A function declared with aninlinefunction specifier is aninline function. Making a
function an inline function suggests that calls to the function be as fast as possible.
1.external linkage
An inline definition of a function with external linkage shall not contain a definition of a
modifiable object with static or thread storage duration, and shall not contain a reference
to an identifier with internal linkage
2.internal linkage
Any function with internal linkage can be an inline function