Xen devel - Aug 2012 - [problem in `xl_cmdimpl.c`] Why pid return by fork() in parent process is not the same with pid returned by getpid()?

Hi all,
    In xen-4.1.2/tools/libxl/xl_cmdimpl.c : create_domain(), it starts up a
child daemon process waiting for the domain death,
there are several lines echo the child pid as follows:

1592        if (child1) {                                  /*it''s in
the
parent*/


1593            printf("Daemon running with PID %d\n", child1);


/**it''s in the child/
1643    LOG("Waiting for domain %s (domid %d) to die [pid %ld]",



1644        d_config.c_info.name, domid, (long)getpid());



After input `xl create xp-101.hvm`, you got `Daemon running with PID 26622`
following the command line in the screen; but the xl log file lies in
/var/log/xen/xl-xp-101.log contains a line `Waiting for domain xp-101
(domid 1) to die [pid 26624]`.
Why 26622 != 26624?  And 26622 could not be found by `ps -ef` command.
What happended to that?!


Thanks in advance!


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On Thu, 2012-08-02 at 10:47 +0100, 马磊 wrote:
>     In xen-4.1.2/tools/libxl/xl_cmdimpl.c : create_domain(), it starts
If you are doing development then target xen-unstable would be
preferable, especially if you are looking at (lib)xl. The xl stuff in
4.1 is effectively a preview and it has change substantially internally
since 4.1.
> After input `xl create xp-101.hvm`, you got `Daemon running with PID
> 26622` following the command line in the screen; but the xl log file
> lies in /var/log/xen/xl-xp-101.log contains a line `Waiting for domain
> xp-101 (domid 1) to die [pid 26624]`.
> Why 26622 != 26624?  And 26622 could not be found by `ps -ef` command.
> What happended to that?!
There's a call to dameon in there, which will involve a double fork
IIRC.

Ian.


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On Thu, Aug 2, 2012 at 5:55 PM, Ian Campbell
<Ian.Campbell@citrix.com>wrote:
> On Thu, 2012-08-02 at 10:47 +0100, 马磊 wrote:
>
> >     In xen-4.1.2/tools/libxl/xl_cmdimpl.c : create_domain(), it starts
>
> If you are doing development then target xen-unstable would be
> preferable, especially if you are looking at (lib)xl. The xl stuff in
> 4.1 is effectively a preview and it has change substantially internally
> since 4.1.
>
> > After input `xl create xp-101.hvm`, you got `Daemon running with PID
> > 26622` following the command line in the screen; but the xl log file
> > lies in /var/log/xen/xl-xp-101.log contains a line `Waiting for domain
> > xp-101 (domid 1) to die [pid 26624]`.
> > Why 26622 != 26624?  And 26622 could not be found by `ps -ef` command.
> > What happended to that?!
>
> There''s a call to dameon in there, which will involve a double
fork
> IIRC.
>
> Ian.
>
>
    What do you mean by `double fork`? In my opinion, the parent exit after
invoking waitpid, and the child goes on toward the rest of the code.
    The PID recorded in xl log file  can''t also be found by `ps -ef`
command.  How should I retrive the PID of the real daemon process?


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On Thu, Aug 2, 2012 at 5:55 PM, Ian Campbell
<Ian.Campbell@citrix.com>wrote:
> On Thu, 2012-08-02 at 10:47 +0100, 马磊 wrote:
>
> >     In xen-4.1.2/tools/libxl/xl_cmdimpl.c : create_domain(), it starts
>
> If you are doing development then target xen-unstable would be
> preferable, especially if you are looking at (lib)xl. The xl stuff in
> 4.1 is effectively a preview and it has change substantially internally
> since 4.1.
>
> > After input `xl create xp-101.hvm`, you got `Daemon running with PID
> > 26622` following the command line in the screen; but the xl log file
> > lies in /var/log/xen/xl-xp-101.log contains a line `Waiting for domain
> > xp-101 (domid 1) to die [pid 26624]`.
> > Why 26622 != 26624?  And 26622 could not be found by `ps -ef` command.
> > What happended to that?!
>
> There''s a call to dameon in there, which will involve a double
fork
> IIRC.
>
> Ian.
>
> I only to see `libxl_fork` in function create_domain() who invokes fork()
once, where is the second fork()?


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On Tue, 2012-08-07 at 09:14 +0100, 马磊 wrote:
>         There's a call to dameon in there, which will involve a double
>         fork
>         IIRC.
>         
>         Ian.
>         
> I only to see `libxl_fork` in function create_domain() who invokes
> fork() once, where is the second fork()?
As I said (but misspelt, sorry) There's a call to daemon() there. Have
you read the manpage for that function?

Ian.


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