glibc && clock_gettime.c

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/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 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 of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 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 variable
because some jokers are already playing with processors with more
than 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_AVAIL
case 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 this
code against multiple execution since all of them should
lead 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 until
we get machines with about 16GHz frequency.  */
tp->tv_nsec = ((tsc % freq) * UINT64_C (1000000000)) / freq;

retval = 0;
}
break;
#endif

default:
__set_errno (EINVAL);
break;
}

return retval;
}