get_clockfreq.c

Glibc 2.3.2源代码(解压后有100多M)

/* Get frequency of the system processor.  IA-64/Linux version.
   Copyright (C) 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 <ctype.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>
#include <libc-internal.h>


hp_timing_t
__get_clockfreq (void)
{
  /* We read the information from the /proc filesystem.  It contains at
     least one line like
	itc MHz    : 733.390988
     We search for this line and convert the number in an integer.  */
  static hp_timing_t result;
  int fd;

  /* If this function was called before, we know the result.  */
  if (result != 0)
    return result;

  fd = open ("/proc/cpuinfo", O_RDONLY);
  if (__builtin_expect (fd != -1, 1))
    {
      /* XXX AFAIK the /proc filesystem can generate "files" only up
         to a size of 4096 bytes.  */
      char buf[4096];
      ssize_t n;

      n = read (fd, buf, sizeof buf);
      if (__builtin_expect (n, 1) > 0)
	{
	  char *mhz = memmem (buf, n, "itc MHz", 7);

	  if (__builtin_expect (mhz != NULL, 1))
	    {
	      char *endp = buf + n;
	      int seen_decpoint = 0;
	      int ndigits = 0;

	      /* Search for the beginning of the string.  */
	      while (mhz < endp && (*mhz < '0' || *mhz > '9') && *mhz != '\n')
		++mhz;

	      while (mhz < endp && *mhz != '\n')
		{
		  if (*mhz >= '0' && *mhz <= '9')
		    {
		      result *= 10;
		      result += *mhz - '0';
		      if (seen_decpoint)
			++ndigits;
		    }
		  else if (*mhz == '.')
		    seen_decpoint = 1;

		  ++mhz;
		}

	      /* Compensate for missing digits at the end.  */
	      while (ndigits++ < 6)
		result *= 10;
	    }
	}

      close (fd);
    }

  return result;
}