How do I get values from a C dll array pointer using ctypes and Python

Question

A DLL provides a pointer to a 1D array in my C code like this:

__int16 *data = Msg_RowAt(surfaceMsg, rowIdx);

//access the values like this
data[iterator]

I'm building a Python project where I need to access the values of the same array. I've tried the following but get an access violation when I try iterate surfaceData.

surfaceDataPtr = Msg_RowAt(surfaceMsg, row)
ptr = ctypes.POINTER(ctypes.c_int16)
surfaceData = ctypes.cast(surfaceDataPtr, ptr)
print("ptr: " + str(surfaceData))
print("val: " + str(surfaceData[1]))

I'm accessing the wrong memory location but I'm not too sure what I've done wrong. Can anyone see what I'm doing wrong?

You probably want to define the argument and return types of Msg_RowAt using argtypes and restype. — 101
– 101, Commented Feb 4, 2021 at 22:41
Casting after the fact is too late. 64-bit pointers are truncated because the default return type is c_int. Define .argtypes and .restype. — Mark Tolonen
– Mark Tolonen, Commented Feb 5, 2021 at 7:06
[SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer). — CristiFati
– CristiFati, Commented Feb 5, 2021 at 13:13

ghchoi · Accepted Answer · 2021-02-06 17:31:15Z

1

You can define restype:

c.Msg_RowAt.restype = ctypes.POINTER(ctypes.c_int16)
surfaceDataPtr = Msg_RowAt(surfaceMsg, row)

Then, you can randomly access to each element:

print(surfaceDataPtr[0])
print(surfaceDataPtr[1])
...

answered Feb 6, 2021 at 17:31

ghchoi

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2 Comments

Velveteen_Brownie Over a year ago

Thanks for the examples. I didn't really understand how to implement the restype.

ghchoi Over a year ago

@Velveteen_Brownie Nothing to implement. Python is not typed. You just assign what restype will be. In this case, Python should know surfaceDataPtr will be a C pointer to short variable.

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