具有相同名称但不同定义的结构：带有 -O2 的分段错误

Question

当两个 C++ 文件一起编译时，我在 C++ 程序中遇到了分段错误，每个文件都包含不同的结构定义（具有相同的名称）。

根据，我了解到结构定义仅限于翻译单元（文件及其包含物）。

但是，在编译时启用 -O1 或更多时我遇到了崩溃。以下最少的代码重现了段错误。

代码在 3 个简短的 C++ 文件和 2 headers:

// td_collision1.cc
#include <iostream>
#include <vector>
#include <cstdlib>
#include "td1.h"

struct Data
{
  long a;
  double m1;
  double m2;
};

void sz1(void) {
    std::cout << "Size of in collision1: " << sizeof(struct Data) << std::endl;
}

void collision1(void) {
    struct Data tmp;
    std::vector<struct Data> foo;
    for (int i=0; i<10; i++) {
        tmp.a = 1;
        tmp.m1 = 0;
        tmp.m2 = 0;
        foo.push_back(tmp);
    }
}
// td1.h
#include <iostream>

void collision1(void);
void sz1(void);

// td_collision2.cc
#include <iostream>
#include <vector>
#include <cstdlib>
#include "td2.h"

struct Data {
  long a;
  double m1; // note that there is one member less here
};

void sz2(void) {
    std::cout << "Size of in collision2: " << sizeof(struct Data) << std::endl;
}

void collision2(void) {
    struct Data tmp2;
    std::vector<struct Data> bar;
    for (int i=0; i<100; i++) {
        tmp2.a = 1;
        tmp2.m1 = 0;
        bar.push_back(tmp2); // errors occur here
    }
}
// td2.h
#include <iostream>

void collision2(void);
void sz2(void);

// td_main.cc
#include <iostream>
#include <cstdlib>
#include "td1.h"
#include "td2.h"

int main(void) {
    sz1();
    sz2();
    collision2();
}

此代码使用带有 -O0 标志的 GCC 6.3 编译，在 valgrind 下运行良好且没有错误。然而，运行它与 -O1 或 O2 导致以下输出：

Size of in collision1: 24
Size of in collision2: 16
==326== Invalid write of size 8
==326==    at 0x400F6C: construct<Data, const Data&> (new_allocator.h:120)
==326==    by 0x400F6C: construct<Data, const Data&> (alloc_traits.h:455)
==326==    by 0x400F6C: push_back (stl_vector.h:918)
==326==    by 0x400F6C: collision2() (td_collision2.cc:22)
==326==    by 0x400FE8: main (td_main.cc:10)
==326==  Address 0x5aba1f0 is 0 bytes after a block of size 96 alloc'd
==326==    at 0x4C2E1FC: operator new(unsigned long) (vg_replace_malloc.c:334)
==326==    by 0x400DE9: allocate (new_allocator.h:104)
==326==    by 0x400DE9: allocate (alloc_traits.h:416)
==326==    by 0x400DE9: _M_allocate (stl_vector.h:170)
==326==    by 0x400DE9: void std::vector<Data, std::allocator<Data> >::_M_emplace_back_aux<Data const&>(Data const&) (vector.tcc:412)
==326==    by 0x400F7E: push_back (stl_vector.h:924)
==326==    by 0x400F7E: collision2() (td_collision2.cc:22)
==326==    by 0x400FE8: main (td_main.cc:10)
==326== 
==326== Invalid write of size 8
==326==    at 0x400F69: construct<Data, const Data&> (new_allocator.h:120)
==326==    by 0x400F69: construct<Data, const Data&> (alloc_traits.h:455)
==326==    by 0x400F69: push_back (stl_vector.h:918)
==326==    by 0x400F69: collision2() (td_collision2.cc:22)
==326==    by 0x400FE8: main (td_main.cc:10)
==326==  Address 0x5aba1f8 is 8 bytes after a block of size 96 alloc'd
==326==    at 0x4C2E1FC: operator new(unsigned long) (vg_replace_malloc.c:334)
==326==    by 0x400DE9: allocate (new_allocator.h:104)
==326==    by 0x400DE9: allocate (alloc_traits.h:416)
==326==    by 0x400DE9: _M_allocate (stl_vector.h:170)
==326==    by 0x400DE9: void std::vector<Data, std::allocator<Data> >::_M_emplace_back_aux<Data const&>(Data const&) (vector.tcc:412)
==326==    by 0x400F7E: push_back (stl_vector.h:924)
==326==    by 0x400F7E: collision2() (td_collision2.cc:22)
==326==    by 0x400FE8: main (td_main.cc:10)
==326== 
==326== 
==326== HEAP SUMMARY:
==326==     in use at exit: 0 bytes in 0 blocks
==326==   total heap usage: 5 allocs, 5 frees, 73,896 bytes allocated
==326== 
==326== All heap blocks were freed -- no leaks are possible
==326== 
==326== For counts of detected and suppressed errors, rerun with: -v
==326== ERROR SUMMARY: 191 errors from 2 contexts (suppressed: 0 from 0)

当 libc 重新分配 std::vector<struct Data> bar 时，push_back() 函数失败。（在我的例子中，它的大小最初是 4 个项目，然后在循环中调用 push_back() 时进一步调整向量的大小。）当 td_collision1.cc 中的 struct Data 与 td_collision2.cc 中的大小相同时，程序不会崩溃。

因此，这两个结构定义之间似乎存在冲突。事实上，如果我重命名一个结构，错误显然就消失了。但是，如上所述，我认为这不可能发生。我误会了什么？另外，如果我去掉函数 collision1()，段错误就会消失（collision1 中的 struct Data 可能因为未使用而被编译器丢弃）

我的理解是，这两个 CC 文件之间存在明显的分离，如果 header.

中不存在结构，则 "crosstalk" 应该是不可能的

编辑：添加缺失的 td2.h

Answer 1

好吧，您链接的是 C 答案，但您的问题是关于 C++ 的。两种语言，两种标准，两种答案。

也就是说，我相信 C 答案应该也是不允许的，根据单一定义规则（两种语言都有）。违反的是未定义的行为，其中包括分段错误。

Answer 2

来自basic.def.odr,（...被我省略了）：

There can be more than one definition of a class type (Clause [class]), ..... Given such an entity named D defined in more than one translation unit, then:

each definition of D shall consist of the same sequence of tokens; and

...

If D is a template and is defined in more than one translation unit, then the preceding requirements shall apply both to names from the template's enclosing scope used in the template definition ([temp.nondep]), and also to dependent names at the point of instantiation ([temp.dep]). If the definitions of D satisfy all these requirements, then the behavior is as if there were a single definition of D. If the definitions of D do not satisfy these requirements, then the behavior is undefined.

在您的程序中，td_collision1.cc 和 td_collision2.cc 中 struct Data 的定义彼此不匹配，因此 struct Data 的定义不满足那些要求，则行为未定义。

Answer 3

你链接的答案是针对C语言的，C不是C++。

在C++中（标准引用自en.cppreference, see ），规则如下：

There can be more than one definition in a program, as long as each definition appears in a different translation unit, of each of the following: class type [...], as long as all of the following is true:

each definition consists of the same sequence of tokens (typically, appears in the same header file)

[...]

If all these requirements are satisfied, the program behaves as if there is only one definition in the entire program. Otherwise, the behavior is undefined.

你的两个定义明显违反了第一个条件，所以行为未定义。

具有相同名称但不同定义的结构：带有 -O2 的分段错误

Structure with same name, different definitions: segmentation fault with -O2

c++

struct

segmentation-fault

translation-unit