Malloc allocates memory more than RAM

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.

Answer 2

This is called as Lazy Allocation.

Most OS like Linux have an Lazy Allocation memory model wherein the returned memory address is a virtual address and the actual allocation only happens at access-time. The OS assumes that it will be able to provide this allocation at access-Time.

The memory allocated by malloc is not backed by real memory until the program actually touches it.

While, since calloc initializes the memory to 0 you can be assured that the OS has already backed the allocation with actual RAM (or swap).

Try using callocand most probably it will return you out of memory unless your swap file/partition is big enough to satisfy the request.