如何从C++释放Ada堆分配Tagged Record？遇双释放错误

问题概述 #

运行代码时触发以下错误：

20
double free or corruption (out)

raised PROGRAM_ERROR : unhandled signal

目标是构建插件架构，以Ada动态库作为特定类型工厂（类似boost.dll），需解决跨语言释放Ada堆分配Tagged Record的问题，确认是否必须依赖Ada的释放机制。

代码示例 #

main.cpp #

#include <iostream>
#include "animal.h"

extern "C" {
    void adainit (void);
    void adafinal (void);
    Animal* new_animal();
    void del_animal(Animal *);
}

int main(void) {
    adainit();
    Animal* A = new_animal();
    std::cout << A->age() << std::endl;
    //delete A;   
    del_animal(A);
    adafinal();
    return 0;
};

alib.ads #

with Interfaces.C;

package ALib is

    type Animal is tagged record
        The_Age : Interfaces.C.int;
    end record;
    pragma Convention (CPP, Animal);

    type Animal_Class_Access is access Animal'Class;

    function New_Animal return access Animal'Class;
    pragma Export(CPP, New_Animal);

    procedure Del_Animal (this : in out Animal_Class_Access);
    pragma Export(CPP, Del_Animal);

    function Age(X : Animal) return Interfaces.C.int;
    pragma Export(CPP, Age);

end ALib;

alib.adb #

with ada.unchecked_deallocation;

package body ALib is

    function New_Animal
        return access Animal'Class is
    begin
        return new Animal'(The_Age => 20);
    end New_Animal;

    procedure Del_Animal (this : in out Animal_Class_Access) is
        procedure Free is new ada.unchecked_deallocation(Animal'Class, Animal_Class_Access);
    begin
        Free(this);
        --null;
    end Del_Animal;

    function Age(X : Animal)
        return Interfaces.C.int is
    begin
        return X.The_Age;
    end Age;

end ALib;

已尝试操作 #

• 为创建Free过程测试多种类型与访问类型组合：
  • Animal + type Animal_Access is access Animal
  • Animal'Class + type Animal_Class_Access is access Animal'Class
  • Animal + type Animal_Access is access Animal'Class
• 考虑使用System.Address作为New_Animal返回值和Del_Animal参数

解决方案 #

必须使用Ada的内存释放机制，核心原因是Ada与C的内存分配器独立，且Ada类访问类型（Animal'Class）的内存布局与C对象不兼容，跨语言直接释放必然触发内存错误。

具体修改步骤 #

1. 修正Ada端接口类型匹配
   将Ada的访问类型改为与C++指针兼容的类型，显式指定导出名称避免命名规则冲突：

   with Interfaces.C;
   
   package ALib is
   
       type Animal is tagged record
           The_Age : Interfaces.C.int;
       end record;
       pragma Convention (CPP, Animal);
   
       type Animal_Access is access Animal;
       pragma Convention (CPP, Animal_Access); -- 确保与C++指针布局兼容
   
       function New_Animal return Animal_Access;
       pragma Export(CPP, New_Animal, "new_animal");
   
       procedure Del_Animal (This : Animal_Access);
       pragma Export(CPP, Del_Animal, "del_animal");
   
       function Age(X : Animal) return Interfaces.C.int;
       pragma Export(CPP, Age, "age");
   
   end ALib;
   

2. 修正Ada端释放逻辑
   使用匹配的访问类型创建Unchecked_Deallocation过程，确保内存释放正确：

   with Ada.Unchecked_Deallocation;
   
   package body ALib is
   
       function New_Animal return Animal_Access is
       begin
           return new Animal'(The_Age => 20);
       end New_Animal;
   
       procedure Free_Animal is new Ada.Unchecked_Deallocation(Animal, Animal_Access);
   
       procedure Del_Animal (This : Animal_Access) is
           Temp : Animal_Access := This;
       begin
           Free_Animal(Temp);
       end Del_Animal;
   
       function Age(X : Animal) return Interfaces.C.int is
       begin
           return X.The_Age;
       end Age;
   
   end ALib;
   

3. C++端保持接口一致性
   确保animal.h的声明与Ada导出接口匹配：

   class Animal {
   public:
       int age() {
           // 对应Ada导出的age函数
           extern "C" int age(Animal&);
           return age(*this);
       }
   };
   

插件架构最佳实践 #

• 所有Ada堆分配对象，必须通过Ada导出的释放函数清理，禁止在C++中直接操作内存。
• 多态插件类型建议用System.Address传递句柄，在Ada内部转换为类访问类型，兼顾类型安全与跨语言兼容性。

内容的提问来源于stack exchange，提问作者shakeysailing