12-19-2016
Quote:
Originally Posted by
SirSalt
Alright, thank you for clarifying that
---------- Post updated 12-18-16 at 09:20 PM ---------- Previous update was 12-17-16 at 10:28 PM ----------
As I meditated on what you said, I thought of something. In your #3 explanation, would the tty be process "A"? Or would the tty in your example be process "B"? I guess I have a slight and subtle confusion about using "fork" and "forked" in the verb context. When you said "You actually forked a child ("B")", was forking the child a result of calling fork() in process "A", or calling fork() in process "B"? I hope you can see what I'm getting at here. :P
In #3, A calls
fork(). At that point you have processes A and B. Then B calls
fork(). At that point you have processes A, B, and C all running the same instructions. Then B calls
exit() leaving you with processes A and C running. Then C calls
execl() (or another function from the
exec family) to replace the instructions C was running with the instructions needed to run the daemon. At that point you then have A running the code it was running and you have C running your daemon.
Note that since C was a child of B and B exited, A cannot use
wait() to determine whether or not C is still running and cannot retrieve the exit status of C using
wait(). (Grandparents do not become the parents of their children's orphaned children when their children die before their grandchildren.)
This User Gave Thanks to Don Cragun For This Post:
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LEARN ABOUT FREEBSD
vfork
VFORK(2) BSD System Calls Manual VFORK(2)
NAME
vfork -- create a new process without copying the address space
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
pid_t
vfork(void);
DESCRIPTION
The vfork() system call can be used to create new processes without fully copying the address space of the old process, which is horrendously
inefficient in a paged environment. It is useful when the purpose of fork(2) would have been to create a new system context for an
execve(2). The vfork() system call differs from fork(2) in that the child borrows the parent's memory and thread of control until a call to
execve(2) or an exit (either by a call to _exit(2) or abnormally). The parent process is suspended while the child is using its resources.
The vfork() system call returns 0 in the child's context and (later) the pid of the child in the parent's context.
The vfork() system call can normally be used just like fork(2). It does not work, however, to return while running in the child's context
from the procedure that called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Be
careful, also, to call _exit(2) rather than exit(3) if you cannot execve(2), since exit(3) will flush and close standard I/O channels, and
thereby mess up the parent processes standard I/O data structures. (Even with fork(2) it is wrong to call exit(3) since buffered data would
then be flushed twice.)
RETURN VALUES
Same as for fork(2).
SEE ALSO
_exit(2), execve(2), fork(2), rfork(2), sigaction(2), wait(2), exit(3)
HISTORY
The vfork() system call appeared in 2.9BSD.
BUGS
To avoid a possible deadlock situation, processes that are children in the middle of a vfork() are never sent SIGTTOU or SIGTTIN signals;
rather, output or ioctl(2) calls are allowed and input attempts result in an end-of-file indication.
BSD
November 13, 2009 BSD