sem_open() error: “undefined reference to sem_open()” on linux (Ubuntu 10.10)

Question

So I am getting the error: \"undefined reference to sem_open()\" even though I have include the semaphore.h header. The same thing is happening for all my pthread function

Answer 1

Including the header does not tell ld about the library. You need to add -lrt to your compilation command line. For threading, you need either -lpthread or -pthread, depending on your platform.

The library is not the header. The header is not the library. This is an important distinction. See What's the difference between a header file and a library?

Answer 2

You need link with pthread lib, using -lpthread option.

Answer 3

The working option in Ubuntu is -lpthread. But if you work on suse or other systems the correct option is -lrt. Also the book Linux Programmin Interface mentions -lrt as the correct option.