How to bring a child process running in the background to the foreground

Question

If I used fork() and execv() to spawn several child processes running in the background and I wanted to bring one of them to the foreground, how could I do that?

I am trying to write a shell that can start processes either in the foreground or background.

score 5 · Answer 1 · answered Apr 26 '11 at 04:43

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"Background" and "foreground" are not terms used generically for processes, but rather only apply to shells which can wait for jobs on demand.

answered Apr 26 '11 at 04:43

Ignacio Vazquez-Abrams

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+1 this is correct. A "foreground" process is one which is receiving stdin and writing stdout/stderr. The OS itself doesn't have a concept of this. – Borealid Apr 26 '11 at 04:46
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Receiving stdin. Even background processes can write to stdout and stderr. – Ignacio Vazquez-Abrams Apr 26 '11 at 04:48
It would be worth adding, that if the original poster meant they need the parent to stop doing anything until a particular child finishes, they just need to `waitpid(2)` for it. – Jan Hudec Apr 26 '11 at 06:28
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The answer is interesting, but doesn't help the OP. – Matt Joiner Apr 27 '11 at 01:27
@Matt: Unless the OP describes *how* they put the processes in the background, there's no way to help. – Ignacio Vazquez-Abrams Apr 27 '11 at 01:42
I already explained how. I use fork and in the child process I use execv to start the process. – node ninja Apr 28 '11 at 06:07
Except **that doesn't put the process in the background.** – Ignacio Vazquez-Abrams Apr 28 '11 at 06:11
After I create the process I don't wait for it. Normally, the parent waits for the child process to finish. This is called running in the foreground. If not waiting is not the same as running in the background, then what does that mean? – node ninja Apr 30 '11 at 05:55
It means that the parent is not waiting. The child still runs, and is connected to the same stdin, stdout, and stderr. – Ignacio Vazquez-Abrams Apr 30 '11 at 06:13
... So are you saying that you want the parent to control the child's access to stdin? – Ignacio Vazquez-Abrams May 01 '11 at 01:40

score 3 · Answer 2 · edited May 23 '17 at 10:34

Complimentarily to Ignacio Vazquez-Abram's answer, I suggest that you emulate the shell foreground/background model.

As far as I can tell, backgrounding a process means suspending it. The easiest way to do this is through SIGSTOP. When you foreground a process, send it SIGCONT. As long as only one of your "jobs" are currently in the foreground, it will be the only one read and writing to the session's tty.

kill(child_pid, SIGSTOP);
kill(child_pid, SIGCONT);

You may want to suspend each process after you fork, and before you execv, and give the user of your shell the option to foreground them later to maintain the invariant.

if (!fork()) { // we are the child
    raise(SIGSTOP); // suspend self
    execv(...); // run the command (after we've been resumed)

Here are some related links I found:

I don't think running a program in the background means suspending it. It means that while the program runs the parent runs. — node ninja, Apr 29 '11 at 21:02
There is no concept of foreground or background when it comes to programs (processes). This is a shell concept. See Ignacio Vazquez-Abrams answer for more on this. — Matt Joiner, Apr 30 '11 at 12:08

score 1 · Answer 3 · answered Apr 26 '11 at 06:13

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you can do use fg to bring process to foreground and bg to put process to background. you should know the pid of the process to bring it to foreground. refer linux manual of fg and bg for more info

answered Apr 26 '11 at 06:13

swd

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How to bring a child process running in the background to the foreground

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