The asm statement puts the pointer (the malloc(5000)+5000's result) into the stack pointer. When the arguments are passed into fprintf(), they're pushed on top

The asm statement puts the pointer (the malloc(5000)+5000's result) into the stack pointer. When the arguments are passed into fprintf(), they're pushed on top of that frame (presumably back into the malloc'd block). In my book, a worthy attempt to grow your stack on the heap, but you've really got to be ready to hit the asm debugger when things go awry. Edit: as mentioned in other comments, the mistake was (at least) in not restoring the stack pointer before main returned.