4.6.2.1 Forking a process

Function: fork

Function: fork parent-proc child-thunk

Interface to the C function fork(), (libc)fork. The behaviour depends upon the number of arguments:

• When called with no arguments: create a new process by forking the current one; if successful return a non–negative fixnum representing the return value of fork(), else raise an exception. The return value is: 0 in the child process; the child process pid in the parent.
• When called with two arguments: create a new process by forking the current one; if successful the parent process applies parent-proc to a fixnum representing the child process pid and return its return value; if successful: the child process evaluates the child-thunk and returns its return value; if an error occurs: an exception is raised.

When forking a process we must remember about input/output ports:

1. Before forking, in the parent process we should call flush-ports-in-close-on-exec-mode; flush-ports-in-close-on-exec-mode, for details.
2. After forking, in the child process we should call close-ports-in-close-on-exec-mode; close-ports-in-close-on-exec-mode, for details.

we must also remember that some file descriptors are marked to be “closed on exec”.

Here is a simple example of forking a process:

#!r6rs
(import (vicare)
  (prefix (vicare posix) px.))

(px.fork
 (lambda (child-pid)
   (printf "in parent pid = ~a, child pid = ~s\n"
	   (px.getpid) child-pid)
   (flush-output-port (current-output-port)))
 (lambda ()
   (printf "in child, pid = ~a\n" (px.getpid))
   (flush-output-port (current-output-port))
   (exit)))

(printf "here we are in the parent\n")
(flush-output-port (current-output-port))