2.4. Making your own Functions

In that last example we defined just one function. To add another function you must generally do two things. First you must define the function, just like we defined main(). Also you you must declare it. Declaring a function is like telling GCC to expect it, we didn't have to declare main() because it is a special function and GCC knows to expect it. The name, or identifier, you give to a function must appear in both the definition and the declaration.

Functions identifiers can be made up of the alphabetic characters "a"-"z" and "A"-"Z", the numeric characters "0"-"9" and the underscore character "_". These can be used in any order so long as the first character of the identifier is not a number. As we said earlier, C is case-sensitive so My_Function is completely different to my_function. A functions identifier must be unique. Identifiers can safely be up to 63 characters long or as short as 1 character.

Along with it's identifier you must give each function a type and a block of code. The type tells the compiler what sort of data it returns. The return value of a function can be ignored, printf() returns an integer saying how many character it displayed to the terminal. This information wasn't important to us so we ignored it in our program. In the next chapter we'll discuss types of data in detail, until then we'll gloss over return values.

Here's a program that defines three functions:

#include <stdio.h>

/* function declarations */
int first_function(void);
int goodbye(void);


int
main()            // function definition
{
  printf("the program begins...\n");
  first_function();
  goodbye();

  return 0;
}


int
first_function()  // function definition
{
  /* this function does nothing */
  return 0;
}


int
goodbye()         // function definition
{
  printf("...and the program ends.\n");

  return 0;
}