This manual page is part of the POSIX Programmer’s Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be 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.1-2017, 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_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:
long tv_nsec Nanoseconds.
The <time.h> header shall also declare the itimerspec structure, which shall include at least the following members:
it_interval Timer period.
struct timespec it_value Timer expiration.
The <time.h> 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 constant.
<time.h> header shall define the following
symbolic constants. The values shall have a type that is
assignment-compatible with clockid_t.
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 time.
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:
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 getdate_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 getdate_err, the behavior is undefined.
The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.
*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);
The <time.h> header shall declare the following as variables:
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.
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 represented.
To obtain the number of clock ticks per second returned by the times() function, applications should call sysconf(_SC_CLK_TCK).
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.
<locale.h>, <signal.h>, <stddef.h>, <sys_types.h>
The System Interfaces volume of POSIX.1-2017, Section 2.2, The Compilation 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_timedwait(), strftime(), strptime(), sysconf(), time(), timer_create(), timer_delete(), timer_getoverrun(), tzset(), utime()
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .
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