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    <p>When you set <code>tx.restype = POINTER(c_ubyte)</code>, the resulting ctypes pointer object is a buffer of either 4 or 8 bytes for the <em>address</em> of the image buffer. You need to create a ctypes array from this address. </p> <p>If you know the size ahead of time, you can set <code>SIZE = WIDTH * HEIGHT * 4; tx.restype = POINTER(c_ubyte * SIZE)</code>. Otherwise set <code>tx.restype = POINTER(c_ubyte)</code>, and cast to the dynamic size, i.e. <code>size = w * h * 4; pbuf = cast(result, POINTER(c_ubyte * size))</code>. Either way, you need to dereference the pointer to get at the array. Use either <code>buf = pbuf[0]</code> or <code>buf = pbuf.contents</code>. Then you can pass <code>buf</code> to <code>Image.frombuffer</code>.</p> <p>That said, it's usually simpler to allocate memory using ctypes. This gives you reference-counted memory management instead of having to manually free memory. The following is a toy example. </p> <p>Assuming a dynamic size, the caller needs to get the image dimensions in order to allocate the array, so I've added a struct that has the width, height, and depth of the image, which gets filled in by the C function <code>get_image_info</code>. The function <code>get_image</code> receives a pointer to the image array to copy in data. </p> <pre><code>import ctypes lib = ctypes.CDLL('imgtest.dll') class ImageInfo(ctypes.Structure): _fields_ = ( ('width', ctypes.c_int), ('height', ctypes.c_int), ('depth', ctypes.c_int), ) pImageInfo = ctypes.POINTER(ImageInfo) pImage = ctypes.POINTER(ctypes.c_ubyte) lib.get_image_info.argtypes = [pImageInfo] lib.get_image_info.restype = ctypes.c_int lib.get_image.argtypes = [pImage] lib.get_image.restype = ctypes.c_int imgnfo = ImageInfo() lib.get_image_info(ctypes.byref(imgnfo)) w, h, d = imgnfo.width, imgnfo.height, imgnfo.depth imgdata = (w * h * d * ctypes.c_ubyte)() lib.get_image(imgdata) from PIL import Image img = Image.frombuffer('RGBA', (w, h), imgdata, 'raw', 'RGBA', 0, 1) </code></pre> <p>C declarations:</p> <pre><code>typedef struct _ImageInfo { int width; int height; int depth; } ImageInfo, *pImageInfo; typedef unsigned char *pImage; int get_image_info(pImageInfo imgnfo); int get_image(pImage img); </code></pre>
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    1. POPIL's Image.frombuffer expected data length when using ctypes array
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    1. CObased on your comment and answer, I moved the buffer allocation from C into Python, and passed into C the pointer to the result buffer. That works as it should. It still doesn't explain *why*, but at least I can move forward. Thanks for the kick in the right direction!
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