How to timeout an ipstream process

[Brian G]

Is it possible to timeout an ipstream process after a defined period?  I am issuing a lpd print queue command and if there is not a response within 5 seconds I want to stop the ipstream process and then call an error routine.

[Jonathan Wakely]

PStreams doesn't currently provide a way of setting a timeout, but you can use an alarm signal to interrupt the process while it's waiting for data. I hope this program demonstrates the idea:

#include <signal.h>
#include <time.h>
#include <unistd.h>
#include <pstream.h>

// whether we timed out or not
volatile sig_atomic_t timed_out = 0;

void signal_handler(int)
{
    timed_out = 1;
}

int main()
{
    using namespace redi;

    sigset_t block_mask;
    sigemptyset(&block_mask);

    struct sigaction action;
    action.sa_handler = signal_handler;
    action.sa_mask = block_mask;
    action.sa_flags = 0;

    // set signal handler for SIGALRM
    sigaction(SIGALRM, &action, 0);

    srand(time(0));
    int timeout = rand() % 4 + 1;
    std::cout << "Time out in " << timeout << " seconds" << std::endl;

    ipstream child("sleep 2 && echo finished");

    alarm(timeout); // raise SIGALRM after timeout seconds

    std::string s;
    std::getline(child, s);

    if (timed_out)
    {
        std::cerr << "Timed out\n";
        child.rdbuf()->kill();
        return EXIT_FAILURE;
    }
    else
    {
        std::cout << s << std::endl;
        return EXIT_SUCCESS;
    }
}

If getline() does not return before the alarm it will be interrupted, timed_out will be set, control will return to the main function and the error handling will be run.

I hadn't planned to make PStreams include this functionality as it might interfere with other signal handling in the program, but since using it would be optional I will reconsider it.

[Jonathan Wakely]

I should add that the example program is incomplete. In a real program you would have to reset the alarm to prevent it timing out after a successful read. You should also restore the default action for the SIGALRM signal.

[Brian G]

Thanks for the response.  A further question:

I am using AIX and its qchk command.

A successful qchk command displays soething similar to the following:

qchk -P R1129
Queue   Dev   Status    Job Files              User         PP %   Blks  Cp Rnk
------- ----- --------- --- ------------------ ---------- ---- -- ----- --- ---
R1129   hp4mv READY   
R1129: JetDirect lpd: no jobs queued on the port Auto

A failed qchk command looks similar to the following:

qchk -P Q1111
Queue   Dev   Status    Job Files              User         PP %   Blks  Cp Rnk
------- ----- --------- --- ------------------ ---------- ---- -- ----- --- ---
Q1111   hp4si READY   

: (WARNING) 0781-375 Connection to server failed.
rembak: errno = 81: A route to the remote host is not available.
Q1111   queue HOST_DOWN

I use the following code to get the output:

ipstream qchk("qchk -P Q1111");
string buffer;

while ( getline(qchk, buffer) ) {
   cout << buffer << endl;
}

A failed qchk command immediately displays:

qchk -P Q1111
Queue   Dev   Status    Job Files              User         PP %   Blks  Cp Rnk
------- ----- --------- --- ------------------ ---------- ---- -- ----- --- ---
Q1111   hp4si READY

and then after 45 seconds displays the remainder of the text (: (WARNING) ... ).

The following code still waits 45 seconds (the remaining code is identical to that which you provided):

...
...
while ( getline(qchk, buffer) )
   {
      if (timed_out)
      {
         cerr << "Timed out\n";
         qchk.rdbuf()->kill(9);
      }
      else
      {
         qchkVct.push_back (buffer);
      }
   }
   if (! timed_out)
   {
      copy(qchkVct.begin(), qchkVct.end(), ostream_iterator<string>(cout, "\n"));
   }

Would you know why it still waiting the 45 seconds?

[Jonathan Wakely]

Hi Brian,

I've never tested PStreams on AIX, so although it *should* work exactly the same I can't guarantee it.

I'm not sure why your program is still pausing, but I'll think about it and see if I can figure it out...

jon