The compiler checks calls to external-language functions in the same way that it handlesordinary C++ functions. However, the compiler typically must generate different code to callfunctions written in other languages. C++ uses linkage directives to indicate the languageused for any non-C++ function.

链接指示：
有两种形式： 单个或者复合的。不能出现在类定义或者函数定义的内部，必须出现在函数的第一次声明上。

头文件cstdlib中声明的一些C函数：
extern "C" size_t strlen(const char*);
extern "C" {
int strcmp(const char*,const );
char * strcat(char*,const char*);
}

The first form consists of the extern keyword followed by a string literal, followed by an"ordinary" function declaration. The string literal indicates the language in which the function iswritten.