System Calls Fork and Exec – When to Use Them

execforksystem-calls

I'm learning about fork() and exec() commands. It seems like fork() and exec() are usually called together. (fork() creates a new child process, and exec() replaces the current process image with a new one.) However, in what scenarios might you call each function on its own? Are there scenarios like these?

Best Answer

Sure! A common pattern in "wrapper" programs is to do various things and then replace itself with some other program with only an exec call (no fork)

#!/bin/sh
export BLAH_API_KEY=blub
...
exec /the/thus/wrapped/program "$@"

A real-life example of this is GIT_SSH (though git(1) does also offer GIT_SSH_COMMAND if you do not want to do the above wrapper program method).

Fork-only is used when spawning a bunch of typically worker processes (e.g. Apache httpd in fork mode (though fork-only better suits processes that need to burn up the CPU and not those that twiddle their thumbs waiting for network I/O to happen)) or for privilege separation used by sshd and other programs on OpenBSD (no exec)

$ doas pkg_add pstree
...
$ pstree | grep sshd
 |-+= 70995 root /usr/sbin/sshd
 | \-+= 28571 root sshd: jhqdoe [priv] (sshd)
 |   \-+- 14625 jhqdoe sshd: jhqdoe@ttyp6 (sshd)

The root sshd has on client connect forked off a copy of itself (28571) and then another copy (14625) for the privilege separation.

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