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14.14.1.17 Callback functions

`ctypes` allows to create C callable function pointers from Python callables. These are sometimes called callback functions.

First, you must create a class for the callback function, the class knows the calling convention, the return type, and the number and types of arguments this function will receive.

The CFUNCTYPE factory function creates types for callback functions using the normal cdecl calling convention, and, on Windows, the WINFUNCTYPE factory function creates types for callback functions using the stdcall calling convention.

Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments.

I will present an example here which uses the standard C library's `qsort` function, this is used to sort items with the help of a callback function. `qsort` will be used to sort an array of integers:

    >>> IntArray5 = c_int * 5
    >>> ia = IntArray5(5, 1, 7, 33, 99)
    >>> qsort = libc.qsort
    >>> qsort.restype = None
    >>>

`qsort` must be called with a pointer to the data to sort, the number of items in the data array, the size of one item, and a pointer to the comparison function, the callback. The callback will then be called with two pointers to items, and it must return a negative integer if the first item is smaller than the second, a zero if they are equal, and a positive integer else.

So our callback function receives pointers to integers, and must return an integer. First we create the `type` for the callback function:

    >>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
    >>>

For the first implementation of the callback function, we simply print the arguments we get, and return 0 (incremental development ;-):

    >>> def py_cmp_func(a, b):
    ...     print "py_cmp_func", a, b
    ...     return 0
    ...
    >>>

Create the C callable callback:

    >>> cmp_func = CMPFUNC(py_cmp_func)
    >>>

And we're ready to go:

    >>> qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +WINDOWS
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    py_cmp_func <ctypes.LP_c_long object at 0x00...> <ctypes.LP_c_long object at 0x00...>
    >>>

We know how to access the contents of a pointer, so lets redefine our callback:

    >>> def py_cmp_func(a, b):
    ...     print "py_cmp_func", a[0], b[0]
    ...     return 0
    ...
    >>> cmp_func = CMPFUNC(py_cmp_func)
    >>>

Here is what we get on Windows:

    >>> qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +WINDOWS
    py_cmp_func 7 1
    py_cmp_func 33 1
    py_cmp_func 99 1
    py_cmp_func 5 1
    py_cmp_func 7 5
    py_cmp_func 33 5
    py_cmp_func 99 5
    py_cmp_func 7 99
    py_cmp_func 33 99
    py_cmp_func 7 33
    >>>

It is funny to see that on linux the sort function seems to work much more efficient, it is doing less comparisons:

    >>> qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +LINUX
    py_cmp_func 5 1
    py_cmp_func 33 99
    py_cmp_func 7 33
    py_cmp_func 5 7
    py_cmp_func 1 7
    >>>

Ah, we're nearly done! The last step is to actually compare the two items and return a useful result:

    >>> def py_cmp_func(a, b):
    ...     print "py_cmp_func", a[0], b[0]
    ...     return a[0] - b[0]
    ...
    >>>

Final run on Windows:

    >>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) # doctest: +WINDOWS
    py_cmp_func 33 7
    py_cmp_func 99 33
    py_cmp_func 5 99
    py_cmp_func 1 99
    py_cmp_func 33 7
    py_cmp_func 1 33
    py_cmp_func 5 33
    py_cmp_func 5 7
    py_cmp_func 1 7
    py_cmp_func 5 1
    >>>

and on Linux:

    >>> qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) # doctest: +LINUX
    py_cmp_func 5 1
    py_cmp_func 33 99
    py_cmp_func 7 33
    py_cmp_func 1 7
    py_cmp_func 5 7
    >>>

It is quite interesting to see that the Windows `qsort` function needs more comparisons than the linux version!

As we can easily check, our array is sorted now:

    >>> for i in ia: print i,
    ...
    1 5 7 33 99
    >>>

Important note for callback functions:

Make sure you keep references to CFUNCTYPE objects as long as they are used from C code. `ctypes` doesn't, and if you don't, they may be garbage collected, crashing your program when a callback is made.