Python ctypes create_string_buffer() Error 0xC0000005

Question

I have a named pipe server with a main loop where I am reading data from a pipe:

while True:
        bytes_available = DWORD()
        ret_code = windll.kernel32.PeekNamedPipe(pipe, None, 0, None, byref(bytes_available))
        if ret_code == 0 and windll.kernel32.GetLastError() == ERROR_BROKEN_PIPE:
            reconnect()
        buf = create_string_buffer(bytes_available.value)
        ret_code = windll.kernel32.ReadFile(pipe, byref(buf), bytes_available.value, None, overlapped_struct_ptr)
        if ret_code != 0 and bytes_available.value > 0:
            # process(buf.raw)
            pass
        elif ret_code == 0 and windll.kernel32.GetLastError() == ERROR_BROKEN_PIPE:
            reconnect()
        windll.kernel32.GetQueuedCompletionStatus(iocp, byref(bytes_transferred), byref(completion_key),
                                                  byref(overlapped_struct_ptr), INFINITE)

Sometimes it crashes on the create_string_buffer() step with the following message:

Process finished with exit code -1073741819 (0xC0000005)

This seems strange to me, because if I replace create_string_buffer(bytes_available.value) with (c_char*bytes_available.value)() everything works fine. And if we look at the create_string_buffer() implementation, we will see, that it allocates the buffer the same way:

def create_string_buffer(init, size=None):
    """create_string_buffer(aString) -> character array
    create_string_buffer(anInteger) -> character array
    create_string_buffer(aString, anInteger) -> character array
    """
    if isinstance(init, (str, unicode)):
        if size is None:
            size = len(init)+1
        buftype = c_char * size
        buf = buftype()
        buf.value = init
        return buf
    elif isinstance(init, (int, long)):
        buftype = c_char * init
        buf = buftype()
        return buf
    raise TypeError(init)

So why does it fail?

windll.kernel32.PeekNamedPipe() call in your code seems to be missing an argument, there should be six, msdn — J.J. Hakala
– J.J. Hakala, Commented Jun 28, 2016 at 13:14
Bingo! I decided to omit the last argument because it was the last one and marked as optional. And I ignored the notice "This parameter can be NULL if no data is to be read." The funny thing is, it worked until I had lots of print statements in the code for debugging puproses. Python has started to crash after I removed print statements for the buffer. Could you add an answer? I will accept it. — wombatonfire
– wombatonfire, Commented Jun 28, 2016 at 19:38

J.J. Hakala · Accepted Answer · 2016-06-28 20:51:34Z

1

windll.kernel32.PeekNamedPipe() call in your code seems to be missing an argument, there should be six; see msdn

Omitting an argument essentially caused undefined behaviour.

answered Jun 28, 2016 at 20:51

J.J. Hakala

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