Hi Dave, > 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 > function.
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 #ifdef __ELF__ Simply, it's an issue to be aware of. I'll suggest a documentation improvement for the GCC manual. Bruno -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple