On Mon, Aug 7, 2023 at 8:52 AM Şahin Duran via Gcc <gcc@gcc.gnu.org> wrote:
>
> Dear GCC Developers,
>
> I think I've just discovered a bug/ undefined situation in the compiler.
> When I try to call a weakly defined function, compiler successfully
> generates the code of calling procedure. However, this calling procedure is
> nothing but branching to address 0 which results in segmentation fault. I
> am not sure if this is the case for the latest version of GCC but it is for
> GCC 4.9.2 and many online compilers. I just thought that maybe including a
> rule that generates compilation error when the user defines a weak function
> and calls it without actually implementing it. You may find the results in
> the attachments.
You need to check the address of weak defined symbol (function) to
make sure it is not a nullptr before calling it.
A weak defined symbol might have the address of nullptr if it is not
defined. And you are running into that.
This is a feature of elf and weak symbols.
Thanks,
Andrew
>
> Kind regards,
> I am looking forward to hearing from you about this.
> Şahin Duran
>
>
> Attachments:
>
> Source Code:
> #include <stdio.h>
> #include <stdlib.h>
> #include "header.h"
>
> __attribute__((weak)) int add(int,int);
>
> int main(int argc, char *argv[]) {
> printf("%x",add);
> add(31,31);
> return 0;
> }
> terminal result : 0
>
> Disassembly (on a 64bit AMD Machine):
> 0x0000000000401530 <+0>: push rbp
> 0x0000000000401531 <+1>: mov rbp,rsp
> 0x0000000000401534 <+4>: sub rsp,0x20
> 0x0000000000401538 <+8>: mov DWORD PTR [rbp+0x10],ecx
> 0x000000000040153b <+11>: mov QWORD PTR [rbp+0x18],rdx
> 0x000000000040153f <+15>: call 0x402100 <__main>
> 0x0000000000401544 <+20>: mov rdx,QWORD PTR [rip+0x2ed5] #
> 0x404420 <.refptr.add>
> 0x000000000040154b <+27>: lea rcx,[rip+0x2aae] # 0x404000
> 0x0000000000401552 <+34>: call 0x402b18 <printf>
> => 0x0000000000401557 <+39>: mov edx,0x1f
> 0x000000000040155c <+44>: mov ecx,0x1f
> 0x0000000000401561 <+49>: call 0x0
> 0x0000000000401566 <+54>: mov eax,0x0
> 0x000000000040156b <+59>: add rsp,0x20
> 0x000000000040156f <+63>: pop rbp
> 0x0000000000401570 <+64>: ret
