How to check what core your code is running on.

gistfile1.c

// The x86 cpuid instruction gives information about the processor
static inline void cpuid(uint32_t code, uint32_t data[4]) {
  asm volatile("cpuid":"=a"(*data),"=b"(*(data+1)),
               "=c"(*(data+2)),"=d"(*(data+3)):"a"(code));
}

// Gets the APIC ID of the processor. Returns a unique identifier for each core.
static inline uint8_t apic_id(void) {
	uint32_t data[4];
	cpuid(1, data);
	uint32_t ebx = data[1];
	uint8_t apic_id = *((uint8_t *)&ebx + 3);
	return apic_id;
}

// More information: http://wiki.osdev.org/Detecting_CPU_Topology_(80x86)