Category 

Storage class specifiers 

Syntax  

extern <data definition> ;
[extern] <function prototype> ;

Description 

Use the extern modifier to indicate that the actual storage and initial value of a variable, or body of a function, is defined in a separate source code module. Functions declared with extern are visible throughout all source files in a program, unless you redefine the function as static. 

The keyword extern is optional for a function prototype. 

Use extern "C" to prevent function names from being mangled in C++ programs.