The extern Storage Class

The extern Storage Class

The extern storage class is used to give a reference of a global variable that is
visible to ALL the program files. When you use 'extern', the variable cannot be
initialized, however, it points the variable name at a storage location that has
been previously defined.
When you have multiple files and you define a global variable or function, which
will also be used in other files, then extern will be used in another file to provide
the reference of defined variable or function. Just for understanding, extern is
used to declare a global variable or function in another file.
The extern modifier is most commonly used when there are two or more files
sharing the same global variables or functions as explained below.

First File: main.c

#include <stdio.h> int count; extern void write_extern(); main() { count = 5; write_extern(); }

Second File: support.c

#include <stdio.h> extern int count; void write_extern(void) { printf("count is %d\n", count); }

Here, extern is being used to declare count in the second file, whereas it has its
definition in the first file, main.c. Now, compile these two files as follows:

$gcc main.c support.c

It will produce the executable program a.out. When this program is executed, it
produces the following result:

5