c - Why does gcc return 0 instead of the address of a stack allocated variable?

Question

As part of an experiment using custom stacks, I wanted a function to return the address of a stack allocated char buffer.

// return pointer to stack variable
void *foo(void)
{
    char sz[10] = "hello";
    return sz;
}

I know that it's illegal to do this in C, and gcc warns too.

gcc -Wall -Wextra -pedantic -std=gnu99 -fomit-frame-pointer -O0 -c foo.c 

foo.c:8:12: warning: function returns address of local variable [-Wreturn-local-addr]
     return sz;

Still, since this is part of an experiment, I want the code as is. The funny thing is that the generated code returns 0 instead of sz's stack address:

boa@localhost:~/tmp$ objdump -dMintel foo.o
0000000000000000 <foo>:
   0:   48 b8 68 65 6c 6c 6f    movabs rax,0x6f6c6c6568
   7:   00 00 00 
   a:   48 89 44 24 f0          mov    QWORD PTR [rsp-0x10],rax
   f:   66 c7 44 24 f8 00 00    mov    WORD PTR [rsp-0x8],0x0
  16:   b8 00 00 00 00          mov    eax,0x0
  1b:   c3                      ret    

As one can see, 0x0 is moved to eax, which is what puzzles me. Why does gcc do this?

Here's a complete source file with another function, bar(), as well as a main function. bar() returns the address as expected.

#include <stdint.h>
#include <stdio.h>

// return pointer to stack variable
void *foo(void)
{
    char sz[10] = "hello";
    return sz;
}

void *bar(void)
{
    char sz[10] = "hello";
    intptr_t i = (intptr_t)sz;
    return (void*)i;
}

int main(void)
{
    printf("foo: %p
", foo());
    printf("bar: %p
", bar());
    return 0;
}

boa@localhost:~/tmp$ make foo && ./foo
cc   foo.o   -o foo
foo: (nil)
bar: 0x7ffce518a268

This is a mystery to me. What may be the logic behind gcc's choice?

Answer 1

GCC deliberately returns NULL in this case as can be seen from the code:

tree zero = build_zero_cst (TREE_TYPE (val));
gimple_return_set_retval (return_stmt, zero);
update_stmt (stmt);

Newer versions of GCC and Clang exploit undefined behavior more aggressively so this check is not suprising. It also makes code fail fast which is a good thing in most cases (not yours, apparently).