Re: [pstreams-users] Empty process output

[Jonathan W. <pst...@ka...>]

On 6 February 2013 11:04, Johannes Wienke wrote:
> Hi,
>
> I am trying to use pstreams to get the output from a system call to the
> Unix pstree command. On stackoverflow I found the following post:
> http://stackoverflow.com/questions/5309020/how-to-get-the-full-stream-output-with-pstreams/5309174
>
> So what I am currently using is this code to read the output:
>
> redi::ipstream in(boost::str(boost::format("pstree -pa %d") % pid));
> std::stringstream ss;
> ss << in.rdbuf();
> std::string pstreeOut = ss.str();
>
> if (pstreeOut.empty()) {
>     throw NoSuchProcessException(pid);
> }
>
> Most of the time this works, but sometimes the exception is triggered. I
> had the suspicion that this code actually doesn't wait for pstree to
> finish. Is anything wrong with the way I us pstrees?

Hi,

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.