clock_gettime.c由glibc提供，在linux下的gcc及几乎所有的编译器或交叉编译器都会包含glibc的库。所以check_gettime函数可以在C/C++程序中调用，在用编译器编译的时候通常需要追加-lrt选项，因为check_gettime函数在librt.a库中实现。
关于glibc：
glibc是gnu发布的libc库，即c运行库。glibc是linux系统中最底层的api，几乎其它任何运行库都会依赖于glibc。
/** Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.This file is part of the GNU C Library.The GNU C Library is free software; you can redistribute it and/ormodify it under the terms of the GNU Lesser General PublicLicense as published by the Free Software Foundation; eitherversion 2.1 of the License, or (at your option) any later version.The GNU C Library is distributed in the hope that it will be useful,but WITHOUT ANY WARRANTY; without even the implied warranty ofMERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNULesser General Public License for more details.You should have received a copy of the GNU Lesser General PublicLicense along with the GNU C Library; if not, write to the FreeSoftware Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA02111-1307 USA. */#include <errno.h>
#include <stdint.h>
#include <time.h>
#include <sys/time.h>
#include <libc-internal.h>
#include <hp-timing.h>#if HP_TIMING_AVAIL
/** Clock frequency of the processor. We make it a 64-bit variablebecause some jokers are already playing with processors with morethan 4GHz. */
static hp_timing_t freq;/** We need the starting time for the process. */
extern hp_timing_t _dl_cpuclock_offset;/** This function is defined in the thread library. */
extern int __pthread_clock_gettime (hp_timing_t freq, struct timespec *tp)__attribute__ ((__weak__));
#endif/** Get current value of CLOCK and store it in TP. */
int
clock_gettime (clockid_t clock_id, struct timespec *tp)
{struct timeval tv;int retval = -1;switch (clock_id){case CLOCK_REALTIME:retval = gettimeofday (&tv, NULL);if (retval == 0)/** Convert into `timespec'. */TIMEVAL_TO_TIMESPEC (&tv, tp);break;#if HP_TIMING_AVAILcase CLOCK_PROCESS_CPUTIME_ID:case CLOCK_THREAD_CPUTIME_ID:{hp_timing_t tsc;if (__builtin_expect (freq == 0, 0)){/** This can only happen if we haven't initialized the `freq'variable yet. Do this now. We don't have to protect thiscode against multiple execution since all of them shouldlead to the same result. */freq = __get_clockfreq ();if (__builtin_expect (freq == 0, 0))/** Something went wrong. */break;}if (clock_id == CLOCK_THREAD_CPUTIME_ID&& __pthread_clock_gettime != NULL){retval = __pthread_clock_gettime (freq, tp);break;}/** Get the current counter. */HP_TIMING_NOW (tsc);/** Compute the offset since the start time of the process. */tsc -= _dl_cpuclock_offset;/** Compute the seconds. */tp->tv_sec = tsc / freq;/** And the nanoseconds. This computation should be stable untilwe get machines with about 16GHz frequency. */tp->tv_nsec = ((tsc % freq) * UINT64_C (1000000000)) / freq;retval = 0;}break;
#endifdefault:__set_errno (EINVAL);break;}return retval;
}