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Top Forums Programming Question about execl, replacing process's contents Post 302987987 by Don Cragun on Monday 19th of December 2016 09:49:59 AM
Old 12-19-2016
Quote:
Originally Posted by SirSalt
Alright, thank you for clarifying that Smilie

---------- 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.)
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FORK(2) 							System Calls Manual							   FORK(2)

NAME
fork - spawn new process SYNOPSIS
fork( ) DESCRIPTION
Fork is the only way new processes are created. The new process's core image is a copy of that of the caller of fork. The only distinc- tion is the fact that the value returned in the old (parent) process contains the process ID of the new (child) process, while the value returned in the child is 0. Process ID's range from 1 to 30,000. This process ID is used by wait(2). Files open before the fork are shared, and have a common read-write pointer. In particular, this is the way that standard input and output files are passed and also how pipes are set up. SEE ALSO
wait(2), exec(2) DIAGNOSTICS
Returns -1 and fails to create a process if: there is inadequate swap space, the user is not super-user and has too many processes, or the system's process table is full. Only the super-user can take the last process-table slot. ASSEMBLER
(fork = 2.) sys fork (new process return) (old process return, new process ID in r0) The return locations in the old and new process differ by one word. The C-bit is set in the old process if a new process could not be cre- ated. FORK(2)
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