Passing a C++ type to a C-style Swift function crashes: malloc: Heap corruption detected ***

[mrousavy]

Description

I have a C-style Swift function that will be called from C++ later. This is my "callback":

func callCppCode() {
  func __callback(value: SharedPtrOfString) {
    let string = String(value.pointee)
    print("Got value from C++: \(value)")
  }
  myCppFunc(__callback)
}

And on the C++ side, myCppFunc looks like this:

#pragma once

#include <string>
#include <memory>

using SharedPtrOfString = std::shared_ptr<std::string>;

inline void myCppFunc(void(* _Nonnull call)(SharedPtrOfString)) {
  auto shared = std::make_shared<std::string>("HELLO!");
  call(shared);
}

When running this, it crashes with the following error:

- SwiftSharedPointerTest(33577,0x1e6e6f840) malloc: Heap corruption detected, free list is damaged at 0x600002b91a10
- *** Incorrect guard value: 1
- SwiftSharedPointerTest(33577,0x1e6e6f840) malloc: *** set a breakpoint in malloc_error_break to debug

Note

When I replaced SharedPtrOfString with Double or even std.string, it works. It's just std::shared_ptr<...> that makes it crash with a heap corruption error.

Reproduction

func callCppCode() {
  func __callback(value: SharedPtrOfString) {
    let string = String(value.pointee)
    print("Got value from C++: \(value)")
  }
  myCppFunc(__callback)
}

#pragma once

#include <string>
#include <memory>

using SharedPtrOfString = std::shared_ptr<std::string>;

inline void myCppFunc(void(* _Nonnull call)(SharedPtrOfString)) {
  auto shared = std::make_shared<std::string>("HELLO!");
  call(shared);
}

Stack dump

Got value from C++: shared_ptr<basic_string<CChar, char_traits<CChar>, allocator<CChar>>>()
SwiftSharedPointerTest(34066,0x1e6e6f840) malloc: Heap corruption detected, free list is damaged at 0x600001726730
*** Incorrect guard value: 7813868916180873059
SwiftSharedPointerTest(34066,0x1e6e6f840) malloc: *** set a breakpoint in malloc_error_break to debug

Expected behavior

I expect the shared_ptr to be free'd fine instead of corrupting the heap.

Environment

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx15.0

Additional information

I have created a reproduction repro here: https://github.com/mrousavy/SwiftSharedPointerBugRepro

Just download and run this (I chose "Run on my Mac (Designed for iPad)") to see the bug.

[ChrisSchofieldCheckatrade]

Possibly related: #67355 (comment)

[mrousavy]

Pretty stupid, but if I just do an unbalanced +1 ref-count increment on the shared_ptr before passing it to the Swift C-Style callback function it works!

It also properly calls the destructor of std::string after the shared_ptr goes out of scope, so I can confirm that the Swift compiler accidentally seems to destroy the shared_ptr twice when it's being passed to a C-style Swift function.

Does anyone have any pointers on where to look in the Swift compiler codebase? I'd love to tackle this myself but can't really seem to grasp the structure here. Not a compiler engineer.

[mrousavy]

So apparently this does not only happen with shared_ptr, but also with any other type - e.g.:

struct SomeType {
  SomeType() {
    printf("constructed!");
  }
  ~SomeType() {
    printf("destructed!");
  }
};