Question

Does this code return an invalid reference to a variable allocated on the stack? Or what:

void *f(size_t sz) {
    return alloca(sz);
}

Or is it a special case that is handled by the alloca implementation / compiler support like f(alloca(size), alloca(size)) would be?

Answer 1

alloca allocates space in the stack frame of f. You can’t do anything useful with it once the function returns (it’s not “reserved” anymore).

The alloca() function allocates size bytes of space in the stack frame
of the caller. This temporary space is automatically freed when the
function that called alloca() returns to its caller.