Question

I’m calling a C shared library from Python on a Linux system.

The problem I’m running into is the function inside the C library takes a pointer to a structure as an argument. It then mallocs the memory for an array of structures, populates the array with data and returns. So I’ve defined the function as

from ctypes import *
class myStruct(Structure):
    _fields_ = [("id", c_uint), "name", c_char*256)]

library.func.argtypes =  [POINTER(myStruct)]

Then I call it like so:

Myfoo = myStruct
Foo = pointer(Myfoo)
Bar = library.func(Foo)
for i in range(Bar):
    print("id = %u, name = %s" % (Foo[i].id, Foo[i].name))

Bar contains the number of structures that were allocated by func.

No matter what I do, I can’t get any data out of Foo. I’ve tried multiple different variations on this for months. I can look at the logs from the C library and I know it’s getting the data and returning it, but I can’t seem to find a way to extract it from Python.

Any thoughts?

Answer 1

Ok, this just needs a few small updates, then.

First, your argument declaration is off, but that probably doesn’t matter. A pointer is a pointer, as far as the python-c value translation machinery is concerned. Most accurate would be POINTER(POINTER(myStruct)), but to make things simpler, let’s just use:

library.func.argtypes = [c_void_p]

Next, you don’t need to bother creating a myStruct instance for your argument to point at; you just need a valid pointer, and a pointer to that. func will allocate the actual myStruct instances. So let’s start with:

Foo = ctypes.POINTER(myStruct)()

Now we can call it. We have a myStruct*, but we’ll pass a pointer to it so func can change it. We don’t need to construct a whole other pointer object, so we’ll use the lighter byref:

Bar = library.func(byref(Foo))

Now you can iterate through Foo[i].

for i in range(Bar):
    print("id = %u, name = %s" % (Foo[i].id, Foo[i].name))