ctypes and how to copy data passed to callback

[waldek]

Hi,

I'm trying to handle data passed to Py Callback which is called from
C dll. Callback passes data to another thread using Queue module and
there the data are printed out.

If data is printed out in a callback itself it's ok. If I put on
queue and next get from queue in another thread script prints some
trash. Looks like the data is released when callback returned. I tired
to make d = copy.deepcopy(data), but it does not work - I got nothing.
Any idea why it's happening ?

--------------------- main thread --------
def callback(data, size):
myqueue.put((data, size))

mydll = cdll.MyDLL
cbproto = CFUNCTYPE(c_int, POINTER(c_char), c_int)
mycallback = cbproto(callback)

mydll.RegisterCallback(mycallback)

---------------------------------- thread listener
----------------------

while True:
data, size = myqueue.get()
print "***", data[:size]

------------------------------------------------------------------------------

 

[Diez B. Roggisch]

Hi,

I'm trying to handle data passed to Py Callback which is called from
C dll. Callback passes data to another thread using Queue module and
there the data are printed out.

If data is printed out in a callback itself it's ok. If I put on
queue and next get from queue in another thread script prints some
trash. Looks like the data is released when callback returned. I tired
to make d = copy.deepcopy(data), but it does not work - I got nothing.
Any idea why it's happening ?

--------------------- main thread --------
def callback(data, size):
myqueue.put((data, size))

mydll = cdll.MyDLL
cbproto = CFUNCTYPE(c_int, POINTER(c_char), c_int)
mycallback = cbproto(callback)

mydll.RegisterCallback(mycallback)

---------------------------------- thread listener
----------------------

while True:
data, size = myqueue.get()
print "***", data[:size]

------------------------------------------------------------------------------

You need to allocate e.g. a bytebuffer using ctypes and then copy the memory
area you get passed into that buffer.

Otherwise I presume whoever invokes the callback releases the originaly
memory block after the callback terminated.

Diez

 

[Thomas Heller]

Hi,

I'm trying to handle data passed to Py Callback which is called from
C dll. Callback passes data to another thread using Queue module and
there the data are printed out.

If data is printed out in a callback itself it's ok. If I put on
queue and next get from queue in another thread script prints some
trash. Looks like the data is released when callback returned. I tired
to make d = copy.deepcopy(data), but it does not work - I got nothing.
Any idea why it's happening ?

--------------------- main thread --------
def callback(data, size):
myqueue.put((data, size))

mydll = cdll.MyDLL
cbproto = CFUNCTYPE(c_int, POINTER(c_char), c_int)
mycallback = cbproto(callback)

mydll.RegisterCallback(mycallback)

---------------------------------- thread listener
----------------------

while True:
data, size = myqueue.get()
print "***", data[:size]

------------------------------------------------------------------------------

I guess your code would work if you change it in this way:

def callback(data, size):
myqueue.put(data[:size])

while True:
data = myqueue.get()
print "***", data

Thomas

 

[waldek]

waldek schrieb:

Hi,

I'm trying to handle data passed to Py Callback which is called from
C dll. Callback passes data to another thread using Queue module and
there the data are printed out.

If data is printed out in a callback itself it's ok. If I put on
queue and next get from queue in another thread script prints some
trash. Looks like the data is released when callback returned. I tired
to make d = copy.deepcopy(data), but it does not work - I got nothing.
Any idea why it's happening ?

--------------------- main thread --------
def callback(data, size):
myqueue.put((data, size))

mydll = cdll.MyDLL
cbproto = CFUNCTYPE(c_int, POINTER(c_char), c_int)
mycallback = cbproto(callback)

---------------------------------- thread listener
----------------------

while True:
data, size = myqueue.get()
print "***", data[:size]

------------------------------------------------------------------------------

I guess your code would work if you change it in this way:

def callback(data, size):
myqueue.put(data[:size])
while True:
data = myqueue.get()
print "***", data

Thomas

Both solutions work fine. The secon is nicer

Thanks