict,
int timer_gettime(timer_t, struct itimerspec *);
int timer_settime(timer_t, int, const struct itimerspec *restrict,
struct itimerspec *restrict);
header shall declare the following as variables:
extern int daylight;
extern long timezone;
extern char *tzname;
Inclusion of the <time.h>
header may make visible all symbols from
The following sections are informative.
The range [0,60] for tm_sec
allows for the occasional leap second.
is a signed value; therefore, years before 1900 may be
To obtain the number of clock ticks per second returned by the times
function, applications should call sysconf
The range [0,60] seconds allows for positive or negative leap seconds. The
formal definition of UTC does not permit double leap seconds, so all mention
of double leap seconds has been removed, and the range shortened from the
former [0,61] seconds seen in earlier versions of this standard.
The System Interfaces volume of POSIX.1‐2008, Section 2.2
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
7, Copyright (C) 2013 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013
Technical Corrigendum 1 applied.) In the event of any discrepancy between this
version and the original IEEE and The Open Group Standard, the original IEEE
and The Open Group Standard is the referee document. The original Standard can
be obtained online at http://www.unix.org/online.html .
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