Extern keyword

1.extern Can be placed in front of variables or functions , A function defined in a file or other variable , The variables or functions used in the following code are foreign , Not defined in this document , Prompt the compiler to look for its definition in other modules when it encounters this variable and function . It's like putting a hat on these foreign variables or functions in this document , Tell all the codes in this document , These guys are not indigenous .

C Language policy , If you omit extern, The default is external function .

extern Whether the modified variable or function is defined or declared ？
answer ： It is a statement.

A.c  The document defines ：

int i = 10;
void fun（void）
{
    
	//code
}

B.c  Used in documents  extern  modification ：

extern int i;				
extern void fun（void）;		

2. as for extern“C” Usage of , It is generally believed to belong to C++ The category of , Let's not discuss it here .

extern "C" The main function of is to realize C++ Code calls other C The language code . add extern "C" after , Will instruct the compiler to press C Compiling of language , instead of C++ Of . because C++ Support function overload , Therefore, the compiler will add the function's parameter type to the compiled code when compiling the function , Not just function names ; and C Language does not support function overloading , So compile C The function of language code will not take the parameter type of function , In general, it includes function names .

This function is very useful , Because in C++ Before appearance , A lot of code is C Written language , And the bottom library is also C Written language , In order to better support the original C Code and what's written C Language library , Need to be in C++ Support as much as possible in C, and extern "C" One of the strategies .