There are a few differences between exit() and _exit()
that become significant when fork(), and especially
vfork(), is used.
The basic difference between exit() and _exit() is that
the former performs clean-up related to user-mode constructs in the
library, and calls user-supplied cleanup functions, whereas the latter
performs only the kernel cleanup for the process.
In the child branch of a fork(), it is normally incorrect to use
exit(), because that can lead to stdio buffers being flushed
twice, and temporary files being unexpectedly removed. In C++ code the
situation is worse, because destructors for static objects may be run
incorrectly. (There are some unusual cases, like daemons, where the
parent should call _exit() rather than the child; the
basic rule, applicable in the overwhelming majority of cases, is that
exit() should be called only once for each entry into
main.)
In the child branch of a vfork(), the use of exit() is
even more dangerous, since it will affect the state of the parent
process.