POSIX Threads: are pthreads_cond_wait() and others systemcalls?

Question

The POSIX standard defines several routines for thread synchronization, based on concepts like mutexes and conditional variables.

my question is now: are these (like e.g. pthreads_cond_init(), pthreads_mutex_init(), pthreads_mutex_lock()... and so on) system calls or just library calls? i know they are included via "pthread.h", but do they finally result in a system call and therefore are implemented in the kernel of the operating system?

Answer 1

On Linux a pthread mutex makes a "futex" system call, but only if the lock is contended. That means that taking a lock no other thread wants is almost free.

In a similar way, sending a condition signal is only expensive when there is someone waiting for it.

So I believe that your answer is that pthread functions are library calls that sometimes result in a system call.

Answer 2

Whenever possible, the library avoids trapping into the kernel for performance reasons. If you already have some code that uses these calls you may want to take a look at the output from running your program with strace to better understand how often it is actually making system calls.