Hello,

I'm writing a program in C which uses a pipe()/fork()/exec() mechanism
to execute a program as a child and make it communicate with the parent
process. At some point, the child process is supposed to stop but
read()
doesn't seem to get an EOF and the parent hangs.
'ps' reports that the child process is defunct.
If I execute the child from a terminal and do exactly the same things
as
the master process does, it exits normally.
What can I do to get things to work properly ?

Thanks in advance for your help.

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Julien wrote:
> Hello,
>
> I'm writing a program in C which uses a pipe()/fork()/exec() mechanism
> to execute a program as a child and make it communicate with the parent
> process. At some point, the child process is supposed to stop but
> read()
> doesn't seem to get an EOF and the parent hangs.
> 'ps' reports that the child process is defunct.
Oh - means it exited. You need call wait/waitpid/or similar
on the child (do it in a signal handler for SIGCHLD). Processes
stick around untill you call wait on them
> If I execute the child from a terminal and do exactly the same things
> as
>  the master process does, it exits normally.
> What can I do to get things to work properly ?
>
> Thanks in advance for your help.
Did you remember to close the writing end (or atleast the one you don't
use) in the child, and close the reading end in the parent ?

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Julien wrote:
> Hello,
>
> I'm writing a program in C which uses a pipe()/fork()/exec() mechanism
> to execute a program as a child and make it communicate with the parent
> process. At some point, the child process is supposed to stop but
> read()
> doesn't seem to get an EOF and the parent hangs.
> 'ps' reports that the child process is defunct.

Which order are things supposed to die?  If the child is defunct, the
parent must have terminated.


--
Ian Collins.

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In article <1139090292.579903@drone2-svc-skyt.qsi.net.nz>,
Ian Collins <ian-news@hotmail.com> wrote:

> Julien wrote: 
>
> Which order are things supposed to die?  If the child is defunct, the
> parent must have terminated.

No, it's the other way around.  Defunct means the child has died, but
the parent hasn't yet called one of the wait() functions to get its
termination status.

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***

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Barry Margolin wrote: 
>
>
> No, it's the other way around.  Defunct means the child has died, but
> the parent hasn't yet called one of the wait() functions to get its
> termination status.
>
Doesn't this also happen if the parent has terminated before the child
and can't call one of the wait() functions, or does init clean up?

--
Ian Collins.

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In article <1139094474.89209@drone2-svc-skyt.qsi.net.nz>,
Ian Collins <ian-news@hotmail.com> wrote:

> Barry Margolin wrote: 
> Doesn't this also happen if the parent has terminated before the child
> and can't call one of the wait() functions, or does init clean up?

When the parent terminates first, init becomes the parent.  Then the
definition becomes the same -- when the child terminates, it will become
a zombie for the brief period until init calls wait().

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***

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Julien wrote:
> 'ps' reports that the child process is defunct.
http://groups.google.com/group/comp...br />
ce&hl=en
Message-ID: <d85eb83f.0312180842.21c629d3@posting.google.com>
ps(1)
wait(2)
etc.

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Thanks for the answers.

[vbcol=seagreen] 


Yes, and I can read and write through the pipes. I actually get what I
expect from the external program that is launched as child.

I did some rewriting and the child process now ends normally (not
defunct anymore) ; waitpid returns the child process' pid.
The while(r=read(...)){...} loop still hangs however because there is
nothing to read in the pipe's buffer. Why doesn't it get EOF ?

I don't know if that has something to do with the problem, but before
terminating, the child spawns another process, which runs in the
background and has init as parent.

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Julien wrote:
> Thanks for the answers.
>
> 
>
>
> Yes, and I can read and write through the pipes. I actually get what I
> expect from the external program that is launched as child.
>
> I did some rewriting and the child process now ends normally (not
> defunct anymore) ; waitpid returns the child process' pid.
> The while(r=read(...)){...} loop still hangs however because there is
> nothing to read in the pipe's buffer. Why doesn't it get EOF ?
>
> I don't know if that has something to do with the problem, but before
> terminating, the child spawns another process, which runs in the
> background and has init as parent.
>

If the grand child inherits a copy of the pipe to the parent,
the pipe will still have an end open when the child exits, so
the parent will not see EOF.   (You can have this same sort of
problem if a parent is spawning multiple children.  A subsequent
child can inherit the parent's end of a pipe to a previous
child, making it impossible for the parent to close() the
pipe to the previous child and have it get EOF.)

--
Clem
"If you push something hard enough, it will fall over."
- Fudd's first law of opposition

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> If the grand child inherits a copy of the pipe to the parent,
> the pipe will still have an end open when the child exits, so
> the parent will not see EOF.   (You can have this same sort of
> problem if a parent is spawning multiple children.  A subsequent
> child can inherit the parent's end of a pipe to a previous
> child, making it impossible for the parent to close() the
> pipe to the previous child and have it get EOF.)

Is there anyway to check that ? It would be strange since the grand
child is supposed to be in the background, so I guess it doesn't need
any stdio.

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