sysdeps/unix/sysv/linux/i386/get_clockfreq.c

   1 /* Get frequency of the system processor.  i386/Linux version.
   2    Copyright (C) 2000-2014 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <ctype.h>
  20 #include <fcntl.h>
  21 #include <string.h>
  22 #include <unistd.h>
  23 #include <libc-internal.h>
  24
  25
  26 hp_timing_t
  27 __get_clockfreq (void)
  28 {
  29   /* We read the information from the /proc filesystem.  It contains at
  30      least one line like
  31         cpu MHz         : 497.840237
  32      or also
  33         cpu MHz         : 497.841
  34      We search for this line and convert the number in an integer.  */
  35   static hp_timing_t result;
  36   int fd;
  37
  38   /* If this function was called before, we know the result.  */
  39   if (result != 0)
  40     return result;
  41
  42   fd = __open ("/proc/cpuinfo", O_RDONLY);
  43   if (__glibc_likely (fd != -1))
  44     {
  45       /* XXX AFAIK the /proc filesystem can generate "files" only up
  46          to a size of 4096 bytes.  */
  47       char buf[4096];
  48       ssize_t n;
  49
  50       n = __read (fd, buf, sizeof buf);
  51       if (__builtin_expect (n, 1) > 0)
  52         {
  53           char *mhz = memmem (buf, n, "cpu MHz", 7);
  54
  55           if (__glibc_likely (mhz != NULL))
  56             {
  57               char *endp = buf + n;
  58               int seen_decpoint = 0;
  59               int ndigits = 0;
  60
  61               /* Search for the beginning of the string.  */
  62               while (mhz < endp && (*mhz < '0' || *mhz > '9') && *mhz != '\n')
  63                 ++mhz;
  64
  65               while (mhz < endp && *mhz != '\n')
  66                 {
  67                   if (*mhz >= '0' && *mhz <= '9')
  68                     {
  69                       result *= 10;
  70                       result += *mhz - '0';
  71                       if (seen_decpoint)
  72                         ++ndigits;
  73                     }
  74                   else if (*mhz == '.')
  75                     seen_decpoint = 1;
  76
  77                   ++mhz;
  78                 }
  79
  80               /* Compensate for missing digits at the end.  */
  81               while (ndigits++ < 6)
  82                 result *= 10;
  83             }
  84         }
  85
  86       __close (fd);
  87     }
  88
  89   return result;
  90 }