Sorry how is this relevant to heap allocation? alloca just allocates memory off the stack - it's literally just bumping the stack register by the number of byte

Sorry how is this relevant to heap allocation? alloca just allocates memory off the stack - it's literally just bumping the stack register by the number of bytes you requested (and hence it's freed when the stack pointer is reset on returning from the function call). Alloca should fail when your current thread runs out of stack space - which will generally be only a few megabytes. The total stack for a new thread is allocated from the heap at its creation, not during its execution and certainly not as part of a call to alloca.