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Every variable declared outside of the functions as a global scope, that is, it can be accessed from any point in the program. The following code shows an example of this situation.

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int number;
int function() 
{
    number++;
    return number;
}

int main(int argc, char *argv[]) 
{
    number = 0;
    function();
    return 0;
}

Variable number is declared in line 1, outside a function, therefore it is global. Next, the variable is accessed in lines 4 and 10.

But in order to access correctly a global variable, there are two requirements to fulfill derived from the way the compiler processes the files. If the variable is declared in the same file, its declaration must precede its use (the compiler reads each file in one pass). If the variable is declared in another file, the same exact definition must be included in the file but with the prefix “extern” (the compiler only remembers information in the file that is processing).

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int number = 10;
int main(int argc, char *argv[]) 
{
    return 1;
}

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extern int number;
int function() 
{
    number++;
    return number;
}

Variable number is defined as global in line 1 of file 1. To access it from file 2, the declaration (without initialization) is replicated in line 1 of file 2 adding the “extern” prefix. The global variable is accessed in line 4 of file 2. If the first line of this file is omitted, the compiler emits an error that the variable number has not been declared. If in file 2 the declaration is included without the “extern”, the compiler emits the error that a variable has been multiply defined.

Any variable declaration may have the prefix “static”. Static variables in C have the following two properties:

As a consequence of these two properties, the following two cases are derived:

The following program shows an example of the behavior of a static variable defined in a function.

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#include <stdio.h>
int function() 
{
    static int number = 10; /* Static variable */
    number++;               /* Maintains the value of the previous execution */
    return number;
}
int main() 
{
    /* Prints the result of two invocations to function */
    printf("%d\n", function()); /* Prints 11 */
    printf("%d\n", function()); /* Prints 12! */
    return 0;
}

Line 4 declares the static variable number and assigns the value 10. Thus, the first time the function is executed, the variable has this value and is increased to 11. This is the value printed when executing line 11. But the function is invoked again, number maintains its value and therefore is increased to 12, which is the value printed when executing line 12 of the program.

The following example shows the behavior of a static variable when defined outside a function.

/* Global variable */
int number = 10;
int main(int argc, char *argv[]) 
{
    number++;
    return 0;
}
/* Available only from this point in this file */
static int coefficient = 20;
void function2() 
{
    number++;
    coefficient++;
}

extern int number;
/* Variable coefficient cannot 
   be accessed in this file */
int function() 
{
    number++;
    return number;
}

The variable number is declared globally and may be accessed from other file when including its declaration with the “extern” prefix. However, the variable coefficient is only accessible from the file in which it is declared and only from that point on (it is not visible in function main.

The problem of “shadowing” appears when a variable is defined in a scope with the same name of another one valid in a higher level scope. The following example shows this situation:

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#include <stdio.h>
int number = 10;
void function() 
{
    int number; /* Colission with global variable*/
    number = 20;
}
int main(int argc, char *argv[]) 
{
    function();
    /* Print the value of number */
    printf("%d\n", number); /* Which value is printed? */
    return 0;
}

The declarations in lines 2 and 5 are identical but they are included in different scopes (global versus local to a function). The compiler allows this declaration. But the shadowing appears when the function is executed: the global variable number cannot be accessed because the name refers to the variable local to the function. Upon function termination, the global variable is no longer “shadowed” and is accessible again, and its value (10) is printed in line 15.