Creating Processes — The fork/exec Model

In Unix, a process is created by an existing process calling fork(). The new process is an almost-exact copy of the parent. To run a different program the child then calls one of the exec family of system calls, which replaces its memory image.

Why fork+exec?

Splitting creation (fork) from program-loading (exec) lets the parent set up file descriptors, environment variables, and pipes for the child before the new program starts. This is how shells implement <, >, and | redirection.

Process Termination

A process ends by calling exit(status). The kernel:

1. Closes all open file descriptors.
2. Releases memory.
3. Keeps a small zombie PCB until the parent calls wait/waitpid to collect the exit status.

If a parent dies before its children, the orphans are reparented to init (PID 1) which immediately reaps them.

Cooperating vs Independent Processes

• Independent — cannot affect each other.
• Cooperating — share data and need synchronization (Unit 2 later).

Inter-process Communication (IPC) Mechanisms

Worked C Example

The code in the snippet below forks a child that prints "hello" while the parent waits for it.

Process Tree

Every process except init has a parent. The whole system is a tree visible via pstree.

Summary

• fork duplicates a process; exec replaces its image; wait reaps the child.
• A child returns 0 from fork; the parent gets the child's PID.
• Zombies and orphans are normal transient states cleaned up by init.