I've been trying to figure out how the fork-exec mechanism is used inside Linux. Everything was going on according to the plan until some web pages started to confuse me.
It is said that a child process should strictly use _exit()
instead of a simple exit()
or a normal return from main()
.
As I know, Linux shell fork-execs every one of the external commands; assuming what I said above is true, the conclusion is that none of these external commands nor any other execution happening inside the Linux shell can do normal return!
Wikipedia & some other webpages claim we've got to use _exit()
just to prevent a child process causing deletion of parent's temporary files while a probable double flushing of stdio buffers may happen. though I understand the former, I have no clues how a double flushing of buffers could be harmful to a Linux system.
I've spent my whole day on this... Thanks for any clarification.
You should use _exit
(or its synonym _Exit
) to abort the child program when the exec
fails, because in this situation, the child process may interfere with the parent process' external data (files) by calling its atexit
handlers, calling its signal handlers, and/or flushing buffers.
For the same reason, you should also use _exit
in any child process that does not do an exec
, but those are rare.
In all other cases, just use exit
. As you partially noted yourself, every process in Unix/Linux (except one, init
) is the child of another process, so using _exit
in every child process would mean that exit
is useless outside of init
.
switch (fork()) {
case 0:
// we're the child
execlp("some", "program", NULL);
_exit(1); // <-- HERE
case -1:
// error, no fork done ...
default:
// we're the parent ...
}