Thread: Function

Code:

/*Demonstrates local variables*/

//C libraries
#include <stdio.h>

//Declare variables
int x = 1, y = 2;

//Declare user defined function
void demo (void); //Used to print a character string

//Main function
int main (void)
{
printf ("\nBefore calling demo (), x = %d and y = %d.", x, y);
demo();
printf ("\nAfter calling demo (), x = %d and y = %d\n.", x, y);
return 0;
}

//Demo function
void demo (void)
{
/*Declares and initializes two local variables*/
int x = 88, y = 99;
/*Display their values*/
printf ("\nWithin demo (), x = %d and y = %d.", x, y);
}

I am learning about functions for the first time, and the author states that the above void demo (void) function does not return any values; yet, in the int main (void) function the void demo (void) function prints the character string "Within demo (), x = 88 and y = 99.": is the printing of that character string not considered a return value, or does the author simply mean that it doesn't return a value that is used by another part of the program (e.g., return to another (not main) function; initialize a variable; etc...)?

... is the printing of that character string not considered a return value ...

No it's not. Just because the demo function provides some form of feedback does not mean it returns a value, at least in programming terms.

When a functions returns a value it should really return something. So when a function is declared as void demo( void ); you know it will never return a value, it will only return to the position it was called from.

See the function below for an example of return values.

Code:

int sum( int x, int y )
{
    return x + y;
}

int result = sum( 3, 6 );//variable result will get the value 9 after calling the function sum

Originally Posted by Euclid365BCE

yet, in the int main (void) function the void demo (void) function prints the character string

This kind of behaviour is called a side effect. More generally:

Originally Posted by C11 Clause 5.1.2.3 Paragraph 2a

Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.

In this case you are effectively modifying a "file" (as represented by the standard output stream stdout) by printing a string, but you may have encountered functions that modify say, an array via a pointer to its first element.