What is the use of extern in C(C语言中extern的用途)?

The "extern" declaration in C is to indicate the existence of, and the type of, a global variable or function. A global variable, or a global function, is one that is available to all C modules (a single C module is typically a single .c file). An extern is something that is defined externally to the current module. In many cases, you can leave off the extern qualifier and not notice any difference because the linker can collapse multiple definitions to one. But the intent is then unclear in the code, and the code is error prone in case of typos. It is much clearer to define the global in one place, and then declare extern references to it in all the other places. When refering to globals provided by a library, especially a shared library, this is even more important in order to ensure you are talking about the correct, common instance of the variable.

Declaring a variable as extern will result in your program not reserving any memory for the variable in the scope that it was declared. For instance (as example) if a program's source code declared the variable var as a global volatile int in foo.c, to properly use it in bar.c you would declare it as extern volatile int var.

It is also not uncommon to find function prototypes declared as extern.

 

Zend/zend.h中有关的定义：

#ifdef __cplusplus
#define BEGIN_EXTERN_C() extern "C" {
#define END_EXTERN_C() }
#else
#define BEGIN_EXTERN_C()
#define END_EXTERN_C()
#endif