Sources of non-determinism [closed]

Question

My supposedly deterministic program produces one of a few slightly different outputs on different runs. The input, compiler and computer are unchanging. I'm not sure which output is right because it always looks reasonable.

Besides a stray call to rand(), how could this be possible?

Answer 1

If your program use float / double, there may be difference in the result if there are context switch on some architecture.

On x86, the FPU use extended precision for intermediary result, but when saved in memory (which happens when there is a context switch either process or thread), such precision is lost. That could cause some small divergence of the result (we've detected such problem in our program). One way to avoid this issue is to ask the compiler not to use FPU but SSE for floating point operations.

http://www.network-theory.co.uk/docs/gccintro/gccintro_70.html

Answer 2

If your output depends on an address allocated on the heap:

int main(int argc, char* argv[])
{
   printf("%p", malloc(42));
   return 0;
}

For each run, the malloc() may return a different virtual address - not to mention NULL in case the allocation failed.