How do I automatically destroy child processes in Windows?

In C++ Windows app, I launch several long running child processes (currently I use CreateProcess(...) to do this.

I want the child processes to be automatically closed if my main processes crashes or is closed.

Because of the requirement that this needs to work for a crash of the "parent", I believe this would need to be done using some API/feature of the operating system. So that all the "child" processes are cleaned up.

How do I do this?

Solution 1:

The Windows API supports objects called "Job Objects". The following code will create a "job" that is configured to shut down all processes when the main application ends (when its handles are cleaned up). This code should only be run once.:

HANDLE ghJob = CreateJobObject( NULL, NULL); // GLOBAL
if( ghJob == NULL)
{
    ::MessageBox( 0, "Could not create job object", "TEST", MB_OK);
}
else
{
    JOBOBJECT_EXTENDED_LIMIT_INFORMATION jeli = { 0 };

    // Configure all child processes associated with the job to terminate when the
    jeli.BasicLimitInformation.LimitFlags = JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE;
    if( 0 == SetInformationJobObject( ghJob, JobObjectExtendedLimitInformation, &jeli, sizeof(jeli)))
    {
        ::MessageBox( 0, "Could not SetInformationJobObject", "TEST", MB_OK);
    }
}

Then when each child process is created, execute the following code to launch each child each process and add it to the job object:

STARTUPINFO info={sizeof(info)};
PROCESS_INFORMATION processInfo;

// Launch child process - example is notepad.exe
if (::CreateProcess( NULL, "notepad.exe", NULL, NULL, TRUE, 0, NULL, NULL, &info, &processInfo))
{
    ::MessageBox( 0, "CreateProcess succeeded.", "TEST", MB_OK);
    if(ghJob)
    {
        if(0 == AssignProcessToJobObject( ghJob, processInfo.hProcess))
        {
            ::MessageBox( 0, "Could not AssignProcessToObject", "TEST", MB_OK);
        }
    }

    // Can we free handles now? Not sure about this.
    //CloseHandle(processInfo.hProcess); 
    CloseHandle(processInfo.hThread);
}

VISTA NOTE: See AssignProcessToJobObject always return "access denied" on Vista if you encounter access-denied issues with AssignProcessToObject() on vista.

Solution 2:

One somewhat hackish solution would be for the parent process to attach to each child as a debugger (use DebugActiveProcess). When a debugger terminates all its debuggee processes are terminated as well.

A better solution (assuming you wrote the child processes as well) would be to have the child processes monitor the parent and exit if it goes away.

Solution 3:

Windows Job Objects sounds like a good place to start. The name of the Job Object would have to be well-known, or passed to the children (or inherit the handle). The children would need to be notice when the parent dies, either through a failed IPC "heartbeat" or just WFMO/WFSO on the parent's process handle. At that point any child process could TermianteJobObject to bring down the whole group.

Solution 4:

You can keep a separate watchdog process running. Its only task is watching the current process space to spot situations like you describe. It could even re-launch the original application after a crash or provide different options to the user, collect debug information, etc. Just try to keep it simple enough so that you don't need a second watchdog to watch the first one.