Some signals are delivered to a process by hardware - e.g. float point exceptions.

But the kernel and any user process can send a signal to another process or to a process group provided the user has privilege to do so.

        #include <sys/types.h>
        #include <signal.h>

        int kill(pid_t pid, int sig);

     

A process can even send a signal to itself:

        kill( getpid(), SIGSTOP);