Convert ctypes byte array to Python list of floating point numbers

Question

I have allocated an array and cast it using the Python ctypes module:

    dataC = ctypes.cast(crt.malloc(size), ctypes.POINTER(ctypes.c_ubyte))

in order to get byte data from a C library:

    someClib.getData(handle, dataC)

Now this array is actually an array of C float types. How can I convert it to a Python list of floating type numbers?

David Heffernan · Accepted Answer · 2013-10-11 22:06:38Z

5

You can cast to a pointer to float:

floatPtr = ctypes.cast(dataC, ctypes.POINTER(ctypes.c_float))

And then use a list comprehension, for example, to pull out the floats:

floatList = [floatPtr[i] for i in range(arrayLength)]

Now, only you know the value of arrayLength but it seems plausible to me that it is equal to size / ctypes.sizeof(ctypes.c_float).

answered Oct 11, 2013 at 22:06

David Heffernan

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3 Comments

Eryk Sun Over a year ago

Backing up a step, allocate the float array to begin with, i.e. dataC = (ctypes.c_float * arrayLength)(). It's sized, so there's no need for a list comprehension. At least use dataC = (ctypes.c_ubyte * size)(). Using the C runtime's malloc is usually the wrong approach.

David Heffernan Over a year ago

@eryksun That sounds sensible. I was answering naively the question as asked.

jensph Over a year ago

@eryksun I'll try your approach - it would simplify the code.

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