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POCython - implementing callbacks
 

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    <p>I have been working with Cython in an attempt to interface with a library written in c++. So far things are going pretty good, and I can effectively use MOST functions within the library. My only problem lies within implementing callbacks. The library has 4 function definitions that look a little something like this:</p> <pre><code>typedef void (*Function1)(const uint16_t *data, unsigned width, unsigned height); void SetCallBack(Function1); </code></pre> <p>So to implement them I figured I would do something like this with cython:</p> <pre><code>ctypedef void (*Function1)(unsigned short *data, unsigned width, unsigned height); cdef extern from "lib.hpp": void SetCallBack(Function1) </code></pre> <p>Which actually compiles correctly, however, I can't for the life of me think how to actually implement that in such a way that the callback would work. I first tried creating a function that would just call that, similar to how you would do it for any other function, coming up with this:</p> <pre><code>def PySetCallBack(Func): SetCallBack(Func) </code></pre> <p>but that gives me the (predictable) error: </p> <p>"Cannot convert Python object to 'Function1'"</p> <p>so yeah, that's where I'm at. If anyone has any experience setting up callbacks in Cython I would be very grateful for any assistance. Thanks.</p> <p><strong>Edit</strong>: Following your advice, I created an intermediate function with a cdef, which looks like this:</p> <pre><code>cdef void cSetCallBack(Function1 function): SetCallBack(function) </code></pre> <p>This seems to have gotten me... Closer? Getting a different error now at least:</p> <pre><code>error: invalid conversion from ‘void (*)(short unsigned int*, unsigned int, unsigned int)’ to ‘void (*)(const uint16_t*, unsigned int, unsigned int)’ </code></pre> <p>Now, far as I can tell those types are identical, so I can't figure what's going on.</p> <p><strong>Edit2</strong>: Fixed that problem by declaring a new typedef:</p> <pre><code>ctypedef unsigned short uint16_t </code></pre> <p>and using that as the argument to call, but apparently that wasn't actually getting any closer, but just taking me around a side track, since when trying to call that function, I get the same "Cannot convert Python object to 'Function1'" error all over again.</p> <p>So, I'm pretty much back where I started. Only thing I can figure to do now is explicitly cast the python object coming in as a c function like that, but, to be honest, I have no idea how I would go about that.</p> <p><strong>Edit the third</strong>: Alright, after dissecting your answer I finally get it, and it works, so hooray and whatnot. What I ended up doing was creating a function like this:</p> <pre><code>cdef void cSetCallback(Function1 function): SetCallback(function) cdef void callcallback(const_ushort *data, unsigned width, unsigned height): global callbackfunc callbackfunc(data,width,height) cSetCallback(callcallback) def PySetCallback(callbackFunc): global callbackfunc callbackfunc = callbackFunc </code></pre> <p>So now the only problem is that it can't convert const_ushort *data into a python object, but that's another problem entirely, so I guess this one is solved, thanks a lot.</p>
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    1. COChange `short unsigned int*` to `const short unsigned int*`.
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    2. COTo the best of my knowledge, Cython has no idea what const means, and whenever I try to use it, I get "Const is not a type identifier".
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