The datatype of a function

1. functions whose role is to just perform a task, and to then return control to the statement (pedantically, the expression) that called it.
   Such functions often have side-effects, such as performing some output, or modifying a global variable so that other statements may access that modified value.
   These functions don't return a specific value to the caller, are termed void functions, and we casually say that they "return void".

2. functions whose role is to calculate a value, and to return that value for use in the expressions that called them. The single value returned will have a type, such as int, char, bool, or float. These functions may also have side-effects.

#include <stdio.h>
#include <stdlib.h>

void output(char ch, int n)        
{
    for(int i=1 ; i<=n ; i=i+1) {
        printf("%c", ch);
    }
}

int main(int argc, char *argv[])
{
    output(' ', 19);
    output('*',  1);
    output('\n', 1);

    return 0;
}

#include <stdio.h>
#include <stdlib.h>

extern double sqrt(double x);

float square(float x)
{
    return x * x;
}

int main(int argc, char *argv[])
{
    if(argc > 2) {
      float a, b, sum;

      a   = atof(argv[1]);
      b   = atof(argv[2]);

      sum = square(a) + square(b);
      printf("hypotenuse = %f\n",
             sqrt(sum) );
    }
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

float square(float x)
{
    return x * x;
}

int main(int argc, char *argv[])
{
    if(argc > 2) {
      float a, b, sum;

      a   = atof(argv[1]);
      b   = atof(argv[2]);

      sum = square(a) + square(b);
      printf("hypotenuse = %f\n",
             sqrt(sum) );
    }
    return 0;
}

In the 3rd example, we're being more correct, by #includ-ing the <math.h> header file - instructing the C compiler find the correct prototype for sqrt() on this system.

In the 2nd and 3rd cases we must compile the examples with:  cc [EXTRAOPTIONS] -o program program.c -lm 

to instruct the linker to search the math library for any missing code (we require the sqrt() function).