Getting the machine serial number and CPU ID using C/C++ in Linux

Here is what the Linux kernel seems to use:

static inline void native_cpuid(unsigned int *eax, unsigned int *ebx,
                                unsigned int *ecx, unsigned int *edx)
{
        /* ecx is often an input as well as an output. */
        asm volatile("cpuid"
            : "=a" (*eax),
              "=b" (*ebx),
              "=c" (*ecx),
              "=d" (*edx)
            : "0" (*eax), "2" (*ecx));
}

which one then can use as e.g.:

#include <stdio.h>

int main(int argc, char **argv)
{
  unsigned eax, ebx, ecx, edx;

  eax = 1; /* processor info and feature bits */
  native_cpuid(&eax, &ebx, &ecx, &edx);

  printf("stepping %d\n", eax & 0xF);
  printf("model %d\n", (eax >> 4) & 0xF);
  printf("family %d\n", (eax >> 8) & 0xF);
  printf("processor type %d\n", (eax >> 12) & 0x3);
  printf("extended model %d\n", (eax >> 16) & 0xF);
  printf("extended family %d\n", (eax >> 20) & 0xFF);

  /* EDIT */
  eax = 3; /* processor serial number */
  native_cpuid(&eax, &ebx, &ecx, &edx);

  /** see the CPUID Wikipedia article on which models return the serial 
      number in which registers. The example here is for 
      Pentium III */
  printf("serial number 0x%08x%08x\n", edx, ecx);

}

Where a good reference on how to use the CPUID instruction is in this Wikipedia article.

EDIT The Wikipedia article says that the serial number was introduced with the Pentium III but was not anymore implemented in later models due to privacy concerns. On a Linux system you can check for the presence of this feature (PSN bit) by doing:

grep -i --color psn /proc/cpuinfo

if this does not show anything, your system does not support a processor serial number.