ctypes, arrays and pointers

[Richard Jones]

Does anyone know how to do the equivalent of this using ctypes?

image_data = malloc(width * height * components);
row_pointers = png_get_rows(png_ptr, info_ptr);
for (y = 0; y < height; y++)
memcpy(&image_data[width * components * y],
row_pointers[height-y-1],
width * components);

That is, I need to get the address of a location *within* an allocated
array. This is what I've got:

image_data = create_string_buffer(width * height * components)
address = addressof(image_data)
row_pointers = libpng.png_get_rows(png_ptr, info_ptr)
for y in xrange(height):
row = string_at(address + width * components * y)
memmove(row, row_pointers[height-y-1], width * components)

but that's not correct. Either the addressof() or string_at() are not
working according to my understanding from the docs, because that code
segfaults. FWIW, I also tried:

row = image_data + width * components * y

but you can't add an int to a c_char_Array_2415600.

Sadly the docs don't seem to be finished

http://docs.python.org/dev/lib/ctypes-arrays-pointers.html

 

[Thomas Heller]

Does anyone know how to do the equivalent of this using ctypes?

image_data = malloc(width * height * components);
row_pointers = png_get_rows(png_ptr, info_ptr);
for (y = 0; y < height; y++)
memcpy(&image_data[width * components * y],
row_pointers[height-y-1],
width * components);

That is, I need to get the address of a location *within* an allocated
array. This is what I've got:

image_data = create_string_buffer(width * height * components)
address = addressof(image_data)
row_pointers = libpng.png_get_rows(png_ptr, info_ptr)
for y in xrange(height):
row = string_at(address + width * components * y)
memmove(row, row_pointers[height-y-1], width * components)

but that's not correct. Either the addressof() or string_at() are not
working according to my understanding from the docs, because that code
segfaults. FWIW, I also tried:

row = image_data + width * components * y

but you can't add an int to a c_char_Array_2415600.

This feature request has now come up the second time, so I guess I will
have to find a solution for it. Someone suggested a byref_at(obj, offset) method
which would return a kind of pointer to 'obj' plus offset, and even provided
a patch for it (on the ctypes-users lists, maybe even in the ctypes or Python
SF tracker). Unfortunately that was after the feature freeze for Python 2.5.

Sadly the docs don't seem to be finished

http://docs.python.org/dev/lib/ctypes-arrays-pointers.html

This problem wouldn't have been described in the docs anyway, but
keep on bugging me about the docs:

Thomas

 

[Richard Jones]

This feature request has now come up the second time, so I guess I will
have to find a solution for it. Someone suggested a byref_at(obj, offset)
method which would return a kind of pointer to 'obj' plus offset, and even
provided a patch for it (on the ctypes-users lists, maybe even in the
ctypes or Python SF tracker). Unfortunately that was after the feature
freeze for Python 2.5.

The following will do what I wanted:

def ptr_add(ptr, offset):
address = addressof(ptr.contents) + offset
return pointer(type(ptr.contents).from_address(address))

Thanks to Alex Holkner for the tip.


Richard