CS110 Lecture 06: Pipes, Signals, and Concurrency

Principles of Computer Systems

Winter 2020

Stanford University

Computer Science Department

Instructors: Chris Gregg and

Nick Troccoli

Process	stdin	stdout
cat	terminal	pipe1[1]
uniq	pipe1[0]	pipe2[1]
sort	pipe2[0]	terminal

CS110 Lecture 06: Pipes, Signals, and Concurrency Principles of Computer Systems Winter 2020 Stanford University Computer Science Department Instructors : Chris Gregg and Nick Troccoli PDF of this presentation

	int main(int argc, char *argv[]) {
	char *args[] = {"/bin/ls", "-l", "/usr/class/cs110", NULL};
	execvp(args[0], args);
	printf("Hello world!\n");
	return 0;
	}

	static int mysystem(char *command) {
	pid_t pidOrZero = fork();
	if (pidOrZero == 0) {
	// If we are the child, execute the shell command
	char *arguments[] = {"/bin/sh", "-c", command, NULL};
	execvp(arguments[0], arguments);
	// If the child gets here, there was an error
	exitIf(true, kExecFailed, stderr, "execvp failed to invoke this: %s.\n", command);
	}

	// If we are the parent, wait for the child
	int status;
	waitpid(pidOrZero, &status, 0);
	if (WIFEXITED(status)) {
	return WEXITSTATUS(status);
	} else {
	return -WTERMSIG(status);
	}
	}

	static int mysystem(char *command) {
	pid_t pidOrZero = fork();
	if (pidOrZero == 0) {
	// If we are the child, execute the shell command
	char *arguments[] = {"/bin/sh", "-c", command, NULL};
	execvp(arguments[0], arguments);
	// If the child gets here, there was an error
	exitIf(true, kExecFailed, stderr, "execvp failed to invoke this: %s.\n", command);
	}

	// If we are the parent, wait for the child
	int status;
	waitpid(pidOrZero, &status, 0);
	if (WIFEXITED(status)) {
	return WEXITSTATUS(status);
	} else {
	return -WTERMSIG(status);
	}
	}

	static int mysystem(char *command) {
	pid_t pidOrZero = fork();
	if (pidOrZero == 0) {
	// If we are the child, execute the shell command
	char *arguments[] = {"/bin/sh", "-c", command, NULL};
	execvp(arguments[0], arguments);
	// If the child gets here, there was an error
	exitIf(true, kExecFailed, stderr, "execvp failed to invoke this: %s.\n", command);
	}

	// If we are the parent, wait for the child
	int status;
	waitpid(pidOrZero, &status, 0);
	if (WIFEXITED(status)) {
	return WEXITSTATUS(status);
	} else {
	return -WTERMSIG(status);
	}
	}

	static int mysystem(char *command) {
	pid_t pidOrZero = fork();
	if (pidOrZero == 0) {
	// If we are the child, execute the shell command
	char *arguments[] = {"/bin/sh", "-c", command, NULL};
	execvp(arguments[0], arguments);
	// If the child gets here, there was an error
	exitIf(true, kExecFailed, stderr, "execvp failed to invoke this: %s.\n", command);
	}

	// If we are the parent, wait for the child
	int status;
	waitpid(pidOrZero, &status, 0);
	if (WIFEXITED(status)) {
	return WEXITSTATUS(status);
	} else {
	return -WTERMSIG(status);
	}
	}

	pid_t pidOrZero = fork();
	// both parent and child run code here onwards
	printf("This is printed by two processes.\n");

	int main(int argc, char *argv[]) {
	char command[kMaxLineLength];
	while (true) {
	printf("> ");
	fgets(command, sizeof(command), stdin);

	// If the user entered Ctl-d, stop
	if (feof(stdin)) {
	break;
	}

	// Remove the \n that fgets puts at the end
	command[strlen(command) - 1] = '\0';

	int commandReturnCode = mysystem(command);
	printf("return code = %d\n", commandReturnCode);
	}

	printf("\n");
	return 0;
	}

	static void executeCommand(char *command, bool inBackground) {
	pid_t pidOrZero = fork();
	if (pidOrZero == 0) {
	// If we are the child, execute the shell command
	char *arguments[] = {"/bin/sh", "-c", command, NULL};
	execvp(arguments[0], arguments);
	// If the child gets here, there was an error
	exitIf(true, kExecFailed, stderr, "execvp failed to invoke this: %s.\n", command);
	}

	// If we are the parent, either wait or return immediately
	if (inBackground) {
	printf("%d %s\n", pidOrZero, command);
	} else {
	waitpid(pidOrZero, NULL, 0);
	}
	}

	int main(int argc, char *argv[]) {
	int fds[2];
	pipe(fds);
	pid_t pid = fork();
	if (pid == 0) {
	close(fds[1]);
	char buffer[6];
	read(fds[0], buffer, sizeof(buffer));
	printf("Read from pipe bridging processes: %s.\n", buffer);
	close(fds[0]);
	return 0;
	}
	close(fds[0]);
	write(fds[1], "hello", 6);
	waitpid(pid, NULL, 0);
	close(fds[1]);
	return 0;
	}

	static const size_t kNumChildren = 5;
	static size_t numDone = 0;

	int main(int argc, char *argv[]) {
	printf("Let my five children play while I take a nap.\n");
	signal(SIGCHLD, reapChild);
	for (size_t kid = 1; kid <= 5; kid++) {
	if (fork() == 0) {
	sleep(3 * kid); // sleep emulates "play" time
	printf("Child #%zu tired... returns to dad.\n", kid);
	return 0;
	}
	}

	// code below is a continuation of that presented on the previous slide
	while (numDone < kNumChildren) {
	printf("At least one child still playing, so dad nods off.\n");
	sleep(5);
	printf("Dad wakes up! ");
	}
	printf("All children accounted for. Good job, dad!\n");
	return 0;
	}

	static void reapChild(int unused) {
	waitpid(-1, NULL, 0);
	numDone++;
	}