fork.2: Make the example more robust

[0mp]

Summary

As noted by kib@:

Using printf around fork is not the best idea, and definitely should not
be provided as a guiding example in the man page. Using stdio safely around
fork() requires at least flushing buffers and ensuring that opened FILEs are
in some consistent state right at fork.

It would work by chance in your example, mostly because you did not use
anything in stdio before fork, but any further changes would result in
very puzzling bugs (for beginners, who are the target of this example).

Reported by: kib

[0mp]

@kib, I am not sure how to check if the FILEs are in a consistent state after fork. I was looking for examples in the base source code but I'm not sure what I'm looking for. Could you explain a bit more what should be in the manual page? Thanks!

[kib]

lib/libc/sys/fork.2

127

In your example, fflush(stdout) should be done before fork, so that buffers are flushed only once. Otherwise e.g. a buffer would be printed both by child and the original process. exit() takes care of flushing.

Alternative is to not use stdio in the child, and to exit child with _exit() (ANSI C _Exit()) instead of exit(), which also does not flush buffers.

[0mp]

I guess this has been superseded by https://reviews.freebsd.org/D31425