Definitions

What is a Process?

A process keeps track of the context of a program, including:

Signals

• Signals are sent via the kernel, triggered by keyboard shortcuts and/or other processes.

  • In most cases the default behavior is to stop the program (e.g., SIGINT, SIGKILL, and yes, SIGSEGV).
  • Some signals suspend it (SIGTSTP)

• A process can define a signal handler via the signal function to do something special (but this is tricky).

• Wikipedia page on signals

Signal Handling

#include <stdio.h>  // printf
#include <time.h>   // sleep
#include <signal.h> // signal

int tried = 0;

void keepGoing(int signal) {
    tried = 1;
}

int main (void) {
    signal(SIGINT, keepGoing);
    while (1) {
        printf("Hahaha!\n");
        if (tried) {
            printf("You can't stop me!\n");
            sleep(0.25);
            tried = 0;
        }
    }
}

Multiprocess Programming

• Any process can be switched out at any moment (but its context will be restored when it gets switched back in).
• Kernel is in charge of which processes get to run when.
• Can create “child” processes if we want to work in parallel:
  • fork creates a new process. Call it once and it returns twice: once to the parent (with the child PID) and once in a new child process (with 0 as the value).
  • exec replaces the running program by loading a new one from a file (gets fresh memory & registers).

Multiprocess Programming

• Processes can communicate via files, signals, ports, etc.
  • Could also coordinate work up-front before starting a child.
• Things can happen in any order: you don’t know who the kernel will pick next.
• Threads offer a mechanism for multiple contexts to share some state; they’re even trickier to get right than processes.
• Parallel programming is hard.
  • It’s also necessary to create programs that perform calculations while also responding to user input.

Multiprocess Programming

• Each process has an ID, called a PID.
• Parent & child processes can communicate:
  • getpid returns the current process’ ID.
  • getppid returns the parent process’ ID.
  • waitpid pauses this process until the target process finishes.
• In the shell:
  • Use man (“manual”) to look at documentation, e.g.:
    man getpid
  • Use ps to see processes, or top for an interactive display.

Multiprocess Examples

#include <stdio.h>
#include <stdlib.h>

int main() {
    int i, pid;
    pid = fork();
    printf("Hello from %d\n", pid);
    exit(0);
}

Hello from 27013
Hello from 0

Hello from 0
Hello from 27296

Hello from 27461
Hello from 0

Multiprocess Examples

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main() {
    int pid = fork();
    if (pid == 0) {
        sleep(3);
        printf("\n\nSurprise!\n");
    }
    exit(0);
}

Multiprocess Examples

#include <stdio.h>
#include <stdlib.h>

int main() {
    int i, pid;
    for (i = 0; i < 2; ++i) {
        pid = fork();
        printf("Hello from %d iter %d\n", pid, i);
    }
    exit(0);
}

Hello from 29195 iter 0
Hello from 0 iter 0
Hello from 29196 iter 1
Hello from 29197 iter 1
Hello from 0 iter 1
Hello from 0 iter 1

Hello from 30035 iter 0
Hello from 30036 iter 1
Hello from 0 iter 1
Hello from 0 iter 0
~/cs240/test > Hello from 30037 iter 1
Hello from 0 iter 1