array output from Python ctypes?

Question

I need to call a C function from Python, using ctypes and to have that function provide one or more arrays back to Python. The arrays will always be of simple types like long, bool, double.

I would very much prefer if the arrays could be dynamically sized. I will know the needed before each call, but different call should use different sizes.

I suppose I should allocate the arrays in Python and let the C code overwrite the contents, so that Python can eventually de-allocate the memory it allocated.

I control both the Python and the C code.

I have this now which does not work:

C:

FOO_API long Foo(long* batch, long bufferSize)
{
    for (size_t i = 0; i < bufferSize; i++)
    {
        batch[i] = i;
    }
    return 0;
}

Python:

print "start test"
FooFunction = Bar.LoadedDll.Foo
longPtrType = ctypes.POINTER(ctypes.c_long)
FooFunction.argtypes = [longPtrType, ctypes.c_long]
FooFunction.restype = ctypes.c_long

arrayType = ctypes.c_long * 7
pyArray = [1] * 7
print pyArray
errorCode = FooFunction(arrayType(*pyArray), 7)
print pyArray
print "test finished"

Produces:

start test
[1, 1, 1, 1, 1, 1, 1]
[1, 1, 1, 1, 1, 1, 1]
test finished

Should Produce:

start test
[1, 1, 1, 1, 1, 1, 1]
[0, 1, 2, 3, 4, 5, 6]
test finished

Why does this not work? Or do I need to do this in a different way?

Maybe try Cython + numpy, see here kbarbary.github.io/blog/cython-and-multiple-numpy-dtypes — mirosval
– mirosval, Commented Dec 10, 2015 at 15:08
@mirosval: No thanks. The solution is complicated enough as it is. — ThomasI
– ThomasI, Commented Dec 10, 2015 at 16:26

falsetru · Accepted Answer · 2015-12-10 15:12:32Z

2

The C array is built using the python list; both are different objects. And the code is print the python list, which is not affected by Foo call.

You need to build the C array, pass it, then use it after the call:

arrayType = ctypes.c_long * 7
array = arrayType(*[1] * 7)
print list(array)
errorCode = FooFunction(array, len(array))
print list(array)

answered Dec 10, 2015 at 15:12

falsetru

371k69 gold badges769 silver badges659 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

ThomasI Over a year ago

Thanks. I arrived at another option, which I have posted, but I am marking your answer as the solution since you got there first and since your answer changes my original code less than my own. I wonder if one is preferable over the other for whatever reason, though? Performance matters.

ThomasI Over a year ago

P.S. i am new to StackOverflow. Is there something I have to do to get email notifications?

ThomasI · Accepted Answer · 2015-12-10 16:21:23Z

0

Thanks to falsetru for the extremely quick answer. I did no notice it right away and in the mean time I arrived at this, which also seems to work. I wonder if one is preferable to the other?

print "start test"
FooFunction = GpGlobals.LoadedDll.Foo
longArrayType = ctypes.c_long * (7)
FooFunction.argtypes = [longArrayType, ctypes.c_long]
FooFunction.restype = ctypes.c_long

pyArray = longArrayType()
for l in pyArray:
    print l        
errorCode = FooFunction(pyArray, 7)
for l in pyArray:
    print l
print "test finished"

I did not initially think that this would work with dynamically sized arrays, but all that I had to do was redefine the argtypes before each call.

answered Dec 10, 2015 at 16:21

ThomasI

451 silver badge7 bronze badges

1 Comment

Eryk Sun Over a year ago

You were better off using ctypes.POINTER(ctypes.c_long) in argtypes since it accepts any long * pointer, including long arrays of any size since they're passed as a pointer. As to iterating the result instead of creating a list, that's preferred if you don't actually need to create a list. There's no need to duplicate the data in two objects.

Collectives™ on Stack Overflow

array output from Python ctypes?

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related