sem_open() error: "undefined reference to sem_open()" on linux (Ubuntu 10.10)
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?