/*
This is an example of how signals and signal handlers work. The key
function in this example is signal(). This will help you implement
the "&" operator.

Normally, when the user types in a command, your shell should fork()
and execvp() the appropriate programs, then waitpid() for them to end
before waiting for the next command. But, if the command has a "&" at
the end, it should present the prompt and wait for the next command
right away. In that case, there is the problem that the parent
process should call wait() or waitpid() as soon as a child finishes,
in order to "clean up" the memory the child held. This is solved with
signals.

When calling signal(), a process  is instructed that it should
execute the appropriate signal handler whenever a specific signal is
delivered to it. Signals are delivered to note various events, like
"interrupt from keyboard" (SIGINT) or "child ended" (SIGCHLD). For
more information, type "man 3 signal" or "man 7 signal".

The idea is that whenever a child finishes, SIGCHLD is
"automatically" delivered to the parent process. So, the parent can
call wait() or waitpid() as soon as it receives SIGCHLD!

In the following example, note the header of signalHandler(). The
header of any signal handler should be like that (any signal handler
should have an int as a parameter and return void).

The example is pretty simple. A signal handler is set for SIGALRM,
SIGUSR1 and SIGCHLD (type "man 7 signal" for details). It sends
SIGALRM to the parent and then finishes (again, sending SIGCHLD).

You can try sending signals to the parent process yourself! Just
try typing:

kill -USR1 <parent_pid>
*/

#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <unistd.h>

void signalHandler(int signal)
{
	printf("Cought signal %d!\n",signal);
	if (signal==SIGCHLD) {
		printf("Child ended\n");
		wait(NULL);
	}
}

int main()
{
	signal(SIGALRM,signalHandler);
	signal(SIGUSR1,signalHandler);
	signal(SIGCHLD,signalHandler);

	if (!fork()) {
		printf("Child running...\n");
		sleep(2);
		printf("Child sending SIGALRM...\n");
		kill(getppid(),SIGALRM); /*send alarm signal to parent*/
		sleep(5);
		printf("Child exitting...\n");
		return 0;
	}
	printf("Parent running, PID=%d. Press ENTER to exit.\n",getpid());
	getchar();
	printf("Parent exitting...\n");
	return 0;
}