Extern function in c file：What is C extern Keyword?

Introduction

The extern keyword in C is used to declare a variable as a global variable such that a variable declared in another scope of the same file or another file can be accessed from anywhere in the program. The variables or functions declared with the extern keyword in C will be linked externally(external linkage).

Declaration

It is important to note that the C extern keyword only declares a variable and does not define a variable.

The C extern keyword declares global variables, mostly in the header files used in other files of a long program.

Declaring a variable implies that the variable is introduced to the compiler[1] along with the data type of the variable. Defining a variable implies that the compiler allocates the storage required for the variable A variable definition can also be considered as a declaration, but only in some cases.

Syntax

The extern keyword is used before the data type of a variable or the return data type of a function. The synt...