Process Management

Every process on Linux descends from init. The process tree is the fundamental organizing structure of the OS.

What Is a Process?

A process is an instance of a running program. It has its own virtual address space, open file descriptors, signal handlers, working directory, and scheduling state. The kernel represents each process with a task_struct.

`fork()` — Creating Processes

fork() creates an exact copy of the calling process. The child gets:

A copy of the parent’s virtual address space (copy-on-write)
Copies of open file descriptors (pointing to the same underlying files)
A new PID; the parent’s PID becomes the child’s PPID

pid_t pid = fork();
if (pid == 0) {
    // child process
} else if (pid > 0) {
    // parent process; pid = child's PID
} else {
    // fork failed
}

`exec()` — Replacing a Process Image

exec replaces the current process’s image with a new program. The PID stays the same; everything else (address space, open FDs, etc.) is replaced. fork + exec is the standard pattern for launching new programs.

`wait()` — Reaping Children

When a child exits, it becomes a zombie — its resources are freed, but its exit status is held until the parent calls wait() or waitpid(). If the parent never waits, the zombie lingers until the parent exits (then init adopts and reaps it).

Process Groups and Sessions

Every process belongs to a process group. Process groups belong to sessions. These groupings are used for job control and signal delivery:

SIGINT (Ctrl-C) goes to the entire foreground process group
SIGHUP goes to all processes in a session when the controlling terminal closes

File Descriptors Across fork

After fork, parent and child share the same underlying open file descriptions (same offset, same flags). This is how pipes work — you fork, then each side closes the end it doesn’t need.

Open Questions

How does clone() differ from fork()? (Hint: threads use clone with CLONE_VM)
What happens to mmap’d regions across fork?