外部"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 ++代码中究竟能做什么?

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

例如:

extern "C" {
   void foo();
}

推荐答案

外部"C"使C ++中的函数名称具有'C'链接(编译器不会更改名称)，以便客户端C代码可以链接到(即使用)您的函数，使用仅包含函数声明的"C"兼容头文件.您的函数定义包含在二进制格式(由C ++编译器编译)中，客户端'C'链接程序随后将使用'C'名称进行链接.

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 (i.e 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 ++重载了函数名，而C没有重载，因此C ++编译器不能仅使用函数名作为要链接的唯一ID，因此它通过添加有关自变量的信息来破坏名称. AC编译器不需要处理名称，因为您不能在C中重载函数名称.当您声明函数在C ++中具有外部"C"链接时，C ++编译器不会将参数/参数类型信息添加到用于链接.

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.

您知道，您可以为每个单独的声明/定义指定"C"链接，也可以使用一个块将声明/定义序列分组以具有一定的链接:

Just so you know, you can specify "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;
}

如果您关心技术方面的问题，它们会在C ++ 03标准的7.5节中列出，这是一个简短的摘要(重点是外部"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"):

• 外部"C"是链接规范
• 需要每个编译器提供"C"链接
• 链接规范应仅在命名空间范围内出现
• 所有函数类型，函数名称和变量名称均具有语言链接
• 具有特定名称的最多一个函数可以具有"C"链接(无论名称空间如何)
• extern"C"强制函数具有外部链接(无法使其静态)
• 从C ++到其他语言定义的对象以及从其他语言到C ++定义的对象的链接是实现定义的，并且取决于语言.两种语言实现的对象布局策略只有足够相似的情况下，才能实现这种链接

• 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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