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times.md

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Time functions

clock

#include <time.h>
clock_t clock(void);

The clock() function shall return the implementation's best
approximation to the processor time used by the process since the
beginning of an implementation-defined era related only to the
process invocation.

C89, C99, POSIX.1-2001.
clock pre seconds => / CLOCKS_PER_SEC => microseconds

arbitrary value at start of the process => t = clock_now - clock_at_start
wrap: 32bit => ~72min
may return children time, not on linux
glibc <= 2.17: implemeted using times
glibc > 2.17: implemented using clock_gettimes

times

#include <sys/times.h>
clock_t times(struct tms *buffer);

SVr4, 4.3BSD, POSIX.1-2001.

struct tms {
clock_t tms_utime;  /* user time */
clock_t tms_stime;  /* system time */
clock_t tms_cutime; /* user time of children */
clock_t tms_cstime; /* system time of children */
};

clock ticks => / sysconf(_SC_CLK_TCK)

getrusage

#include <sys/resource.h>
int getrusage(int who, struct rusage *r_usage);

SVr4, 4.3BSD.  POSIX.1-2001.
struct timeval ru_utime; /* user CPU time used */
struct timeval ru_stime; /* system CPU time used */
RUSAGE_SELF
RUSAGE_CHILDREN

Linux: RUSAGE_THREAD (since Linux 2.6.26)

clock_gettime

#include <time.h>
int clock_getres(clockid_t clk_id, struct timespec *res);
int clock_gettime(clockid_t clk_id, struct timespec *tp);

SUSv2, POSIX.1-2001.
struct timespec {
time_t   tv_sec;        /* seconds */
long     tv_nsec;       /* nanoseconds */
};
_POSIX_C_SOURCE >= 199309L

can check define _POSIX_TIMERS (<unistd.h>)

CLOCK_REALTIME (mandatory) wall-clock since Epoch
if _POSIX_MONOTONIC_CLOCK => CLOCK_MONOTONIC seconds / nanoseconds since arbitrary
if _POSIX_CPUTIME => CLOCK_PROCESS_CPUTIME_ID => CPU
if _POSIX_THREAD_CPUTIME => CLOCK_THREAD_CPUTIME_ID => Thread CPU

Linux
=> CLOCK_MONOTONIC
CLOCK_REALTIME_COARSE (since Linux 2.6.32) A faster but less precise version of CLOCK_REALTIME
CLOCK_MONOTONIC_COARSE (since Linux 2.6.32) A  faster  but less precise version of CLOCK_MONOTONIC
CLOCK_MONOTONIC_RAW (since Linux 2.6.28) Similar to CLOCK_MONOTONIC but not subject to NTP/adjtime
CLOCK_BOOTTIME (since Linux 2.6.39) Identical to CLOCK_MONOTONIC, except it also includes any time that the system is suspended
CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12) Per-process CPU-time clock
CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12) Thread-specific CPU-time clock