extern“C"的作用是什么?在 C++ 中? [英] What is the effect of extern "C" in C++?

问题描述

将 extern "C" 放入 C++ 代码中究竟有什么作用?

What exactly does putting extern "C" into C++ code do?

例如:

extern "C" {
   void foo();
}

推荐答案

extern "C" 使 C++ 中的函数名具有 C 链接(编译器不会破坏名称)以便客户端C 代码可以使用仅包含函数声明的 C 兼容头文件链接到(使用)您的函数.您的函数定义包含在二进制格式中(由您的 C++ 编译器编译)，然后客户端 C 链接器将使用 C 名称链接到该格式.

extern "C" makes a function-name in C++ have C linkage (compiler does not mangle the name) so that client C code can link to (use) your function using a C compatible header file that contains just the declaration of your function. Your function definition is contained in a binary format (that was compiled by your C++ compiler) that the client C linker will then link to using the C name.

由于 C++ 有函数名的重载而 C 没有，C++ 编译器不能只使用函数名作为唯一的 id 来链接，所以它通过添加有关参数的信息来破坏名称.AC 编译器不需要修改名称，因为您不能在 C 中重载函数名称.当您声明一个函数在 C++ 中具有 extern "C" 链接时，C++ 编译器不会添加参数/用于链接的名称的参数类型信息.

Since C++ has overloading of function names and C does not, the C++ compiler cannot just use the function name as a unique id to link to, so it mangles the name by adding information about the arguments. A C compiler does not need to mangle the name since you can not overload function names in C. When you state that a function has extern "C" linkage in C++, the C++ compiler does not add argument/parameter type information to the name used for linkage.

正如您所知，您可以显式指定 extern "C" 链接到每个单独的声明/定义，或者使用块将一系列声明/定义分组以具有特定链接:

Just so you know, you can specify extern "C" linkage to each individual declaration/definition explicitly or use a block to group a sequence of declarations/definitions to have a certain linkage:

extern "C" void foo(int);
extern "C"
{
   void g(char);
   int i;
}

如果您关心技术细节，它们在 C++03 标准的第 7.5 节中列出，这里是一个简短的总结(重点是 extern C"):

If you care about the technicalities, they are listed in section 7.5 of the C++03 standard, here is a brief summary (with emphasis on extern "C"):

• extern "C" 是一个链接规范
• 每个编译器都要求提供C"联动
• 链接规范只能出现在命名空间范围内
• 所有函数类型、函数名和变量名都有语言链接 参见Richard的评论:只有具有外部链接的函数名和变量名才有语言链接
• 具有不同语言链接的两种函数类型是不同类型，即使在其他方面相同
• 联动规格嵌套，内部决定最终联动
• extern "C" 被类成员忽略
• 至多一个具有特定名称的函数可以有C"链接(不考虑命名空间)
• extern "C" 强制函数具有外部链接(不能使其成为静态) 见理查德的评论: staticextern "C" 内有效；如此声明的实体具有内部链接，因此没有语言链接
• 从 C++ 到用其他语言定义的对象以及从其他语言到用 C++ 定义的对象的链接是实现定义和语言相关的.只有两种语言实现的对象布局策略足够相似，才能实现这种联动

• extern "C" is a linkage-specification
• Every compiler is required to provide "C" linkage
• A linkage specification shall occur only in namespace scope
• All function types, function names and variable names have a language linkage See Richard's Comment: Only function names and variable names with external linkage have a language linkage
• Two function types with distinct language linkages are distinct types even if otherwise identical
• Linkage specs nest, inner one determines the final linkage
• extern "C" is ignored for class members
• At most one function with a particular name can have "C" linkage (regardless of namespace)
• extern "C" forces a function to have external linkage (cannot make it static) See Richard's comment: static inside extern "C" is valid; an entity so declared has internal linkage, and so does not have a language linkage
• Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved

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