ctypes and flexible (zero-length) arrays

Dear lazyweb, I’ve been racking my brain trying to get information out of the following C struct in a nice way when using ctypes:

struct Foo
    unsigned int length;
    char name[];

I wrote a little C function to get instance of the struct easily into python:

struct Foo *
put_name(char *name)
    struct Foo *foo = malloc(sizeof(struct Foo) + strlen(name));
    foo->length = strlen(name);
    memcpy(foo->name, name, foo->length);

Now, how do I actually get the full contents of Foo.name in python? The only way I could think of that actually works is to create a dummy-struct type in order to get the length out and then use it to dynamically create a new sub-class of ctypes.Structure, then create an instance based on the address of what was returned. I think the following shows it pretty clearly:

class FOO(Structure):
    _fields_ = [('length', c_uint), ('name', c_char * 0)]
liblib = cdll.LoadLibrary('./_build/liblib.so')
c_put_name = liblib.put_name
c_put_name.argtypes = [c_char_p]
c_put_name.restype = POINTER(FOO)
def put_name(str):
    f = c_put_name(str)
    K = type('FOO%s' % f.contents.length, (Structure,),
                        {'_fields_' : [('length', c_uint), ('name', c_char * f.contents.length)]})
    return K.from_address(addressof(f.contents))

I still think there ought to be some other way that ‘feels’ nicer. I mean the use of “short arrays” (declared as here with [], or zero size as supported by GCC, or the more portable array of size one) seems to be common enough to warrant some support from ctypes, right?

Any suggestions for improvements?


One Comment

  1. Pingback: ctypes and flexible (zero-length) arrays

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