Python calling C Shared Library with ctypes custom structures

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?

Accepted Answer

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))
```

Python calling C Shared Library with ctypes custom structures

Solution

Problem