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Re: weak symbols on Cygwin
- From: Bruno Haible <bruno at clisp dot org>
- To: Dave Korn <dave dot korn dot cygwin at googlemail dot com>
- Cc: Eric Blake <eblake at redhat dot com>, cygwin at cygwin dot com
- Date: Wed, 7 Apr 2010 11:06:10 +0200
- Subject: Re: weak symbols on Cygwin
- References: <4BBB31C6.firstname.lastname@example.org> <email@example.com> <4BBBF5C2.firstname.lastname@example.org>
> on ELF platforms, you can leave undefined references in an executable, to
> be filled in by ld.so at runtime according to what the loader finds present
> once it's loaded libc.
> On windows ... There's no run-time process of
> scanning all available symbols from all loaded libraries against undefined
> references in the executable.
> on Windows, all the library functions imported
> from libc come in the form of import stubs from the (static) import library.
> So weak references won't cause any of those symbols to be pulled in, even if
> the library does provide them. But if something else in the program causes
> them to be pulled in, the weak references will resolve to the imported
Thanks for these explanations. So, "#pragma weak" is very different on PE
systems than on ELF systems.
> > 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 whole concept isn't going to translate directly to Windows platforms.
> There's no interposing or providing missing definitions at runtime; if it's
> not already there at final link time, it's not going to be there at runtime.
> The only way windows can determine this sort of thing is through dlsym()ing
> all the open libraries, I think.
I was implicitly hoping that
- gcc would collect all weak-declared symbols in some table, and
generate code so that the reference to these symbols makes an indirection
through this table,
- the Cygwin runtime would call dlsym or GetProcAddress at startup and
fill in addresses into said table.
> What's the larger-scale goal you're trying to support by this method? Are
> you after implementing some sort of plug-in architecture or something?
I've got some code, written for ELF platforms, that detects whether the pthread
library (often a separate library from libc) is linked, by doing
#pragma weak pthread_cancel
bool pthread_in_use = (pthread_cancel != NULL);
This code breaks in Cygwin 1.7, because the autoconf test determines that gcc
accepts #pragma weak. Not a big problem - I can surround that code with
Simply, it's an issue to be aware of. I'll suggest a documentation improvement
for the GCC manual.
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