Question

I just executed a program that mallocs 13 MB in a 12 MB machine (QEMU Emulated!) . Not just that, i even browsed through the memory and filled junk in it…

void 
large_mem(void) 
{
  #define LONGMEM  13631488
  long long *ptr = (long long *)malloc(LONGMEM);
  long long i;
  if(!ptr) {
     printf("%s(): array allocation of size %lld failed.\n",__func__,LONGMEM);
     ASSERT(0);
  }
  for(i = 0 ; i < LONGMEM ; i++ ) { 
    *(ptr+i)=i;
  }
  free(ptr);
}

How is it possible ? I was expecting a segmentation fault.

Answer 1

It’s called virtual memory which is allocated for your program. It’s not real memory which you call RAM.

There is a max limit for virtual memory as well, but it’s higher than RAM. It’s implemented (and defined) by your operating system.