Update values of a struct pointer in ctypes python without using a C function

Question

I'm having a C function that returns a pointer to an struct:

struct iperf_test *
iperf_new_test()
{
    struct iperf_test *test;

    test = (struct iperf_test *) malloc(sizeof(struct iperf_test));
    ...
    return test;
}

This function is called from Python in the following way:

self.lib = cdll.LoadLibrary("lib.so")
self._test = self.lib.iperf_new_test()

The struct has some values such as:

struct iperf_test
{
    int       server_port;
    int       bind_port; 
};

The examples I see on the internet shows that I need to use a function that receives a pointer to alter the values, for example in python:

self.lib.iperf_set_test_server_port(self._test, int(port))

And in C:

void
iperf_set_test_server_port(struct iperf_test *ipt, int srv_port)
{
    ipt->server_port = srv_port;
}

Is there a way to change the value bind_port directly without using a C function?

Answer 1

Yes. This is why ctypes supports defining your own structs , and defining prototypes for functions.

You'd need to make a Python level definition of your structure, eg:

from ctypes import Structure, c_int, POINTER

class iperf_test(Structure):
    _fields_ = [("server_port", c_int),
                ("bind_port", c_int)]

Then, before calling your C function, you set its restype correctly:

# Load library as normal
self.lib = cdll.LoadLibrary("lib.so")
# New, so Python knows how to interpret result
self.lib.iperf_new_test.restype = POINTER(iperf_test)
# Call, and Python returns pointer to Python definition of struct
self._test = self.lib.iperf_new_test()

Now you can use it by dereferencing (done with [0] since Python lacks a * pointer dereference operator) and setting attributes on the dereferenced struct directly:

self._test[0].bind_port = new_bind_port