passing null structure pointer ctypes

Question

I am writing a Python wrapper for cpp APIs in that for one API I am trying to pass a NULL structure pointer as a parameter. Not sure how we can achieve that in Python.

Below is my sample implementation:

cpp_header.hpp

typedef enum {
    E_FLAG_ON = 0,
    E_FLAG_OFF
} option;

typedef struct {
    float *a;
    float b;
    char *file_path; 
    option flag;
} inputs;

// API
int op_init(const inputs*);

This is what happening inside the init API:

Implementation.cpp

int op_init(const inputs* usr_ptr) {
    internal_opration_read_set(&local_struct) { // local struct variable
         // read one input file and update the structure values
    }
    if (usr_prt !=  NULL) {
        internal_opration_update_set(usr_ptr, &local_struct) {
            // update the values only if i send NOT NULL structure
        }
    }
}

From cpp test application I'm passing NULL structure to initialize.

test.cpp

int main() {
    inputs *usr_cfg = NULL;
    op_init(usr_cfg);
}

ctypes_imple.py

From ctypes import *

class inputs(Structure):
    _fields_ = [('a', POINTER(c_float)),
                ('b', c_float),
                ('file_path', c_char_p),
                ('flag', option)]

# loading so file
so_lib = CDLL('some so')
# how we can initialize NULL structure pointer here?
so_lib.op_init() # how to send structure pointer as argument?

NOTE: this API reads inputs from a file and updates values to a structure in C. I am clueless how we can achieve the same in a Python wrapper? I mean updating values from so file runtime to a ctypes Python class.

Mark Tolonen · Accepted Answer · 2022-07-08 19:11:48Z

1

Use None to pass a null pointer:

so_lib.op_init(None)

To send the actual structure instantiate one and send it. Best to define .argtypes and restype as well so ctypes doesn't have to guess and can perform better error checking:

so_lib.op_init.argtypes = POINTER(inputs),
so_lib.op_init.restype = c_int

arg = inputs() # all initialized to zero/null by default.
so_lib.op_init(arg)

edited Jul 8, 2022 at 19:11

answered Jul 8, 2022 at 12:50

Mark Tolonen

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2 Comments

soundarrajan v Over a year ago

I tried giving 'None', the memory passed from here is not same when i debug it from c source side. Since memory is not same code is not working as expected. Any way that we can pass same address to c source? Address in python: {} <cparam 'P' (0xffffb39b1270)> {} <core.LP_sc_app_config_t object at 0xffffb39b1440> {} <cparam 'P' (0xffffb39b1540)> {} <core.LP_sc_app_config_t object at 0xffffb39b13b0> Address received in C source: [PY WRAPPER DEBUG] Recevied address: 0xffffb39b1420

Mark Tolonen Over a year ago

@soundarrajanv None is a null pointer and will be address zero on C side. None of the above relates to your question. Edit your question with a minimal reproducible example if you want more help. Addresses of Python objects won’t match the addresses of the C-translated memory. Use ctypes.addressof() to print the C address.

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