7.1 Extracting Information from /proc

Most of the entries in /proc provide information formatted to be readable by humans, but the formats are simple enough to be easily parsed. For example, /proc/cpuinfo contains information about the system CPU (or CPUs, for a multiprocessor machine). The output is a table of values, one per line, with a description of the value and a colon preceding each value.

For example, the output might look like this:

% cat /proc/cpuinfo 

processor       : 0 

vendor_id       : GenuineIntel 

cpu family      : 6 

model           : 5 

model name      : Pentium II (Deschutes) 

stepping        : 2 

cpu MHz         : 400.913520 

cache size      : 512 KB 

fdiv_bug        : no 

hlt_bug         : no 

sep_bug         : no 

f00f_bug        : no 

coma_bug        : no 

fpu             : yes 

fpu_exception   : yes 

cpuid level     : 2 

wp              : yes 

flags           : fpu vme de pse tsc msr pae mce cx8 apic sep 

mtrr pge mca cmov pat pse36 mmx fxsr 

bogomips        : 399.77 

We'll describe the interpretation of some of these fields in Section 7.3.1, "CPU Information."

A simple way to extract a value from this output is to read the file into a buffer and parse it in memory using sscanf. Listing 7.1 shows an example of this. The program includes the function get_cpu_clock_speed that reads from /proc/cpuinfo into memory and extracts the first CPU's clock speed.

Listing 7.1 (clock-speed.c) Extract CPU Clock Speed from /proc/cpuinfo

#include <stdio.h> 

#include <string.h> 

/* Returns the clock speed of the system's CPU in MHz, as reported by 

   /proc/cpuinfo. On a multiprocessor machine, returns the speed of 

   the first CPU. On error returns zero.  */ 

float get_cpu_clock_speed () 

{

   FILE* fp; 

   char buffer[1024]; 

   size_t bytes_read; 

   char* match; 

   float clock_speed; 

   /* Read the entire contents of /proc/cpuinfo into the buffer.  */ 

   fp = fopen ("/proc/cpuinfo", "r"); 

   bytes_read = fread (buffer, 1, sizeof (buffer), fp); 

   fclose (fp); 

   /* Bail if read failed or if buffer isn't big enough.  */ 

   if (bytes_read == 0 || bytes_read == sizeof (buffer)) 

     return 0; 

   /* NUL-terminate the text.  */ 

   buffer[bytes_read] == '\0'; 

   /* Locate the line that starts with "cpu MHz".  */ 

   match = strstr (buffer, "cpu MHz"); 

   if (match == NULL) 

     return 0; 

   /* Parse the line to extract the clock speed.  */ 

   sscanf (match, "cpu MHz  :  %f", &clock_speed); 

   return clock_speed; 

} 

 int main () 

{

   printf ("CPU clock speed: %4.0f MHz\n", get_cpu_clock_speed ()); 

   return 0; 

} 

Be aware, however, that the names, semantics, and output formats of entries in the /proc file system might change in new Linux kernel revisions. If you use them in a program, you should make sure that the program's behavior degrades gracefully if the /proc entry is missing or is formatted unexpectedly.