fork, exec, wait — The Process Triple

One call that returns twice

fork() (POSIX) duplicates the calling process. It returns once per process: 0 in the child, the child's pid in the parent. Everything else — memory, open files, cwd — is copied or shared copy-on-write. The child usually immediately calls one of the exec*() functions, which replaces the process image with a new program; a successful exec never returns.

Collecting the result

The parent reaps the child with waitpid(pid, &status, 0). The status word is not the exit code — it is a bitfield decoded by macros:

• WIFEXITED(status) — did the child exit normally?
• WEXITSTATUS(status) — the low 8 bits the child passed to exit(), _exit(), or return.

Two consequences to internalize:

1. Exit codes are 8 bits. _exit(256) is observed as 0; _exit(-1) as 255. Never encode data beyond 0–255 in an exit code.
2. Killed children did not exit. If a signal terminated the child, WIFEXITED is false and WEXITSTATUS is meaningless — check WIFSIGNALED and WTERMSIG.

Every command-line tool you have ever chained with && was orchestrating exactly this triple.

POSIX vs ISO C, again

Nothing in this lesson exists in ISO C. fork, exec, waitpid, _exit are POSIX. Portable ISO-only programs cannot create processes — and that limitation is itself part of C's story: the standard library stops at system() (which returns an implementation-defined status), and everything richer is platform work.