I copied some rows from the vfork man page.
"The vfork() function can normally be used just like fork(). It does not work, however, to return while running in the child's context from the caller of vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Be careful, also, to call _exit() rather than exit() if you cannot exec, since exit() flushes and closes standard I/O channels, thereby damaging the parent process' standard I/O data structures. (Even with fork(), it is wrong to call exit(), since buffered data would then be flushed twice.)"

I don't understand the difference between _exit and exit could some one please tell me?
Thanks moonz

Found this in a Google search:
http://linux.about.com/library/cmd/blcmdl2_exit.htm

Hi thank you for your replie!
But according to this document there isn't any difference, but there is.
I found this searching the web with google
For a discussion on the effects of an exit, the transmission of exit
status, zombie processes, signals sent, etc., see exit(3).

The function _exit() is like exit(), but does not call any functions
registered with atexit() or on_exit(). Whether it flushes standard I/O
buffers and removes temporary files created with tmpfile(3) is imple-
mentation dependent. On the other hand, _exit() does close open file
descriptors, and this may cause an unknown delay, waiting for pending
output to finish. If the delay is undesired, it may be useful to call
functions like tcflush() before calling _exit(). Whether any pending
I/O is cancelled, and which pending I/O may be cancelled upon _exit(),
is implementation-dependent.

At this location http://man-wiki.net/index.php/2:_exit
Sorry that i wasted your time!
Moonz

It is often the case, in the Linux/Unix world, that names beginning with "_" refer to internal or primitive routines.