Hi,

I'm currently experimenting with QSharedMemory and QProcess in PyQt. So I wrote a small application to launch a process. From this process I create a shared memory segment and write data to it. The application reads the data back when the process writes on the output. Unfortunately I get this error when the application attempts to attach to the shared memory segment: QSharedMemory::handle: doesn't exist.

Is it possible to do this sort of things ? Or maybe I should use a different approach to achieve this ?

Thanks for the help!
-mab

My application code:
Qt Code: 
  1. from PyQt4 import QtGui, QtCore
  2. import sys
  3. import pickle
  4.  
  5. class Widget(QtGui.QWidget):
  6.     def __init__(self):
  7.         super(Widget,self).__init__()
  8.  
  9.         # create process
  10.         self.p = QtCore.QProcess(self)    
  11.  
  12.         # Connect to process output 
  13.         self.p.readyReadStandardOutput.connect( self.on_process_output )
  14.  
  15.         self.key = 'share_mem_key'
  16.         self.shmem = QtCore.QSharedMemory( key )
  17.  
  18.         self.p.start( 'python.exe test_process.py "%s"' % self.key )
  19.  
  20.     def on_process_output(self):
  21.         self.shmem = QtCore.QSharedMemory( self.key )
  22.         if not self.shmem.isAttached() and not self.shmem.attach():
  23.             # Error when trying to attach to the shared memory segment
  24.             print 'ERROR %s ' % self.shmem.errorString()
  25.  
  26.         self.shmem.lock()
  27.         try:
  28.             print pickle.loads(self.shmem.data().asstring())
  29.         finally:
  30.             self.shmem.unlock()
  31.  
  32. # application loop    
  33. app = QtGui.QApplication(sys.argv)
  34. widget = Widget()
  35.  
  36. widget.show()
  37. app.exec_()
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The external process code:
Qt Code: 
  1. from PyQt4 import QtCore
  2. import ctypes
  3. import ctypes.util
  4. import pickle
  5. import sys
  6.  
  7. CLIB = ctypes.cdll.LoadLibrary(ctypes.util.find_library('c'))
  8.  
  9. def main(argv):   
  10.     key = argv[1]
  11.  
  12.     # write to shared memory      
  13.     data = range(50000)
  14.     data_bytes = pickle.dumps( data, pickle.HIGHEST_PROTOCOL)
  15.     data_len = len(data_bytes)      
  16.  
  17.     shmem = QtCore.QSharedMemory(key)
  18.     if not shmem.create(data_len):
  19.         sys.stderr.write( 'ERROR: shared memory creation' )
  20.         sys.stderr.flush()        
  21.         return
  22.  
  23.     if not shmem.isAttached() and not shmem.attach():
  24.         sys.stderr.write( 'ERROR: shared memory access' )
  25.         sys.stderr.flush()        
  26.         return 
  27.  
  28.     shmem.lock()
  29.     try:
  30.         CLIB.memcpy(int(shmem.data()), data_bytes, data_len)
  31.     finally:
  32.         shmem.unlock()
  33.  
  34.     sys.stdout.write( 'done' )
  35.     sys.stdout.flush()
  36.  
  37. if __name__ == '__main__':
  38.     main(sys.argv)
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