# Why does popen3 fork a grandchild? **URL:** \ **Category:** Ruby\ **Created:** [March 15, 2007, 8:35am UTC](https://www.ruby-forum.com/t/why-does-popen3-fork-a-grandchild/88878 "2007-03-15T08:35:24Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![greg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/71c47a/32.png) [@greg](https://www.ruby-forum.com/u/greg)\ **Post date:** [March 15, 2007, 8:35am UTC](https://www.ruby-forum.com/t/why-does-popen3-fork-a-grandchild/88878/1 "2007-03-15T08:35:24Z")
So I wanted to know why the global variable $? wasn’t being set to reflect the exit status of the subprocess when using popen3; I checked the list archives and saw that this has come up many times and that the cause is that popen3 forks a child, then from the child forks another, effectively a “grandchild” process. One of the consequences is that the exit status is effectively thrown away. The workarounds suggested in the archives are: - don’t use popen3; find an alternative - instead of checking $? make sure that nothing is printed to the standard err - append the exit code to the standard error as the last line and extract it later - write a popen3 replacement that uses a child rather than a grandchild So I’ve tried the latter option but I’m still left with the curiosity… why does popen3 use this child/grandchild model? What are the advantages of it? Cheers, Greg ---
**Author:** ![greg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/71c47a/32.png) [@greg](https://www.ruby-forum.com/u/greg)\ **Post date:** [March 16, 2007, 12:46am UTC](https://www.ruby-forum.com/t/why-does-popen3-fork-a-grandchild/88878/2 "2007-03-16T00:46:01Z")
Greg H. wrote: > why does popen3 use this child/grandchild model? It orphans the child process, so its parent doesn’t need to clean it up when it dies. Instead, the process is adopted by “init”, which is process-id 1, which takes responsibility for the cadaver. To run completely isolated from the calling process’ environment, it must become a leader of a new process group and drop its controlling terminal. In BSD-derived system, the latter can be done by calling the TIOCNTTY ioctl, and the latter via setgprp(). On AT&T UNIX systems however, setpgrp performs both functions, but only if it’s the first time this call has been made in this process. As a result, it must first fork(), then setpgrp(), then fork again. I’m not sure whether all this is completely relevant to popen3, but there might be subtle issues on some systems if the code was changed to use only one fork. For example, on some Unix systems, a terminal (tty) device doesn’t become available until the no process has that tty as a controlling tty - so “login” can’t issue a login prompt. If you pipe output to a command and then exit while it continues to process that data, then the user logs out, no-one won’t be able to log in again on that tty until the background process exits. All this was thoroughly examined by Dave Lennert in his classic 1987 posting entitled “How to write a Unix Daemon” to the HP internal newsgroup “hp.unix”, which I archived and still have. The paper was also published in “;login:”, Volume 12 No 4, July/August 1987. The paper seems rather hard to find online now. Clifford H… ---
**Author:** ![greg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/71c47a/32.png) [@greg](https://www.ruby-forum.com/u/greg)\ **Post date:** [March 16, 2007, 1:26am UTC](https://www.ruby-forum.com/t/why-does-popen3-fork-a-grandchild/88878/3 "2007-03-16T01:26:04Z")
On Fri, 16 Mar 2007, Clifford H. wrote: > It orphans the child process, so its parent doesn’t need to clean > it up when it dies. Instead, the process is adopted by “init”, > which is process-id 1, which takes responsibility for the cadaver. Thanks for the substantive reply, saves me the bother of doing a poorer one… > All this was thoroughly examined by Dave Lennert in his classic > 1987 posting entitled “How to write a Unix Daemon” to the HP > internal newsgroup “hp.unix”, which I archived and still have. > The paper was also published in “;login:”, Volume 12 No 4, > July/August 1987. The paper seems rather hard to find online now. Can you post a link, or email me a copy? Thanks John C. Phone : (64)(3) 358 6639 Tait Electronics Fax : (64)(3) 359 4632 PO Box 1645 Christchurch Email : removed\_email\_address@domain.invalid New Zealand ---
**Author:** ![greg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/71c47a/32.png) [@greg](https://www.ruby-forum.com/u/greg)\ **Post date:** [March 16, 2007, 8:12am UTC](https://www.ruby-forum.com/t/why-does-popen3-fork-a-grandchild/88878/4 "2007-03-16T08:12:31Z")
John C. wrote: > Can you post a link, or email me a copy? [http://cjh.polyplex.org/software/daemon.pdf](http://cjh.polyplex.org/software/daemon.pdf) I had to dig up the “mm” macros, then groff to ps, then ps2pdf 🙂 There seem to be unnecessary page breaks, but I didn’t change Dave Lennert’s source. Clifford H… ---
**Author:** ![greg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/71c47a/32.png) [@greg](https://www.ruby-forum.com/u/greg)\ **Post date:** [March 16, 2007, 10:45pm UTC](https://www.ruby-forum.com/t/why-does-popen3-fork-a-grandchild/88878/5 "2007-03-16T22:45:52Z")
Greg H. wrote: > Many thanks for sharing this, Clifford! You’re welcome. I always thought it was a “keeper”. ---
**Author:** ![greg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/71c47a/32.png) [@greg](https://www.ruby-forum.com/u/greg)\ **Post date:** [March 16, 2007, 1:27pm UTC](https://www.ruby-forum.com/t/why-does-popen3-fork-a-grandchild/88878/6 "2007-03-16T13:27:11Z")
On 16 mar, 08:08, Clifford H. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I had to dig up the “mm” macros, then groff to ps, then ps2pdf 🙂 > There seem to be unnecessary page breaks, but I didn’t change > Dave Lennert’s source. Many thanks for sharing this, Clifford! Cheers, Greg