Allocation of Memory with python to pass to dll

Listing [Python.Docs]: ctypes – A foreign function library for Python.

Issues:

• I shallowly browsed Functional Mock-up Interface , but I didn’t see any reference to the API. Anyway, this hi, lo approach is poor (looks like it’s from from the 16bit (segment, offset) era). Pointers exist for decades, and that’s precisely their purpose: handling memory addresses

  • There’s no size limit ("how far can one go" from the (start) address), meaning that the API consumers might end up accessing memory that they don’t own (Undefined Behavior – very likely to crash)

  The above make me think there’s a (big) misunderstanding about the API

• Don’t mix Python and CTypes object addresses, they are not the same! [Python.Docs]: Built-in Functions – id(object) returns the Python wrapper object (PyObject) address, not the actual pointer (that you care about)

• A very common one (missing functions argtypes, restype): [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati’s answer), but it’s not affecting the current scenario

• Since you’re on Win, take a look at [MS.Docs]: ULARGE_INTEGER union (winnt.h), no need to reinvent the wheel. Anyway, I removed that to be as platform independent as possible

• Combining the 2 ints in a unsigned long long might have disastrous effects for a negative value (sign bit set) of hi

• More minor ones (that don’t worth mentioning individually)

I prepared a small example.

dll00.c:

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


#if defined(_WIN32)
#  define DLL00_EXPORT_API __declspec(dllexport)
#else
#  define DLL00_EXPORT_API
#endif

#define BYTE unsigned char


static uint32_t addrLo = 0;
static uint32_t addrHi = 0;


#if defined(__cplusplus)
extern "C" {
#endif

DLL00_EXPORT_API void setAddrLo(int lo);
DLL00_EXPORT_API void setAddrHi(int hi);
DLL00_EXPORT_API void modifyArray();

#if defined(__cplusplus)
}
#endif


void setAddrLo(int lo) {
    addrLo = (uint32_t)lo;
}

void setAddrHi(int hi) {
    addrHi = (uint32_t)hi;
}

static BYTE* toPtr(uint32_t hi, uint32_t lo) {
#if SIZE_MAX == 0xffffffffffffffffull
    uint64_t quad = ((uint64_t)hi << 32) + lo;
    printf("C  - Addr: 0x%016llX, Hi: 0x%08X, Lo: 0x%08Xn", quad, hi, lo);
    return (BYTE*)quad;
#elif SIZE_MAX == 0xfffffffful
    printf("C  - Addr: 0x%016llX, Hi: 0x%08X, Lo: 0x%08Xn", lo, hi, lo);
    return (BYRE*)lo;
#else
#  error "Neither 64bit nor 32bit architecture"
#endif
}

void modifyArray() {  // A 'size' argument would be make sense to specify maximum array index.
    BYTE *addr = toPtr(addrHi, addrLo);
    if (addr == NULL) {
        printf("C  - NULL pointer!n");
        return;
    }
    addr[0] *= 2;
    addr[1] = 2;
    addr[10] = addr[0];
}

code00.py:

#!/usr/bin/env python

import sys
import ctypes as ct


DLL_NAME = "./dll00.{:s}".format("dll" if sys.platform[:3].lower() == "win" else "so")
BUF_LEN = 20


def main(*argv):
    dll = ct.CDLL(DLL_NAME)
    set_addr_lo = dll.setAddrLo
    set_addr_lo.argtypes = (ct.c_int,)
    set_addr_lo.restype = None
    set_addr_hi = dll.setAddrHi
    set_addr_hi.argtypes = (ct.c_int,)
    set_addr_hi.restype = None
    modify_array = dll.modifyArray
    modify_array.argtypes = ()
    modify_array.restype = None

    b = b"x20x03x20"
    Array = ct.c_char * BUF_LEN
    buf = Array(*b)
    print("PY - Array: {:}".format(list(ord(i) for i in buf)))
    addr = ct.addressof(buf)  # The reverse of what's done in the .dll (toPtr) - doesn't make much sense
    if sys.maxsize > 0x100000000:
        lo = addr & 0xffffffff
        hi = (addr >> 32) & 0xffffffff
    else:
        hi = 0
        lo = addr
    print("PY - Addr: 0x{:016X}, Hi: 0x{:08X}, Lo: 0x{:08X}".format(addr, hi, lo))
    set_addr_lo(lo)
    set_addr_hi(hi)
    modify_array()
    print("PY - Array: {:}".format(list(ord(i) for i in buf)))


if __name__ == "__main__":
    print("Python {:s} {:03d}bit on {:s}n".format(" ".join(elem.strip() for elem in sys.version.split("n")),
                                                   64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    rc = main(*sys.argv[1:])
    print("nDone.")
    sys.exit(rc)

Output:

[cfati@CFATI-5510-0:e:WorkDevStackOverflowq068304564]> sopr.bat
### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###

[prompt]> "c:Installpc032MicrosoftVisualStudioCommunity2019VCAuxiliaryBuildvcvarsall.bat" x64 >nul

[prompt]> dir /b
code00.py
dll00.c

[prompt]>
[prompt]> cl /nologo /MD /DDLL dll00.c  /link /NOLOGO /DLL /OUT:dll00.dll
dll00.c
   Creating library dll00.lib and object dll00.exp

[prompt]> dir /b
code00.py
dll00.c
dll00.dll
dll00.exp
dll00.lib
dll00.obj

[prompt]>
[prompt]> "e:WorkDevVEnvspy_pc064_03.08.07_test0Scriptspython.exe" code00.py
Python 3.8.7 (tags/v3.8.7:6503f05, Dec 21 2020, 17:59:51) [MSC v.1928 64 bit (AMD64)] 064bit on win32

PY - Array: [32, 3, 32, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
PY - Addr: 0x00000203275B23B0, Hi: 0x00000203, Lo: 0x275B23B0
C  - Addr: 0x00000203275B23B0, Hi: 0x00000203, Lo: 0x275B23B0
PY - Array: [64, 2, 32, 0, 0, 0, 0, 0, 0, 0, 64, 0, 0, 0, 0, 0, 0, 0, 0, 0]

Done.