尝试释放CString数组时发生内存泄漏

Question

以下（MCVE如果编译为cdylib称为libffitest，则需要libc作为依赖项）说明了问题：

use libc::{c_char, c_void, size_t};
use std::ffi::CString;
use std::mem;
use std::slice;

#[repr(C)]
#[derive(Clone)]
pub struct Array {
    pub data: *const c_void,
    pub len: size_t,
}

#[no_mangle]
pub unsafe extern "C" fn bar() -> Array {
    let v = vec![
        CString::new("Hi There").unwrap().into_raw(),
        CString::new("Hi There").unwrap().into_raw(),
    ];
    v.into()
}

#[no_mangle]
pub extern "C" fn drop_word_array(arr: Array) {
    if arr.data.is_null() {
        return;
    }
    // Convert incoming data to Vec so we own it
    let mut f: Vec<c_char> = arr.into();
    // Deallocate the underlying c_char data by reconstituting it as a CString
    let _: Vec<CString> = unsafe { f.iter_mut().map(|slice| CString::from_raw(slice)).collect() };
}

// Transmute to array for FFI
impl From<Vec<*mut c_char>> for Array {
    fn from(sl: Vec<*mut c_char>) -> Self {
        let array = Array {
            data: sl.as_ptr() as *const c_void,
            len: sl.len() as size_t,
        };
        mem::forget(sl);
        array
    }
}

// Reconstitute from FFI
impl From<Array> for Vec<c_char> {
    fn from(arr: Array) -> Self {
        unsafe { slice::from_raw_parts_mut(arr.data as *mut c_char, arr.len).to_vec() }
    }
}

我认为，通过将传入的Array重组为一个片，将它的所有权作为Vec，然后将元素重组为CString，我释放了所有分配的内存，但我显然做错了什么。执行这个Python脚本告诉我它试图释放一个未分配的指针： python(85068,0x10ea015c0) malloc: *** error for object 0x7ffdaa512ca1: pointer being freed was not allocated

import sys
import ctypes
from ctypes import c_void_p, Structure, c_size_t, cast, POINTER, c_char_p

class _FFIArray(Structure):
    """
    Convert sequence of structs to C-compatible void array

    """
    _fields_ = [("data", c_void_p),
                ("len", c_size_t)]


def _arr_to_wordlist(res, _func, _args):
    ls = cast(res.data, POINTER(c_char_p * res.len))[0][:]
    print(ls)
    _drop_wordarray(res)


prefix = {"win32": ""}.get(sys.platform, "lib")
extension = {"darwin": ".dylib", "win32": ".dll"}.get(sys.platform, ".so")
lib = ctypes.cdll.LoadLibrary(prefix + "ffitest" + extension)

lib.bar.argtypes = ()
lib.bar.restype = _FFIArray
lib.bar.errcheck = _arr_to_wordlist
_drop_wordarray = lib.drop_word_array

if __name__ == "__main__":
    lib.bar()

Answer 1

嗯，那是一个有趣的经历。你知道吗

您最大的问题是以下转换：

impl From<Array> for Vec<c_char> {
    fn from(arr: Array) -> Self {
        unsafe { slice::from_raw_parts_mut(arr.data as *mut c_char, arr.len).to_vec() }
    }
}

从FFI边界出来的字符串数组开始（即*mut *mut c_char）。出于某种原因，您突然决定，它是Vec<c_char>，而不是Vec<*const c_char>，正如您所期望的CString转换。这就是UB#1-免费后使用的原因。你知道吗

由于类型之间的不断变换，不必要的复杂转换使得事情变得更加混乱。如果您的FFI边界是Vec<CString>，为什么要将返回拆分为两个单独的调用？事实上，这是在呼唤灾难。你知道吗

考虑以下几点：

impl From<Array> for Vec<CString> {
    fn from(arr: Array) -> Self {
        unsafe {
          slice::from_raw_parts(
            arr.data as *mut *mut c_char,
            arr.len
          )
            .into_iter().map(|r| CString::from_raw(*r))
            .collect()
        }
    }
}

这提供了一个一步FFI边界转换（无需在方法中使用第二个unsafe块）、干净类型和无泄漏。你知道吗