linker - is pthread in glibc.so implemented by weak symbol to provide pthread stub functions?

Question

is pthread in glibc.so implemented by weak symbol to provide pthread stub functions?

I know there is pthread.so to provide the functions similar with pthread in glibc.so somebody said pthread in glibc provide stub only and will be replace when explicit linking to lpthread.

So my question is how to support it? using weak symbol or other tech?

is libssl similar with pthread in glibc?

Answer 1

Yes, glibc uses a stub implementation of various pthread functions, so that single threaded programs do not have to waste cycles doing things like locking and unlocking mutexes, and yet do not have to link to a different C library (like what is done in the Microsoft world, for instance).

For instance, according to POSIX, every time you call fputc(ch, stream), there is mutex lock and unlock. If you don't want that, you call fputc_unlocked. But when you do that, you're using a POSIX extension related to threading; it's not an appropriate workaround for programs that don't use POSIX or don't use the threading API.

The overriding of the stub pthread functions with the real ones (in the dynamic glibc) is not based on weak symbols. The shared library mechanism makes it possible to override non-weak definitions.

Weak symbols are a mechanism which allows for symbol overriding under static linking.

If you want a source for the above statement, here it is:

"Note that a definition in a DSO being weak has no effects. Weak definitions only play a role in static linking." [Ulrich Drepper, "How To Write Shared Libraries"].

If you run nm on the static glibc on your system (if you have one), libc.a, you will note that functions like pthread_mutex_lock are marked weak. In the dynamic version, libc.so.<whatetever>, the functions are not marked weak.

Note: you should use nm -D or nm --dynamic to look at the symbols in a shared library. nm will not produce anything on a shared library that is stripped. If it does, you're looking at the debug symbols, not the dynamic symbols.