Processes and Threads

A process is a program in execution. It has a code section, data, heap, stack and a Process Control Block (PCB).

Process States

Process state transition diagram

Process Control Block

Process ID, process state, program counter, CPU registers, scheduling info (priority), memory-management info (page tables), accounting info and I/O status.

Creating a Process: fork()

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

int main() {
    pid_t pid = fork();
    if (pid == 0) {
        printf("Child: my PID is %d\n", getpid());
    } else {
        wait(NULL);                     // parent waits for child
        printf("Parent: child %d finished\n", pid);
    }
    return 0;
}

Child: my PID is 4122 Parent: child 4122 finished

Exam pointA program calling fork() n times (sequentially, unconditionally) produces 2ⁿ processes, so 2ⁿ − 1 are children.

Process vs Thread

Process	Thread
Independent, with its own address space	Lightweight; shares the process's code, data and files
Expensive context switch	Cheap context switch
Communicates via IPC	Communicates via shared memory

User-level threads are managed by a library; kernel-level threads are managed by the OS. Mapping models: many-to-one, one-to-one and many-to-many.

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