Program: Executable instructions

Process: Running instance of a program

• Process Control Block: A data structure for each process to store PID, $pc, $sp, file table, signal table, etc.
• PID: Unique process ID in PCB

Process States

• Running: Processes that are actively running on a CPU core
• Ready: Processes that needs to be run
• Blocked/Sleeping: Processes that are waiting for an event

Multi-processing: Forking

Fork: Create a new process on the exact point of execution

• By default, the parent doesn’t wait for its children
• Zombie Process: A child that finished but the parent hasn’t wait()ed for its exit status
• Orphan Process: A child that’s still running after parent exits, the parent will be changed to PID 1

Init Process: PID 1, calls wait() in a loop to collect orphan processes

// fork() returns:
// * >1 child's pid for parent process
// * 0 for child process
// * -1 on failure
int fork = fork();
if (fork == -1) perror("fork");

// wait(&status) waits for one child process to finish, returns -1 on failure
pid_t pid;
int result;
if ((pid = wait(&result)) == -1) perror("wait");
else {
	if (WIFEXITED(status))
		printf("Child %d exited with code %d\\n", pid, WEXITSTATUS(status));
	else if (WIFSIGALED(status))
		printf("Child %d exited with signal %d\\n", pid, WTERMSIG(status));
}

Running Different Programs

Exec: Replace the current process with the new program

// exec Variants:
// * l: Parameters as vararg, v: Parameters as array
// * p: Search for binary in paths
// * e: Specify environment variables
// Example: execvp(char* executable, char** null-terminated argv)
// * The code following exec will only execute on error
execl("./hello", NULL);
perror("exec");

Pipe: Subprocess Communication

• Producer: Process writing data | Consumer: Process reading data