Python Bytes to CTypes Void Pointer

Question

I have a c function that takes as arguments a void * pointer and an integer length for the size of the buffer pointed to.

e.g.

char* myfunc(void *mybuffer, int buflen)

On the python side I have a bytes object of binary data read from a file.

What I am trying to figure out is the right conversions to be able to call the c function from python, and am struggling a bit.

I understand the conversions for dealing with simple string data (e.g. encoding to utf-8 and using a char_p type) but dealing with a bytes object has been a bit of a struggle....

Thanks in advance!

The answer depends on the function. Is the return value a null-terminated string? An allocated string that must be freed later? Can it contain embedded nulls? For the mybuffer parameter, will the contents be mutable? Provide a simple implementation in C of the function, and your attempt at calling it with Python. See minimal reproducible example. — Mark Tolonen
– Mark Tolonen, Commented Jun 15, 2022 at 0:41
Also, if your data comes from a binary file, you shouldn't use c_char_p (but POINTER(c_char)). — CristiFati
– CristiFati, Commented Jun 15, 2022 at 5:55
@MarkTolonen yes, the return will be a null terminated string with no embedded nulls (the return value is not a problem to handle in other cases where I've used c-python interactions). The challenge I'm having is identifying the proper argument as well as conversions for arbitrary binary data as input to the called c-function. No, the input data will not be mutated in anyway by either caller or callee. I'll update to a more complete example later today. — K.F. Adams
– K.F. Adams, Commented Jun 15, 2022 at 17:33

Mark Tolonen · Accepted Answer · 2022-06-15 18:15:57Z

2

Given your commented description, you can just use the obvious types if you don't need to free the returned char* memory. You can pass a bytes object to a void*. Here's a quick demo:

test.c

#include <stdio.h>
#include <stdint.h>

#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

API char* myfunc(void *mybuffer, int buflen) {
    const uint8_t* tmp = (const uint8_t*)mybuffer;
    for(int i = 0; i < buflen; ++i) // show the passed bytes
        printf("%02X\n", tmp[i]);
    return "output"; // static string no deallocation required
}

test.py

import ctypes as ct
import os

dll = ct.CDLL('./test')
dll.myfunc.argtypes = ct.c_void_p, ct.c_int
dll.myfunc.restype = ct.c_char_p

buf = bytes([1,2,0,0xaa,0x55])  # including embedded null
ret = dll.myfunc(buf, len(buf))
print(ret)

Output:

01
02
00
AA
55
b'output'

answered Jun 15, 2022 at 18:15

Mark Tolonen

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5 Comments

K.F. Adams Over a year ago

Thanks. I didn't realize you could hand in a straight bytes object with no other changes like that to a void* arg. Much appreciated.

Vadim Kantorov Over a year ago

If I have ctypes.POINTER(ctypes.c_int16) in argtypes definition, can I pass in a buf = bytes(...)-object as ctypes.cast(buf, ctypes.POINTER(ctypes.c_int16))? Will it marshal the pointer correctly?

Mark Tolonen Over a year ago

@VadimKantorov That should normally just be submitted as a question, but in this case just try it (hint: it works).

Vadim Kantorov Over a year ago

I tried it, it seems to work okay, but I first was confused that

a = bytes([1,2,3,4]); print(b := ctypes.cast(a, ctypes.POINTER(ctypes.c_int16))); print(c := ctypes.cast(a, ctypes.POINTER(ctypes.c_int16)));

prints two different addresses <__main__.LP_c_short object at 0x7f6f02a2a540>, <__main__.LP_c_short object at 0x7f6f02a2a240>, but ctypes.cast(b, ctypes.c_void_p).value and ctypes.cast(c, ctypes.c_void_p).value do print the same, so I'm hoping that no buffer re-allocations happened behind the scenes...

Mark Tolonen Over a year ago

@VadimKantorov Again, ask a question. Code in comments is not readable. Those addresses are of the python objects, not the buffers they represent

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