time.h(0P) POSIX Programmer's Manual time.h(0P)PROLOG
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NAMEtime.h — time types
SYNOPSIS
#include <time.h>
DESCRIPTION
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 2.2,
The Compilation Environment) to enable the visibility of these symbols
in this header.
The <time.h> header shall define the clock_t, size_t, time_t, types as
described in <sys/types.h>.
The <time.h> header shall define the clockid_t and timer_t types as
described in <sys/types.h>.
The <time.h> header shall define the locale_t type as described in
<locale.h>.
The <time.h> header shall define the pid_t type as described in
<sys/types.h>.
The tag sigevent shall be declared as naming an incomplete structure
type, the contents of which are described in the <signal.h> header.
The <time.h> header shall declare the tm structure, which 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.
The <time.h> header shall declare the timespec structure, which shall
include at least the following members:
time_t tv_sec Seconds.
long tv_nsec Nanoseconds.
The <time.h> 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.
The <time.h> header shall define the following macros:
NULL As described in <stddef.h>.
CLOCKS_PER_SEC
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 sys‐
tems. However, it may be variable on other systems, and
it should not be assumed that CLOCKS_PER_SEC is a com‐
pile-time constant.
The <time.h> header shall define the following symbolic constants. The
values shall have a type that is assignment-compatible with clockid_t.
CLOCK_MONOTONIC
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.
CLOCK_PROCESS_CPUTIME_ID
The identifier of the CPU-time clock associated with the
process making a clock() or timer*() function call.
CLOCK_REALTIME
The identifier of the system-wide clock measuring real
time.
CLOCK_THREAD_CPUTIME_ID
The identifier of the CPU-time clock associated with the
thread making a clock() or timer*() function call.
The <time.h> header shall define the following symbolic constant:
TIMER_ABSTIME Flag indicating time is absolute. For functions taking
timer objects, this refers to the clock associated with
the timer.
The <time.h> header shall provide a declaration or definition for get‐
date_err. The getdate_err symbol shall expand to an expression of type
int. It is unspecified whether getdate_err is 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 get‐
date_err, 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);
clock_t clock(void);
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,
timer_t *restrict);
int timer_delete(timer_t);
int timer_getoverrun(timer_t);
int timer_gettime(timer_t, struct itimerspec *);
int timer_settime(timer_t, int, const struct itimerspec *restrict,
struct itimerspec *restrict);
void tzset(void);
The <time.h> 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
<signal.h> header.
The following sections are informative.
APPLICATION USAGE
The range [0,60] for tm_sec allows for the occasional leap second.
tm_year is a signed value; therefore, years before 1900 may be repre‐
sented.
To obtain the number of clock ticks per second returned by the times()
function, applications should call sysconf(_SC_CLK_TCK).
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
<locale.h>, <signal.h>, <stddef.h>, <sys_types.h>
The System Interfaces volume of POSIX.1‐2008, Section 2.2, The Compila‐
tion Environment, asctime(), clock(), clock_getcpuclockid(),
clock_getres(), clock_nanosleep(), ctime(), difftime(), getdate(),
gmtime(), localtime(), mktime(), mq_receive(), mq_send(), nanosleep(),
pthread_getcpuclockid(), pthread_mutex_timedlock(),
pthread_rwlock_timedrdlock(), pthread_rwlock_timedwrlock(), sem_timed‐
wait(), strftime(), strptime(), sysconf(), time(), timer_create(),
timer_delete(), timer_getoverrun(), tzset(), utime()COPYRIGHT
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 Electri‐
cal 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 .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 time.h(0P)
