How can you find the processor number a thread is running on?

For XP, a quick google as revealed this:

https://www.cs.tcd.ie/Jeremy.Jones/GetCurrentProcessorNumberXP.htm Does this help?

If all you want to do is avoid contention, you don't need to know the current CPU. You could just randomly pick a heap. Or you could have a heap per thread. Although you may get more or less contention that way, you would avoid the overhead of polling the current CPU, which may or may not be significant. Also check out Intel Thread Building Block's scalable_allocator, which may have already solved that problem better than you will.

From output of man sched_getcpu:

NAME
       sched_getcpu - determine CPU on which the calling thread is running

SYNOPSIS
       #define _GNU_SOURCE
       #include <utmpx.h>

       int sched_getcpu(void);

DESCRIPTION
   sched_getcpu() returns the number of the CPU
   on which the calling thread is currently executing.

RETURN VALUE
   On success, sched_getcpu() returns a non-negative CPU number.
   On error, -1 is returned and errno is set to indicate the error.

SEE ALSO
   getcpu(2)

Unfortunately, this is Linux specific. I doubt there is a portable way to do this.

In addition to Antony Vennard's answer and the code on the cited site, here is code that will work for Visual C++ x64 as well (no inline assembler):

DWORD GetCurrentProcessorNumberXP() {
   int CPUInfo[4];   
   __cpuid(CPUInfo, 1);
   // CPUInfo[1] is EBX, bits 24-31 are APIC ID
   if ((CPUInfo[3] & (1 << 9)) == 0) return -1;  // no APIC on chip
   return (unsigned)CPUInfo[1] >> 24;
}

A short look at the implementation of GetCurrentProcessorNumber() on Win7 x64 shows that they use a different mechanism to get the processor number, but in my (few) tests the results were the same for my home-brewn and the official function.