Processes and Threads

Process​

A process is a program in execution: code, data, heap, stack and OS bookkeeping stored in a process control block (PCB) (PID, state, registers, memory maps, open files).

Process states​

New -> Ready -> Running -> Terminated
         ^        |
         |        v
         +----- Waiting (I/O or event)

A context switch saves the state of the running process into its PCB and loads another one. It is pure overhead, so schedulers try not to switch too often.

Creating processes (POSIX)​

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

int main(void) {
    pid_t pid = fork();          // duplicates the calling process
    if (pid == 0) {
        printf("child\n");
    } else {
        wait(NULL);              // parent waits for the child
        printf("parent\n");
    }
    return 0;
}

fork() returns 0 in the child and the child's PID in the parent. exec() replaces the current program image.

Threads​

A thread is a unit of execution inside a process. Threads of one process share code, heap and open files, but each has its own stack and registers.

Inter-process communication (IPC)​

• Pipes and named pipes (FIFOs)
• Message queues
• Shared memory (fastest, needs synchronisation)
• Sockets (also across machines)
• Signals (simple notifications)