How to return a pointer to a structure in ctypes?

Question

I try to pass a pointer of a structure which is given me as a return value from the function 'bar' to the function 'foo_write'. But I get the error message 'TypeError: must be a ctypes type' for line 'foo = POINTER(temp_foo)'. In the ctypes online help I found that 'ctypes.POINTER' only works with ctypes types. Do you know of another way? What would you recommend?

C:

typedef struct FOO_{
    int i;
    float *b1;
    float (*w1)[];
}FOO;

foo *bar(int foo_parameter) {...
void foo_write(FOO *foo)

Python with ctypes:

class foo(Structure):
    _fields_=[("i",c_int),
              ("b1",POINTER(c_int)),
              ("w1",POINTER(c_float))]

temp_foo=foo(0,None,None)
foo = POINTER(temp_foo)
foo=myclib.bar(foo_parameter)
myclib.foo_write(foo)

Solution 1

Your bar function has an incorrect definition, I guess you mean it is struct FOO_ *bar(int);?

The Python code is wrong in the sense that foo_parameter is never declared, so I'm not 100% sure what you want to do. I assume you want to pass a parameter of your python-declared foo, which is an instance of a struct FOO_, into the C bar(int) and get back a pointer to struct FOO_.

You don't need POINTER to do that, the following will work:

#!/usr/bin/env python
from ctypes import *

class foo(Structure):
    _fields_=[("i",c_int),
              ("b1",POINTER(c_int)),
              ("w1",POINTER(c_float))]

myclib = cdll.LoadLibrary("./libexample.so")
temp_foo = foo(1,None,None)
foovar = myclib.bar(temp_foo.i)
myclib.foo_write(foovar)

Since CTypes will wrap the return type of bar() in a pointer-to-struct for you.

Solution 2

Change

foo = POINTER(temp_foo)

to

foo = pointer(temp_foo)

can solve the problem.

Please see http://docs.python.org/library/ctypes.html#ctypes-pointers for more information.

Comments

[Framester]

Hi rq, i chose you as you pointed out, that I don't need ctypes.POINTER at all.

[fadedbee]

Is foo a classname, or a variable here? How does c_types know that the return type of bar is 'foo'?

[Overdrivr]

foo is both a class and a variable name ?

[Ben Moss]

FWIW the above 2 comments about class vs variable name are obsolete after the last edit.