How set a C char** pointer in Python

Question

I want to set a C char** pointer, called results, in Python. The variable is in a dll I have loaded. I want to set results so that it points to a string in Python. I want to get the string I created in Python (or at least a copy of it since ctypes does a lot of copying) to be pointed to by the C variable results. So I have in Python product_class = (ctypes.c_char_p)(b"321"). I want to set results to the value "321".

Here is the code I have written. It does not work. It does not even change the C-variable results.

# py_parse_pdl_func function is a callback which is called from a c dll which has been loaded into the python prorgram.
# Here is the declaration of the callback in c
# typedef int (*tsl_pdl_cb_t)(void *pz_prv, const char **results, const char* query);
# so am trying to set results to point to a string "321"


def py_parse_pdl_func(pz_prv, py_results, query):
    global product_class_void
    product_class = (ctypes.c_char_p)(b"321")
    product_class_void = ctypes.cast(product_class, ctypes.c_void_p)
    py_results.contents = ctypes.c_long(product_class_void.value)
  
    return 1

Mark Tolonen · Accepted Answer · 2023-02-02 16:15:50Z

1

Here's a reproducible example. You may need to keep a reference to the string returned since Python could deallocate it at any time.

test.c

#include <stdio.h>

typedef int (*tsl_pdl_cb_t)(void *pz_prv, const char **results, const char* query);

__declspec(dllexport)
int function_using_callback(tsl_pdl_cb_t callback) {
    char* results = NULL;
    int retval = 0;

    if(callback)
        retval = callback(NULL, &results, "the query");
    printf("results = '%s'\n", results);
    return retval;
}

test.py

import ctypes as ct

CALLBACK = ct.CFUNCTYPE(ct.c_int, ct.c_void_p, ct.POINTER(ct.c_char_p), ct.c_char_p)

dll = ct.CDLL('./test')
dll.function_using_callback.argtypes = CALLBACK,
dll.function_using_callback.restype = ct.c_int

@CALLBACK
def py_parse_pdl_func(pz_prv, py_results, query):
    py_results[0] = b'321'
    return 1

retval = dll.function_using_callback(py_parse_pdl_func)
print('retval =', retval)

Output:

results = '321'
retval = 1

answered Feb 2, 2023 at 16:15

Mark Tolonen

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5 Comments

drlolly Over a year ago

Superb. That worked for me also. One question. My code was identical to yours except I used '(results.contents).contents = b"321"' ' instead of 'results[0] = b"321"'. Why do you think that did not work?. I have not seen that 'indexing syntax' in the ctypes documentation. I thought you had to use contents attribute to de-reference,

Mark Tolonen Over a year ago

@drlolly .contents reads the pointer, but it returns a new object of the contents because altering it wouldn't change the original char* result passed in. Using indexing directly writes to the original object. I tracked down the problem by printing the pointers of &results and results before and after the callback in C, and comparing to the addresses received and changed in Python.

Mark Tolonen Over a year ago

The documentation mentions this: "Note that ctypes does not have OOR (original object return), it constructs a new, equivalent object each time you retrieve an attribute:"

drlolly Over a year ago

Your comment ‘Using indexing directly accesses the original object’, is the key to this problem. One point is I don’t understand where in the ctypes documentation that is stated. The documentation hints at this when it says ‘You can access or change arbitrary memory locations. ‘ But it does not explicitly say that the location is in the c module rather than the python module.

drlolly Over a year ago

For others information: I have used double indexing [0][0] to access a ptr-to-prt-int in c. Doing this, the value of the ptr-to-int did not change, it pointed to the same integer. The value of the integer was changed.

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