Clock gettime с linux

int clock_getres(clockid_t clk_id , struct timespec * res );

int clock_gettime(clockid_t clk_id , struct timespec * tp );

int clock_settime(clockid_t clk_id , const struct timespec * tp );

Feature Test Macro Requirements for glibc (see feature_test_macros (7)):

clock_getres (), clock_gettime (), clock_settime (): _POSIX_C_SOURCE >= 199309L

DESCRIPTION

The functions clock_gettime () and clock_settime () retrieve and set the time of the specified clock clk_id .

The res and tp arguments are timespec structures, as specified in :

The clk_id argument is the identifier of the particular clock on which to act. A clock may be system-wide and hence visible for all processes, or per-process if it measures time only within a single process.

All implementations support the system-wide real-time clock, which is identified by CLOCK_REALTIME . Its time represents seconds and nanoseconds since the Epoch. When its time is changed, timers for a relative interval are unaffected, but timers for an absolute point in time are affected.

More clocks may be implemented. The interpretation of the corresponding time values and the effect on timers is unspecified.

Sufficiently recent versions of glibc and the Linux kernel support the following clocks: CLOCK_REALTIME System-wide real-time clock. Setting this clock requires appropriate privileges. CLOCK_MONOTONIC Clock that cannot be set and represents monotonic time since some unspecified starting point. CLOCK_PROCESS_CPUTIME_ID High-resolution per-process timer from the CPU. CLOCK_THREAD_CPUTIME_ID Thread-specific CPU-time clock.

RETURN VALUE

ERRORS

CONFORMING TO

AVAILABILITY

NOTES

Note for SMP systems

If the CPUs in an SMP system have different clock sources then there is no way to maintain a correlation between the timer registers since each CPU will run at a slightly different frequency. If that is the case then clock_getcpuclockid(0) will return ENOENT to signify this condition. The two clocks will then only be useful if it can be ensured that a process stays on a certain CPU.

The processors in an SMP system do not start all at exactly the same time and therefore the timer registers are typically running at an offset. Some architectures include code that attempts to limit these offsets on bootup. However, the code cannot guarantee to accurately tune the offsets. Glibc contains no provisions to deal with these offsets (unlike the Linux Kernel). Typically these offsets are small and therefore the effects may be negligible in most cases.

Источник

Clock gettime с linux

int clock_getres(clockid_t clk_id, struct timespec *res);

int clock_gettime(clockid_t clk_id, struct timespec *tp);

int clock_settime(clockid_t clk_id, const struct timespec *tp);

Компонуется при указании параметра -lrt (только для glibc до версии 2.17).

Требования макроса тестирования свойств для glibc (см. feature_test_macros(7)):

clock_getres(), clock_gettime(), clock_settime():

ОПИСАНИЕ

Функции clock_gettime() и clock_settime() получают и устанавливают время указанных часов clk_id.

Аргументы res и tp представляют структуру timespec, определённую в :

Аргумент clk_id представляет идентификатор определённых часов, над которыми производится действие. Часы могут использоваться системой в целом и поэтому видимы всем процессам или конкретного процессу, если они отсчитывают время только в пределах одного процесса.

Все реализации поддерживают системные часы реального времени, которые имеют идентификатор CLOCK_REALTIME. Их время представляется в секундах и наносекундах с начала Эпохи. Когда их время изменяется, на таймеры с относительными интервалами это никак не влияет, но таймеры с абсолютной точкой во времени учитывают это.

Может быть реализовано много часов. Представление соответствующих значений времени и влияние на таймеры не определено.

В последних версиях glibc и ядра Linux поддерживаются следующие часы:

CLOCK_REALTIME Часы системы, отсчитывающие реальное (т. е., бытовое) время. Для настройки этих часов требуются соответствующие права. Данные часы подвержены скачкам системного времени (например, если системный администратор вручную изменяет время) и постепенной подгонке, выполняемой adjtime(3) и NTP. CLOCK_REALTIME_COARSE (начиная с Linux 2.6.32; есть только в Linux) Более быстрая, но менее точная версия CLOCK_REALTIME. Используйте, если нужны не очень точные метки времени, но быстро. CLOCK_MONOTONIC Часы, которые не могут быть настроены и показывают монотонный ход времени отсчитываемой с некой неопределённой начальной точки. Эти часы не подвержены скачкам системного времени (например, системный администратор вручную изменил время), но на них влияет постепенная подгонка, выполняемая adjtime(3) и NTP. CLOCK_MONOTONIC_COARSE (начиная с Linux 2.6.32; есть только в Linux) Более быстрая, но менее точная версия CLOCK_MONOTONIC. Используйте, если нужны не очень точные метки времени, но быстро. CLOCK_MONOTONIC_RAW (начиная с Linux 2.6.28; есть только в Linux) Похожи на CLOCK_MONOTONIC, но предоставляют прямой доступ к аппаратным часам, которые не подводятся NTP или постепенной подгонкой, выполняемой adjtime(3). CLOCK_BOOTTIME (начиная с Linux 2.6.39; есть только в Linux) Идентичны CLOCK_MONOTONIC, но также содержат любое время, на которое система была приостановлена (suspended). Это позволяет приложениям получить учитывающие приостановку монотонные часы без обращения к сложностям CLOCK_REALTIME, которые могут быть неоднородны, если время изменили с помощью settimeofday(2). CLOCK_PROCESS_CPUTIME_ID (начиная с Linux 2.6.12) Настраиваемые для каждого процесса часы ЦП (измеряют время ЦП, затраченное всеми нитями процесса). CLOCK_THREAD_CPUTIME_ID (начиная с Linux 2.6.12) Часы, работающие на ЦП, для каждой нити.

Источник

Описание функций языка Си

All | _ | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

clock_gettime – определение текущего времени с высокой точностью.

#include
int clock_gettime(clockid_t clock_id, struct timespec *t_time);

clock_id – идентификатор часов.
t_time – структура, в которую будет записано определенное время.

0 — при успешном завершении функции.

-1 — если обнаружена ошибка, при этом переменной errno присваивается код ошибки [EINVAL] — аргумент clock_id не является идентификатором часов.

Функция clock_gettime() записывает текущие показания часов, определенных идентификатором clock_id в структуру, на которую указывает t_time.

Система всегда содержит часы с идентификатором CLOCK_REALTIME (системные часы). Значение этих часов содержит время в секундах и наносекундах, истекшее с 0 часов 1 января 1970 года.

При наличии соответствующего оборудования при конфигурации системы могут быть созданы дополнительные часы.

Структура timespec t_time содержит два элемента: time_t tv_sec – секунды, long tv_nsec – наносекунды (время истекшее с последней полной секунды).

Точность времени, записанного в переменную tv_nsec, определяется разрешающей способностью часов, и может отличаться от наносекунд. Узнать разрешающую способность часов можно с помощью функции clock_getres.

Для компиляции программы использующей данную функцию под ОС типа linux, требуется ключ – lrt. Например, для компиляции файла test.c c созданием объектного файла ts.o необходимо в консоле перейти в папку с файлом test.c и набрать команду: gcc test.c -o ts — lrt

В примере определяется текущее время, затем значение секунд и наносекунд выводится в консоль. После вывода снова определяется время и рассчитывается разница между двумя засечками времени в наносекундах (время, затраченное на вывод в консоль) и результат выводится на консоль.

Секунды: 1400251717
Наносекунды: 83548887
Затрачено время: 83082 нс

Источник

clock_gettime(3) — Linux man page

clock_getres, clock_gettime, clock_settime — clock and time functions

Synopsis

int clock_getres(clockid_t clk_id, struct timespec *res);
int clock_gettime(clockid_t clk_id, struct timespec *tp);
int clock_settime(clockid_t clk_id, const struct timespec *tp);

Description

The function clock_getres() finds the resolution (precision) of the specified clock clk_id, and, if res is non-NULL, stores it in the struct timespec pointed to by res. The resolution of clocks depends on the implementation and cannot be configured by a particular process. If the time value pointed to by the argument tp of clock_settime() is not a multiple of res, then it is truncated to a multiple of res.

The functions clock_gettime() and clock_settime() retrieve and set the time of the specified clock clk_id.

The res and tp arguments are timespec structs, as specified in : The clk_id argument is the identifier of the particular clock on which to act. A clock may be system-wide and hence visible for all processes, or per-process if it measures time only within a single process.

All implementations support the system-wide realtime clock, which is identified by CLOCK_REALTIME. Its time represents seconds and nanoseconds since the Epoch. When its time is changed, timers for a relative interval are unaffected, but timers for an absolute point in time are affected.

More clocks may be implemented. The interpretation of the corresponding time values and the effect on timers is unspecified.

Sufficiently recent versions of GNU libc and the Linux kernel support the following clocks: CLOCK_REALTIME System-wide realtime clock. Setting this clock requires appropriate privileges. CLOCK_MONOTONIC Clock that cannot be set and represents monotonic time since some unspecified starting point. CLOCK_PROCESS_CPUTIME_ID High-resolution per-process timer from the CPU. CLOCK_THREAD_CPUTIME_ID Thread-specific CPU-time clock.

Return Value

clock_gettime(), clock_settime() and clock_getres() return 0 for success, or -1 for failure (in which case errno is set appropriately).

Errors

tp points outside the accessible address space.

The clk_id specified is not supported on this system.

clock_settime() does not have permission to set the clock indicated.

Most systems require the program be linked with the librt library to use these functions.

NOTE for SMP systems

The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks are realized on many platforms using timers from the CPUs (TSC on i386, AR.ITC on Itanium). These registers may differ between CPUs and as a consequence these clocks may return bogus results if a process is migrated to another CPU.

If the CPUs in an SMP system have different clock sources then there is no way to maintain a correlation between the timer registers since each CPU will run at a slightly different frequency. If that is the case then clock_getcpuclockid(0) will return ENOENT to signify this condition. The two clocks will then only be useful if it can be ensured that a process stays on a certain CPU.

The processors in an SMP system do not start all at exactly the same time and therefore the timer registers are typically running at an offset. Some architectures include code that attempts to limit these offsets on bootup. However, the code cannot guarantee to accurately tune the offsets. Glibc contains no provisions to deal with these offsets (unlike the Linux Kernel). Typically these offsets are small and therefore the effects may be negligible in most cases.

Availability

On POSIX systems on which these functions are available, the symbol _POSIX_TIMERS is defined in to a value greater than 0. The symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID, CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)

Источник

clock_gettime(2) — Linux man page

clock_getres, clock_gettime, clock_settime — clock and time functions

Synopsis

int clock_getres(clockid_t clk_id, struct timespec *res);

int clock_gettime(clockid_t clk_id, struct timespec *tp);

int clock_settime(clockid_t clk_id, const struct timespec *tp);

Link with -lrt (only for glibc versions before 2.17).

Feature Test Macro Requirements for glibc (see feature_test_macros(7)): clock_getres(), clock_gettime(), clock_settime(): _POSIX_C_SOURCE >= 199309L

Description

The function clock_getres() finds the resolution (precision) of the specified clock clk_id, and, if res is non-NULL, stores it in the struct timespec pointed to by res. The resolution of clocks depends on the implementation and cannot be configured by a particular process. If the time value pointed to by the argument tp of clock_settime() is not a multiple of res, then it is truncated to a multiple of res.

The functions clock_gettime() and clock_settime() retrieve and set the time of the specified clock clk_id.

The res and tp arguments are timespec structures, as specified in : The clk_id argument is the identifier of the particular clock on which to act. A clock may be system-wide and hence visible for all processes, or per-process if it measures time only within a single process.

All implementations support the system-wide real-time clock, which is identified by CLOCK_REALTIME. Its time represents seconds and nanoseconds since the Epoch. When its time is changed, timers for a relative interval are unaffected, but timers for an absolute point in time are affected.

More clocks may be implemented. The interpretation of the corresponding time values and the effect on timers is unspecified.

Sufficiently recent versions of glibc and the Linux kernel support the following clocks: CLOCK_REALTIME System-wide clock that measures real (i.e., wall-clock) time. Setting this clock requires appropriate privileges. This clock is affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), and by the incremental adjustments performed by adjtime(3) and NTP. CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific) A faster but less precise version of CLOCK_REALTIME. Use when you need very fast, but not fine-grained timestamps.

CLOCK_MONOTONIC Clock that cannot be set and represents monotonic time since some unspecified starting point. This clock is not affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), but is affected by the incremental adjustments performed by adjtime(3) and NTP. CLOCK_MONOTONIC_COARSE (since Linux 2.6.32; Linux-specific) A faster but less precise version of CLOCK_MONOTONIC. Use when you need very fast, but not fine-grained timestamps. CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific) Similar to CLOCK_MONOTONIC, but provides access to a raw hardware-based time that is not subject to NTP adjustments or the incremental adjustments performed by adjtime(3). CLOCK_BOOTTIME (since Linux 2.6.39; Linux-specific) Identical to CLOCK_MONOTONIC, except it also includes any time that the system is suspended. This allows applications to get a suspend-aware monotonic clock without having to deal with the complications of CLOCK_REALTIME, which may have discontinuities if the time is changed using settimeofday(2). CLOCK_PROCESS_CPUTIME_ID High-resolution per-process timer from the CPU. CLOCK_THREAD_CPUTIME_ID Thread-specific CPU-time clock.

Return Value

clock_gettime(), clock_settime() and clock_getres() return 0 for success, or -1 for failure (in which case errno is set appropriately).

Errors

tp points outside the accessible address space.

The clk_id specified is not supported on this system.

clock_settime() does not have permission to set the clock indicated.

Conforming To

Availability

On POSIX systems on which these functions are available, the symbol _POSIX_TIMERS is defined in to a value greater than 0. The symbols _POSIX_MONOTONIC_CLOCK, _POSIX_CPUTIME, _POSIX_THREAD_CPUTIME indicate that CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID, CLOCK_THREAD_CPUTIME_ID are available. (See also sysconf(3).)

Notes

Note for SMP systems The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks are realized on many platforms using timers from the CPUs (TSC on i386, AR.ITC on Itanium). These registers may differ between CPUs and as a consequence these clocks may return bogus results if a process is migrated to another CPU.

If the CPUs in an SMP system have different clock sources then there is no way to maintain a correlation between the timer registers since each CPU will run at a slightly different frequency. If that is the case then clock_getcpuclockid(0) will return ENOENT to signify this condition. The two clocks will then only be useful if it can be ensured that a process stays on a certain CPU.

The processors in an SMP system do not start all at exactly the same time and therefore the timer registers are typically running at an offset. Some architectures include code that attempts to limit these offsets on bootup. However, the code cannot guarantee to accurately tune the offsets. Glibc contains no provisions to deal with these offsets (unlike the Linux Kernel). Typically these offsets are small and therefore the effects may be negligible in most cases.

Источник