Variable Declaration in C

Variable Declaration in C

A variable declaration provides assurance to the compiler that there exists a
variable with the given type and name so that the compiler can proceed for
further compilation without requiring the complete detail about the variable. A
variable declaration has its meaning at the time of compilation only, the
compiler needs actual variable declaration at the time of linking the program.
A variable declaration is useful when you are using multiple files and you define
your variable in one of the files which will be available at the time of linking the
program. You will use the keyword extern to declare a variable at any place.
Though you can declare a variable multiple times in your C program, it can be
defined only once in a file, a function, or a block of code.

Example
Try the following example, where variables have been declared at the top, but
they have been defined and initialized inside the main function:

#include <stdio.h> // Variable declaration: extern int a, b; extern int c; extern float f; int main () { /* variable definition: */ int a, b; int c; float f; /* actual initialization */ a = 10; b = 20; c = a + b; printf("value of c : %d \n", c); f = 70.0/3.0; printf("value of f : %f \n", f); return 0; }

When the above code is compiled and executed, it produces the following result:

value of c : 30 value of f : 23.333334

The same concept applies on function declaration where you provide a function
name at the time of its declaration and its actual definition can be given
anywhere else. For example:

// function declaration int func(); int main() { // function call int i = func(); } / / function definition int func() { return 0; }