Weak symbols are a poor man's version of linker land polymorphism. A weak symbol can be overriden by a strong symbol when a linker loads an executable or shared object. But in the absence of a strong sym, the weak version is used without any errors. And that's how it has worked for a long long time in ELF binary land.
But then dlopen() went and changed the rules. When you load a shared library with RTLD_GLOBAL, the symbols became available to all the other shared objects. And the libc rtld.c had the runtime magic required to make this happen (and the unloading was even harder).
Then one fine day, Weak Symbols were empowered. In dynamic shared objects (DSO), there was no difference between weak and strong. It has been so since the glibc-2.1.91 release.
Now let me backtrack to my original problem. Once upon a time, there was a php extension which used an apache function, ap_table_set() to be precise. But for the same php extension to be loadable (though not necessarily useful) inside a php command line executable - the external symbol had to be resolved. That's where a weak symbol proves itself invaluable - a libapstubs.a could be created with a weak ap_table_set, so that as long as the extension is run outside apache, the stub function will get called.
But it wasn't working on linux (works on FreeBSD). And except for the extension, we weren't able to write a single test case which would show the problem. And then I ran into a neat little env variable - LD_DYNAMIC_WEAK. Just set it to 1 and the rtld relegates the weak symbols back into the shadows of the strong ones. But that raised a few other problems elsewhere and I personally was lost.
But now I know where exactly this went wrong. Php was using a glibc dl flag called RTLD_DEEPBIND (introduced in zend.h,1.270). This seems to be a glibc feature and the flag bit is ignored by the FreeBSD libc - which was running the php module happily. As you can read in that mail, it looks up the local shared object before it starts looking in the global scope. Since libapstubs was a static .a object, the local scope of the ext .so did contain the dummy ap_table_set() and since glibc rltd was ignoring the weak flags, that function was called instead of the real apache one.
I'm perfectly aware that RTLD_DEEPBIND can save a large amount of lookup time for shared objects built with -fPIC, because of the PLT (we've met before). But if you are trying to use it to load random binaries (like extension modules), here's a gotcha you need to remember.
Now to get back to doing some real work :)--
A wise person makes his own decisions, a weak one obeys public opinion.