Variable declaration in C Programming and Rules for writing the variable name

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 definition has its meaning at the time of compilation only, the compiler needs actual variable definition at the time of linking the program.

#include <stdio.h>
extern int a, b;
extern int c;
extern float f;
int main ( )
{
int a , b ;
int c ;
float f ;
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;
}

Output:
value of c: 30
value of f: 23.333333


Rules for writing the variable name in C

1. A variable name can have letters (both uppercase and lowercase letters), digits and underscore only.
2. The first letter of a variable should be either a letter or an underscore. However, it is discouraged to start the variable name with an underscore. It is because the variable name that starts with an underscore can conflict with a system name and may cause an error.
3. There is no rule on how long a variable can be. However, the first 31 characters of a variable are discriminated by the compiler. So, the first 31 letters of two variables in a program should be different.