为什么我们需要extern“C”{#include< foo.h> }在C ++？ [英] Why do we need extern "C"{ #include <foo.h> } in C++?

问题描述

为什么我们需要使用：

  externC{
 #include< foo.h> ; 
} 
  

具体来说：

• 我们应该何时使用它？


<级别要求我们使用它吗？



• 如何在编译/链接方面如何解决需要我们使用它的问题？

解决方案

C和C ++在表面上类似，但每个都编译成一组非常不同的代码。当您使用C ++编译器包含头文件时，编译器需要C ++代码。然而，如果它是一个C头，那么编译器期望包含在头文件中的数据被编译为某种格式--C ++'ABI'或'应用程序二进制接口'，因此链接器阻塞。这是比传递C ++数据到期望C数据的函数更好的。

（为了进入真正的细节，C ++的ABI通常 /方法，因此调用 printf（）而不将原型标记为C函数，C ++实际上将生成调用 _Zprintf

所以：use externC{...}; 当包括ac标题时，就是这么简单。否则，编译后的代码会出现不匹配，链接器会阻塞。然而，对于大多数头文件，你甚至不需要 extern ，因为大多数系统C头文件已经说明它们可能被C ++代码包含并已经 extern 他们的代码。

Why do we need to use:

extern "C" {
#include <foo.h>
}

Specifically:

• When should we use it?

• What is happening at the compiler/linker level that requires us to use it?

• How in terms of compilation/linking does this solve the problems which require us to use it?

解决方案

C and C++ are superficially similar, but each compiles into a very different set of code. When you include a header file with a C++ compiler, the compiler is expecting C++ code. If, however, it is a C header, then the compiler expects the data contained in the header file to be compiled to a certain format—the C++ 'ABI', or 'Application Binary Interface', so the linker chokes up. This is preferable to passing C++ data to a function expecting C data.

(To get into the really nitty-gritty, C++'s ABI generally 'mangles' the names of their functions/methods, so calling printf() without flagging the prototype as a C function, the C++ will actually generate code calling _Zprintf, plus extra crap at the end.)

So: use extern "C" {...}; when including a c header—it's that simple. Otherwise, you'll have a mismatch in compiled code, and the linker will choke. For most headers, however, you won't even need the extern because most system C headers will already account for the fact that they might be included by C++ code and already extern their code.

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