CODEWITHSUNDEEP
pythonmultithreadingpyqt
karensantana
karensantana

Reputation: 1629

Segmentation fault (core dumped), executing a Thread

I was trying to run a program in python that opens a program and read from its stdout. When i run the program code i got a segmentation Fault error, but when i put the code inside the function Myfunc out of the thread it work perfectly! I dont understand what is happening: Here is my code:

class Workspace(QMainWindow, Ui_MainWindow):
    """ This class is for managing the whole GUI `Workspace'.
        Currently a Workspace is similar to a MainWindow
    """

    def __init__(self):
        #p= subprocess.Popen(["java -Xmx256m -jar bin/HelloWorld.jar"],cwd=r'/home/karen/sphinx4-1.0beta5-src/sphinx4-1.0beta5/', shell=True, stdout=subprocess.PIPE, bufsize= 4024)
        try:
            from Queue import Queue, Empty
        except ImportError:
            while True:
    #from queue import Queue, Empty  # python 3.x
                print "error"

        ON_POSIX = 'posix' in sys.builtin_module_names

        def enqueue_output(out, queue):
            for line in iter(out.readline, b''):
                queue.put(line)
            out.close()

        p= Popen(["java -Xmx256m -jar bin/HelloWorld.jar"],cwd=r'/home/karen/sphinx4-1.0beta5-src/sphinx4-1.0beta5/',stdout=PIPE, shell=True, bufsize= 4024)
        q = Queue()
        t = threading.Thread(target=enqueue_output, args=(p.stdout, q))
        #t = Thread(target=enqueue_output, args=(p.stdout, q))
        t.daemon = True # thread dies with the program
        t.start()

# ... do other things here
        def myfunc(q):
            while True:

                try: line = q.get_nowait()
         # or q.get(timeout=.1)
                except Empty:
                    print('Vacio')
                else: # got line
    # ... do something with line

                    print line  


        thread = threading.Thread(target=myfunc, args=(q,))
        thread.start()

The error:

Segmentation fault (core dumped)

Upvotes: 4

Views: 4849

Answers (1)

Eugene Sajine
Eugene Sajine

Reputation: 8200

I have experienced the same problem recently. I have quite a few calls of shell command via subprocess from dependent GUI threads that are working just fine, but one just refuses to work correctly and segfaults. The difference I have came to is exactly in the fact that I'm trying to run it from the main GUI thread and it was segfaulting when i was trying to emit a signal that was usually emitted from subthread!

My solution to avoid segfaulting was to move the part of dialog that required some shell involvement to a separate QThread, effectively inheriting the same common class that other threads in my app are using. The problem is gone! QThread is the key!

Code schema:

class Dialog(QtGui.QDialog):

     def __init__(self):
         ...
         QtCore.QObject.connect(self.ui.listwidget_a, QtCore.SIGNAL("itemClicked(QListWidgetItem)", self.do_work)
         ...

     def do_work(self, qlistwidgetitem):
         ...
         wc = WorkerClass(str(qlistwidgetitem.text()))
         wc.start()
         wc.wait()
         my_result = wc.getResult()


 class WorkerClass(CommonClass):
     string = ""
     result = ""
     def __init__(string)
         super(WorkerClass, self).__init__()
         self.string = string

     def run():
         self.execute_shell(self.string)
         self.result = self.shell_output

     def get_result(self):
         return self.result

  class CommonClass(QtCore.QThread):

     self.shell_output = ""

     def execute_shell(string):
         ...
         p = Popen(...)
         self.shell_output, self.shell_error = p.communicate()
         self.output_ready.emit(self.shell_output) 
# that last line is where i was segfaulting 
# before I made WorkerClass (my dialog class was 
# inherinting CommonClass directly but it wasn't 
# working in separate thread)

I hope that will help!

Upvotes: 5

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