How do I retrieve the number of processors on C / Linux?

I am writing a small C application that use some threads for processing data. I want to be able to know the number of processors on a certain machine, without using system() & in combination to a small script.

The only way i can think of is to开发者_开发技巧 parse /proc/cpuinfo. Any other useful suggestions ?

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As others have mentioned in comments, this answer is useful:

numCPU = sysconf( _SC_NPROCESSORS_ONLN );

Leaving as a solution for folks that might skip over comments...

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Why not use sys/sysinfo.h?

#include <sys/sysinfo.h>
#include <stdio.h>
void main () {
   printf ("You have %d processors.\n", get_nprocs ());
}

Way more information can be found on the man page

$ man 3 get_nprocs

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machine:/sys/devices/system/cpu$ ls
cpu0  cpu3  cpu6     kernel_max  perf_counters  sched_mc_power_savings
cpu1  cpu4  cpu7     offline     possible
cpu2  cpu5  cpuidle  online      present

If you have a machine with sysfs, take a look in /sys/devices/system/cpu.

Make sure you're asking for what you want -- CPUs, cores, hyperthreads, etc.

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The following was the code that I used to figure number of cores.....it might help you

//Finding the number of cores(logical processor) using cpuid instruction.....
    __asm
    {
        mov eax,01h //01h is for getting number of cores present in the processor
        cpuid
        mov t,ebx
    }

(t>>16)&0xff contains the number cores........

I guess this could help you http://lists.gnu.org/archive/html/autoconf/2002-08/msg00126.html

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#include <stdio.h>

void getPSN(char *PSN)
{int varEAX, varEBX, varECX, varEDX;
   char str[9];
   //%eax=1 gives most significant 32 bits in eax 
   __asm__ __volatile__ ("cpuid": "=a" (varEAX), "=b" (varEBX), "=c" (varECX), "=d" (varEDX) : "a" (1));
   sprintf(str, "%08X", varEAX); //i.e. XXXX-XXXX-xxxx-xxxx-xxxx-xxxx
   sprintf(PSN, "%C%C%C%C-%C%C%C%C", str[0], str[1], str[2], str[3], str[4], str[5], str[6], str[7]);
   //%eax=3 gives least significant 64 bits in edx and ecx [if PN is enabled]
   __asm__ __volatile__ ("cpuid": "=a" (varEAX), "=b" (varEBX), "=c" (varECX), "=d" (varEDX) : "a" (3));
   sprintf(str, "%08X", varEDX); //i.e. xxxx-xxxx-XXXX-XXXX-xxxx-xxxx
   sprintf(PSN, "%s-%C%C%C%C-%C%C%C%C", PSN, str[0], str[1], str[2], str[3], str[4], str[5], str[6], str[7]);
   sprintf(str, "%08X", varECX); //i.e. xxxx-xxxx-xxxx-xxxx-XXXX-XXXX
   sprintf(PSN, "%s-%C%C%C%C-%C%C%C%C", PSN, str[0], str[1], str[2], str[3], str[4], str[5], str[6], str[7]);
}

int main()
{
    char PSN[30]; //24 Hex digits, 5 '-' separators, and a '\0'
    getPSN(PSN);
    printf("%s\n", PSN); //compare with: lshw | grep serial:
    return 0;
}

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Here's a minimal example of how to get physical cores and virtual threads:

#include <stdio.h>

...

unsigned int thread_count, core_count;
FILE *cpu_info = fopen("/proc/cpuinfo", "r");
while (!fscanf(cpu_info, "siblings\t: %u", &thread_count))
  fscanf(cpu_info, "%*[^s]");
while (!fscanf(cpu_info, "cpu cores\t: %u", &core_count))                   
  fscanf(cpu_info, "%*[^c]");                                                                                          
fclose(cpu_info);

It's more portable than _SC_NPROCESSORS_ONLN as it doesn't require glibc extensions.

Note that you don't need to check for EOF in this example as fscanf will return EOF if reached. This will cause the loop to stop safely.

Also, this example doesn't contain error checking to see if fopen failed. This should be done however you see fit.

This fscanf technique was derived from here: https://stackoverflow.com/a/43483850