Question

I have very primitive web front-end for my C++ application. Client (web browser) is enters php site and fills form with parameters. Than (after post submit) php calls exec and application does its work.
Application can work longer than minute and requires pretty large amount of RAM.

Is there any possibility to detect disconnecting from client (for example closure of tab in web browser). I want to do this, because after disconnecting client will not be able to see result of computations, so I can kill application and free some RAM on server.

Thanks for any help or suggestions.

Answer 1

As long as the C++ program produces output while running, rather than generating all the output just prior to termination, use passthru() instead of exec().

This causes PHP to flush the output to the client as the content is produced, which allows PHP to detect when clients disconnect. PHP will terminate when the client disconnects and kill the child process immediately (as long as ignore_user_abort() is not set).

Example:

<?php

  function exec_unix_bg ($cmd) {
    // Executes $cmd in the background and returns the PID as an integer
    return (int) exec("$cmd > /dev/null 2>&1 & echo $!");
  }
  function pid_exists ($pid) {
    // Checks whether a process with ID $pid is running
    // There is probably a better way to do this
    return (bool) trim(exec("ps | grep \"^$pid \""));
  }

  $cmd = "/path/to/your/cpp arg_1 arg_2 arg_n";

  // Start the C++ program
  $pid = exec_unix_bg($cmd);

  // Ignore user aborts to allow us to dispatch a signal to the child
  ignore_user_abort(1);

  // Loop until the program completes
  while (pid_exists($pid)) {

    // Push some harmless data to the client
    echo " ";
    flush();

    // Check whether the client has disconnected
    if (connection_aborted()) {
      posix_kill($pid, SIGTERM); // Or SIGKILL, or whatever
      exit;
    }

    // Could be done better? Only here to prevent runaway CPU
    sleep(1);

  }

  // The process has finished. Do your thang here.

To collect the program’s output, redirect the output to a file instead of /dev/null. I suspect you will need pcntl installed as well as posix for this, since the PHP manual indicates the SIGxxx constants are defined by the pcntl extension – although I have never had one installed without the other so I’m not sure either way.