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From: Bruno Haible <bruno AT clisp DOT org>
To: Eric Blake <eblake AT redhat DOT com>,
   Dave Korn <dave DOT korn DOT cygwin AT googlemail DOT com>, cygwin AT cygwin DOT com
Subject: Re: weak symbols on Cygwin
Date: Wed, 7 Apr 2010 01:41:35 +0200
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Hi Dave,

Dave Korn wrote:
> >> These were all due to the fact that gcc 4.3.x on Cygwin 1.7.2
> >> accepts "#pragma weak foo", but the symbol foo then evaluates
> >> to the NULL address, even if foo is defined in libc.
> > 
> > Dave, are weak symbols something that should work on cygwin with new
> > enough binutils/gcc?  Or is this an indicator of a gcc bug, for silently
> > accepting #pragma weak foo that it can't support?
> 
>   Weak symbols work on Cygwin, but the semantics of undefined weak symbols
> aren't identical to ELF platforms: a weak reference won't pull in an archive
> member that wouldn't otherwise be linked; the implications in relation to
> import libraries should be fairly obvious.

I don't know what semantics is implemented by "#pragma weak" on Cygwin.

On ELF platforms, I use "#pragma weak" in order to detect whether a symbol
is defined in the libraries which are linked in with the executable
(including libc). This does not work on Cygwin: this program

======================================================
   #include <stdio.h>
   extern void gurky (void);
   #pragma weak fputs
   #pragma weak gurky
   int main ()
   {
     printf ("fputs %s, gurky %s\n",
             fputs != NULL ? "present" : "missing",
             gurky != NULL ? "present" : "missing");
     return 0;
   }
======================================================

compiled and run with
  $ gcc -o foo foo.c -Wall
  $ ./foo

prints on glibc systems:
  fputs present, gurky missing

but on Cygwin 1.7.2:
  fputs missing, gurky missing

With this inability to distinguish present from missing libc symbols,
"#pragma weak" is useless to me on Cygwin.

Bruno

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