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POPython and C interop: Dynamic resizing of ctypes
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copied!<p>I'm trying to interop with C in Python, and I have the following function prototype:</p> <pre><code>int testout(unsigned char *test, int *size) </code></pre> <p>The size of the unsigned char is determined by the size parameter. So I need a way to resize the buffer in Python dynamically to adjust to the size returned from the function call. This has been very difficult for me to figure out. </p> <p>From the <a href="http://docs.python.org/library/ctypes.html#variable-sized-data-types" rel="nofollow">Python ctypes manual</a>:</p> <p>"This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:</p> <pre><code>>>> short_array[:] [0, 0, 0, 0] >>> short_array[7] Traceback (most recent call last): ... IndexError: invalid index >>> </code></pre> <p>Another way to use variable-sized data types with ctypes is to use the dynamic nature of Python, and (re-)define the data type after the required size is already known, on a case by case basis."</p> <p>Great! I'm getting IndexError:invalid index, just as the example shows. And they decided not to show how to do this properly! :)</p> <p>Does anyone know how to resize a ctype properly? </p> <p>Here is my example code that works, except for the resizing:</p> <pre><code>from ctypes import * lib = "test.so" dll = cdll.LoadLibrary(lib) print "Testing pointer output" dll.testout.argtypes = [POINTER(c_ubyte), POINTER(c_int)] sizeout = c_int(0) mem = (c_ubyte * 20)() dll.testout(mem, byref(sizeout)) print "Sizeout = " + str(sizeout.value) for i in range(0,sizeout.value): print "Item " + str(i) + " = " + str(mem[i]) </code></pre>

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