problem with unistd.h and wait.h library.

[hecram]

Hi I am doing some changes in some files. I am trying to execute a fork() and waitpid in the ts.c file inside the /modules/demux/ directory. To achieve that I have introduced the unistd.h and sys/wait.h library to compile the file.
The problem is the make don´t let compile the file telling me the following:

.../modules/demux/ts.c: undefined reference to '_fork';
.../modules/demux/ts.c: undefined reference to '_waitpid'

I don´t know why it is giving me that problem if I have introduce the libraries at the beginning of the file:

#include <unistd.h>
#include <sys/wait.h>

If anyone could tell me which is being my programmation error I would be very grateful.
thank you.