title: Posix Semaphore compilation error in Makefile
body: <p>This is my makefile:</p>
  
  <pre><code>program : program.o
            gcc -o program program.o
  program.o : program.c library.h
              gcc -c program.c
  </code></pre>
  
  <p>In "library.h" I've got the headers, but I have a problem with the semaphores.
  It says "undefined reference to sem_open , sem_post, sem_wait. . ."</p>