Where is the man page for "pthread_mutex_lock"?

I can't find the manual for pthread_mutex_lock with man pthread_mutex_lock, is that something wrong with manpages-dev ? I got that package installed, but missing some manual.

Should I fire a bug to Ubuntu ?

The documentation for pthreads is in the glibc-doc package.

Ubuntu (and Debian) include the desired man pages in the manpages-posix-dev package.

Just for completeness, here a version we can enjoy in our web-browser:

http://manpages.ubuntu.com/manpages/wily/man3/pthread_mutex_lock.3.html

I know, links might get outdated, but if we feed our search-engine we will get a lot of alternative links which deliver the desired information.

Advantage: Better readability, comfortability with use in GUI. We don't need to install anything.

Disadvantage: Requires Internet connection (at least for the time we need to download the site).

Just install the glibc-doc package, the missing man pages will be there:

sudo apt install glibc-doc