How to use malloc and free with python ctypes?

Question

I have a function in my C library, say runsim() which takes pointer to struct repdata as one of the arguments, where struct repdata is

Answer 1

You can allocate buffers using ctypes and assign them to the pointers. Once the Python ctypes objects have no references they will be freed automatically. Here's a simple example (with a Windows DLL...don't have a Linux machine handy, but the idea is the same) and a Python wrapper.

create_string_buffer allocates a writable buffer that can be passed from Python to C that ctypes will marshal as a char*.

You can also create writable arrays of ctypes types with the syntax:

variable_name = (ctypes_type * length)(initial_values)

x.h

#ifdef DLL_EXPORTS
#define DLL_API __declspec(dllexport)
#else
#define DLL_API __declspec(dllimport)
#endif

struct example {
    char* data;
    int len;          // of data buffer
    double* doubles;
    int count;        // of doubles
};

DLL_API void func(struct example* p);

x.c

#include <stdio.h>
#define DLL_EXPORTS
#include "x.h"

void func(struct example* p)
{
    int i;
    strcpy_s(p->data,p->len,"hello, world!");
    for(i = 0; i < p->count; i++)
        p->doubles[i] = 1.1 * (i + 1);
}

x.py

import ctypes

class Example(ctypes.Structure):

    _fields_ = [
        ('data',ctypes.POINTER(ctypes.c_char)),
        ('len',ctypes.c_int),
        ('doubles',ctypes.POINTER(ctypes.c_double)),
        ('count',ctypes.c_int)]

    def __init__(self,length,count):
        self.data = ctypes.cast(ctypes.create_string_buffer(length),ctypes.POINTER(ctypes.c_char))
        self.len = length
        self.doubles = (ctypes.c_double * count)()
        self.count = count

    def __repr__(self):
        return 'Example({},[{}])'.format(
            ctypes.string_at(self.data),
            ','.join(str(self.doubles[i]) for i in range(self.count)))

class Dll:

    def __init__(self):
        self.dll = ctypes.CDLL('x')
        self.dll.func.argtypes = [ctypes.POINTER(Example)]
        self.dll.func.restype = None

    def func(self,ex):
        self.dll.func(ctypes.byref(ex))

d = Dll()
e = Example(20,5)
print('before:',e)
d.func(e)
print ('after:',e)

Output

before: Example(b'',[0.0,0.0,0.0,0.0,0.0])
after: Example(b'hello, world!',[1.1,2.2,3.3000000000000003,4.4,5.5])