I'm using a List of QProcess objects to keep track of some processes that need to be start/stopped at user defined intervals.
I'm able to start and stop the processes OK. But the issue arises when I stop a process using the following methods (Pseudo code):
process->start("PathToProcess","Some Arguments");
//Do some stuff.
process->terminate();
However, if I try to start the process again at another time, I get the error:
QProcess::start: Process is already running
I can do a ps -ef|grep processName and find that it is indeed dead, but it's sitting in a defunct state which I think is preventing me from starting it again.
What do I need to do to prevent this defunct state, or remove the defunct method so I can start my process again without reconstruction?
Figured out what was causing the error.
In qprocess_unix.cpp, you'll find a class called QProcessManager. Essentially this class has signal handlers that watch for child processes that have died. When a child dies, the QProcessManager sends a message across a pipe that lets the QProcess class know that it terminated/died.
In a unrelated part of my code, I had set up some signal catching statements that I used for various puposes. However, these signal catches were catching my SIGCHLD event and thus the QProcessManager was never being triggered to pipe to the QProcess that it died.
In my case, my only options are to either watch for the death of the child manually or to remove the signal catching I'm performing in my other sections of code.
For future reference, if you have this problem, you may be better off doing POSIX calls for kills and terminates, and checking the return value of those calls manually. If success, perform a:
process->setProcessState(ProcessState::NotRunning);//Specify the process is no longer running
waitpid(process->pid(),NULL,WNOHANG); //Clear the defunct process.
Thanks all.
Call process->waitForFinished() after calling process->terminate() in order to reap the zombie process. Then you can reuse the process object.
Related
In my Qt C++ program I created a process as follows:
myProcess = new QProcess();
myProcess->start(programpath, arguments);
Terminating the process is handled as follows:
myProcess->terminate();
Terminating the child process with QProcess::kill(),terminate() or close() works, but I don't want to use it because it doesn't give the child process a chance to clean up before exiting.
Is there any other way to exit the process? Thanks!
The polite way would be for the parent process to politely ask the child process to go away. Then when the child process exits (of its own volition), the QProcess object will emit a finished(int, QProcess::ExitStatus) signal, and you can have a slot connected to that signal that will continue your process (e.g. by deleting the QProcess object at that time). (Or if you don't mind blocking your Qt event loop for a little while, you could just call waitForFinished() on the QProcess object after asking it to exit, and waitForFinished() won't return until the process has gone away or the timeout period has elapsed)
Of course for the above to work you need some way to ask the child process to exit. How you go about doing that will depend on what the child process is running. If you're lucky, you are in control of the child process's code, in which case you can modify it to exit in response to some action of the parent process -- for example, you could code the child process to exit when its stdin descriptor is closed, and have the parent process call closeWriteChannel() on the QProcess object to cause that to happen. Or if you're running under Linux/Unix you could send a SIGINT signal to the child process and the child process could set up a handler that would catch the signal and start an orderly shutdown. Or if you want something really stupid-quick and dirty, have the child process periodically check for the presence of a file at a well-known location (e.g. "/tmp/hey-child-process-PIDNUMBERHERE-go-away.txt" or something) and the parent process would create such a file when it wants the child to go away. Not that I'd recommend that last method as I don't think it would be very robust, except maybe as a proof of concept.
terminate actually gives the process an chance to clean up. The program being terminated just has to take that chance i.e. the system sents a SIGTERM and the application and it can ignore that and exit cleanly on its own. If this is still not nice enough then you have to implement your own way of asking the application to quit. Jeremy Friesner made some good successions. If the application code is not written by yourself you'll have to read the documentation for that program closer, maybe its documented how to do that.
My windows/QT application is not exiting consistently.
After the object clean up, I am trying to delete the QProcess object. But it's not cleaning up the process and it can be shown in the task manager.
delete process;
process = 0;
I think delete process is not working because some threads are running at the background. I want to ensure that all threads are closed gracefully, so the process will exit.
Please let me know the right way to do this.
The task manager does not show your QProcess object, and deleting that object will not affect the task manager. If there is still code running in your process, the process will continue to exist according to the OS.
If you really want to exit the process, you can call the Win32 function ExitProcess(). Just make sure you've saved everything you want to.
Qt will normally exit the process when all your threads stop running.
One problem may be that the QProcess object doesn't represent a child thread, it represents an independent process, you probably want to use the QThread object.
With the QThread object you can use some combination of the quit, exit, wait, and terminate depending on the exact content of you child thread, check the documentation for the differences.
I'm currently in the process of building a small shell within C++.
A user may enter a job at the prompt such as exe1 && exe2 &. Similar to the BASH shell, I will only execute exe2 if exe1 exits successfully. In addition, the entire job must be performed in the background (as specified by the trailing & operator).
Right now, I have a jobManager which handles execution of jobs and a job structure which contains the job's executable and their individual arguments / conditions. A job is started by calling fork() and then calling execvp() with the proper arguments. When a job ends, I have a signal handler for SIGCHLD, in which I perform wait() to determine which process has just ended. When exe1 ends, I observe its exit code and make a determination as to whether I should proceed to launch exe2.
My concern is how do I launch exe2. I am concerned that if I use my jobManager start function from the context of my SIGCHLD handler, I could end up with too many SIGCHLD handler functions hanging out on the stack (if there were 10 conditional executions, for instance). In addition, it just doesn't seem like a good idea to be starting the next execution from the signal handler, even if it is occurring indirectly. (I tried doing something similar 1.5 years ago when I was just learning about signal handling -- I seem to recall it failing on me).
All of the above needs to be able to occur in the background and I want to avoid having the jobManager sitting in a busy wait just waiting for exe1 to return. I would also prefer to not have a separate thread sitting around just waiting to start the execution of another process. However, instructing my jobManager to begin execution of the next process from the SIGCHLD handler seems like poor code.
Any feedback is appriciated.
I see two ways:
1)Replace you sighandler with loop that call "sigwait" (see man 3 sigwait)
then in loop
2)before start create pipe, and in mainloop of your program use "select" on pipe handle to wait
events. In signal handler write to pipe, and in mainloop handle situation.
Hmmm that's a good one.
What about forking twice, once per process? The first one runs, and the second one stops. In the parent SIGCHLD handler, send a SIGCONT to the second child, if appropriate, which then goes off and runs the job. Naturally, you SIGKILL the second one if the first one shouldn't run, which should be safe because you won't really have set anything up.
How does that sound? You'll have a process sitting around doing nothing, but it shouldn't be for very long.
I am invoking several processes in my main and I can get the pid of that processes. Now I want to wait until all this processes have been finished and then clear the shared memory block from my parent process. Also if any of the process not finished and segfaulted I want to kill that process. So how to check from the pid of processes in my parent process code that a process is finished without any error or it gave broke down becoz of runtime error or any other cause, so that I can kill that process.
Also what if I want to see the status of some other process which is not a child process but its pid is known.
Code is appreciated( I am not looking for script but code ).
Look into waitpid(2) with WNOHANG option. Check the "fate" of the process with macros in the manual page, especially WIFSIGNALED().
Also, segfaulted process is already dead (unless SIGSEGV is specifically handled by the process, which is usually not a good idea.)
From your updates, it looks like you also want to check on other processes, which are not children of your current process.
You can look at /proc/{pid}/status to get an overview of what a process is currently doing, its either going to be:
Running
Stopped
Sleeping
Disk (D) sleep (i/o bound, uninterruptable)
Zombie
However, once a process dies (fully, unless zombied) so does its entry in /proc. There's no way to tell if it exited successfully, segfaulted, caught a signal that could not be handled, or failed to handle a signal that could be handled. Not unless its parent logs that information somewhere.
It sounds like your writing a watchdog for other processes that you did not start, rather than keeping track of child processes.
If a program segfaults, you won't need to kill it. It's dead already.
Use the wait and waitpid calls to wait for children to finish and check the status for some idea of how they exiting. See here for details on how to use these functions. Note especially the WIFSIGNALED and WTERMSIG macros.
waitpid() from SIGCHLD handler to catch the moment when application terminates itself. Note that if you start multiple processes you have to loop on waitpid() with WNOHANG until it returns 0.
kill() with signal 0 to check whether the process is still running. IIRC zombies still qualify as processes thus you have to have proper SIGCHLD handler for that to work.
I have a program which needs to invoke a process to perform an operation and wait for it to complete the operation. The problem is that the invoked process clones itself and exits, which causes the wait api to return when the process exits. How can I wait for the cloned process to finish execution and return?
I am using the windows JOB object as mentioned in http://www.microsoft.com/msj/0399/jobkernelobj/jobkernelobj.aspx, But I am not sure if this is the best way.
umm, I'm pretty sure you can can the spawner process id from any process. I'd iterate through all the processes, find the one's who's parent id matches the one of the process you spawned, and wait for it to die.
alternatively (I mean, thats pretty hack) what is the child child process doing? is there some other way you could detect when it has finished doing what it is meant to do?
a hack way to get a process's parent id
http://www.codeguru.com/cpp/w-p/win32/article.php/c1437
takes a handle, and using the method in the code above, returns the parent id.
http://msdn.microsoft.com/en-us/library/ms684280(VS.85).aspx
OpenProcess takes an id, gets a handle to it (if you're lucky)
http://msdn.microsoft.com/en-us/library/ms684320(VS.85).aspx
GetProcessId takes a handle, gets it's id.
http://msdn.microsoft.com/en-us/library/ms683215(VS.85).aspx
GetExitCodeProcess takes a handle, returns whether the process is done or not.
http://msdn.microsoft.com/en-us/library/ms683189(VS.85).aspx
so appart from using hidden nt calls that it expressly tells you not to, you would basically have to create your process, get it's id, then spam all the process, opening them and checking their parent ids against the id of the process you created, if you didn't find one, then it's done, if you do, spam it with GetExitCodeProcess until its done.
I haven't tested any of this, but it looks like A way to do it. though if it's THE BEST way to do it I might just have to loose all faith in windows...
+1 for using job objects ;)
Assuming the process that you're running isn't spawning the cloned version of itself in such a way that it breaks out of the job...
You should be able to simply monitor the job events and act on JOB_OBJECT_MSG_ACTIVE_PROCESS_ZERO (see JOBOBJECT_ASSOCIATE_COMPLETION_PORT and SetInformationJobObject()). Monitoring the job in this way will also give you notifications of the processId's of new processes created within the job and details of when they exit.
If you have control over the source of invoked process, one possible solution would be to make it wait for the process it spawns by cloning itself.