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Re: calling an undefined weak symbol

On 14 Oct 2007, Metuki Sabhe stated:

> Hi list,
> In the function __libc_start_main we have those two lines:
> extern void __nptl_deallocate_tsd (void) __attribute ((weak));
> __nptl_deallocate_tsd ();
> If I understand correctly, the function will call __nptl_deallocate_tsd,
> which will have the value of the __nptl_deallocate_tsd function
> if libpthread will be linked in, or zero if not (because this is a weak
> symbol).
> But, if libpthread is not linked in, then __libc_start_main will call
> a value of zero which will make a Segmentation Fault !

Indeed: only, as you suspected, this can never happen.

> Can you please explain this to me (I don't really think glibc creates
> a segfault, I am sure I missed something here) ?

Look a few lines up:

  int not_first_call;
  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
  if (__builtin_expect (! not_first_call, 1))

The code you note is in an else branch of that if block. i.e., that code
is only invoked if the cancel_jmp_buf is jumped to, and as its name
indicates this is part of the cancellation point mechanism, and is only
otherwise referenced by NPTL code.

So it's quite safe, and if we enter that else branch we *must* be in a
program linked with -lpthread.

(The code you note is actually the !SHARED branch; the SHARED branch
makes the same assumptions, but does it by way of a thunk through an
NPTL structure filled with function pointers.)

`Some people don't think performance issues are "real bugs", and I think 
such people shouldn't be allowed to program.' --- Linus Torvalds

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