Re: [pstreams-users] Empty process output

[Johannes W. <jw...@te...>]

Hi,

Thanks for the quick reply!

On 02/06/2013 02:19 PM, Jonathan Wakely wrote:
> The expression 'ss << in.rdbuf()' will read from the pstreambuf until
> End-Of-File is reached, which will only happen when the child process
> closes its end of the pipe, which in your program will happen after
> 'pstree' exits. So it should always wait for 'pstree' to finish, as
> demonstrated by this program which takes 5 seconds to print the
> output, because it has to wait for the child to finish sleeping and
> exit:
> 
> #include <pstreams/pstream.h>
> #include <sstream>
> #include <iostream>
> 
> int main(int argc, char** argv)
> {
>     std::string pid = argc > 1 ? argv[1] : "1";
>     redi::ipstream in("pstree -pa " + pid + " && sleep 5");
>     std::stringstream ss;
>     ss << in.rdbuf();
>     std::string pstreeOut = ss.str();
> 
>     if (pstreeOut.empty()) {
>         throw std::runtime_error("No such process: " + pid);
>     }
>     std::cout << pstreeOut << '\n';
> }
> 
> My guess would be that the process you are trying to inspect has
> exited and so pstree produces no output.  On my GNU/Linux system
> 'pstree' doesn't print an error or exit with non-zero status for a
> non-existent PID, which isn't very helpful.  If the processes you are
> trying to view are owned by you then you could use this command:
> 
> boost::format("kill -s 0 %d 2>&1 && pstree -pa %d") % pid % pid
> 
> The kill command will not send a signal, but will print an error and
> exit with a non-zero status if the process doesn't exist.

Actually the process I am inspecting is still alive, but there seems to
be a bug in pstree when a thread of that process goes away. In that case
pstree tries to access the thread's stat-file and exists with "No such
file or directory" on stderr. Not exactly what I expected from such a
tool... I will search for a different way to find all subprocesses.

Thanks,
Johannes