Comment by kazinator

> So why is there not a fork-exec combo?

posix_spawn

> Why would anyone ever want fork as a primitive?

With fork you can very easily write a sever like mini_httpd:

https://acme.com/software/mini_httpd/

Or, in Unix shells:

  # function1 and funtion2 are shell functions

  $ function1 | grep foo | function2 

here, the shell must fork a process (without exec) to run one of these functions.

For instance function1 might run in a fork, the grep is a fork and exec of course, and function2 could be in the shell's primary process.

In the POSIX shell language, fork is so tightly integrated that you can access it just by parenthesizing commands:

  $ (cd /path/to/whatever; command) && other command

Everything in the parentheses is a sub-process; the effect of the cd, and any variable assignments, are lost (whether exported to the environment or not).

In Lisp terms, fork makes everything dynamically scoped, and rebinds it in the child's context: except for inherited resources like signal handlers and file descriptors.

Imagine every memory location having *earmuffs* like a defvar, and being bound to its current value by a giant let, and imagine that being blindingly efficient to do thanks to VM hardware.