Why doesn't this simple program print a thing?

Question

I have written a simple program that is supposed to add 1 and 53, and return 54. But apparantly it does nothing. No error, just nothing.

The file is saved as firstEx.c, I compiled and run like this

$ gcc -o firstEx firstEx.c
$ ./firstEx

All goes fine, except I see no output. Can anyone help? (I'm totally new to C.)

Here's the code:

#include <stdio.h>

int main()
{ 
  int var1, var2;
  var1 = 1; 
  var2 = 53;
  void add_two_numbers (int var1, int var2);
  return 0;
}

void add_two_numbers (int a, int b) 
{ 
  int c;
  c = a + b; 
  printf ("%d\n", c); 
}

Answer 1

The line

void add_two_numbers (int var1, int var2);

is a function prototype, not a call to said function. It's unusual to see it in the middle of a function but it's certainly valid, since it's just a style of declaration as per the standard (ISO C11 6.7).

To actually call the function, you have to use something like:

add_two_numbers (var1, var2);

You should also ensure the function prototype is within scope when you call it, by either providing a prototype beforehand or simply defining the function before you call it. I'd start with something like:

#include <stdio.h>

void add_two_numbers (int a, int b) {
    int c;
    c = a + b;
    printf ("%d\n", c);
}

int main (void) {
    int var1, var2;
    var1 = 1;
    var2 = 53;
    add_two_numbers (var1, var2);
    return 0;
}

---

If you wanted to keep the order of functions the same, a prototype approach could be done as follows:

#include <stdio.h>

void add_two_numbers (int, int);

int main (void) {
    int var1, var2;
    var1 = 1;
    var2 = 53;
    add_two_numbers (var1, var2);
    return 0;
}

void add_two_numbers (int a, int b) {
    int c;
    c = a + b;
    printf ("%d\n", c);
}

---

A couple of other things just to keep you thinking:

• You should name your functions based on intent. Most code cutters will expect a function called add_two_numbers to add the numbers and return the result, rather than print out the sum. Perhaps a better name would have been print_sum_of_two_numbers.
• It's not necessary to always use variables, unless you're intending the value to vary :-) The entire add_two_numbers function could be whittled down to the single line printf ("%d", a + b);.
• Similarly, the first four lines of main could be reduced to add_two_numbers (1, 53);.
• As of C99, you no longer have to explicitly return zero from main, it will return zero "automagically" if you drop out the bottom. Some people may still prefer to be explicit, and quite a few oldies like myself will put it in just from force of habit, but it's no longer necessary.

With all those changes, you can end up with the far more succinct:

#include <stdio.h>

void print_sum_of_two_numbers (int a, int b) {
    printf ("%d\n", a + b);
}

int main (void) {
    print_sum_of_two_numbers (1, 53);
}