Functions with Arguments Functions are an integral part of coding within the C language. An entire system can be coded with functions that have no parameters (arguments), relying instead on global variables and return types. However, this type of approach seriously impacts the reusability, maintainability and clarity of the code as it is not obvious what functions affect what variables. A much neater and more flexible approach is to use functions with arguments as this allows modular, recursive, externalised code to be developed. Functions can receive their arguments in 2 ways Pass by value, or Pass by reference the difference is great and must be understood to get the most out of functions with arguments. Pass by Value Pass by value arguments are simple. Consider the following code: int y; int inc_by_10(int a) { a = a + 10; return a; } ... y = 5; y = inc_by_10(y); ... Most definitely a very trivial function but it serves to illustrate the point, all that the function does is to use whatever value was contained in the int argument a, add 10 to it and return that value back as an int. So in this case we assign the value 5 to y and then use y as the argument to the function, assigning the return value back to y again. An expensive way to add 10. If we had assigned the return value to a variable other than y the value of y would not have changed. This is because we used a pass by value argument, that is, an argument that contains a directly usable value. The value of y is copied into the function argument a which has no relationship to y at all except that they contain the same value. This means that when the function manipulates a it is using a totally separate variable from y and therefore y remains untouched. Pass by Reference The C language does not really have pass by reference arguments in the true sense unlike PASCAL, Fortran or Visual Basic for example. Languages that do have pass by reference arguments actually do some work for you in the background to make the mechanism work. However, C does allow you to implement the mechanism yourself in order to get pass by reference arguments. So what is a pass by reference argument. Well, it is an argument that allows you to alter the value of the variable used as the argument. Let us consider another version of the inc_by_10 function: int y; int inc_by_10(int *a) { *a = *a + 10; return *a; } ... y = 5; y = inc_by_10(&(y)); ... Notice the slight differences in the function. Instead of using a straight int we now use an int * (a pointer to an int) as the argument. This means that we must pass the address of an int variable as the argument value. Yes we are still really using pass by value, but in this case we do not want to use that value directly, instead we treat it as a reference to the real value. This means that whenever we want to get or set the value of the argument we have to prefix it with a * character. In order to change the value of the variable referenced in the argument, y in this case, we did not have to re-assign the return value of the function to it, as the first line of the function has already changed the value. So we could re-write the example thus: int y; void inc_by_10(int *a) { *a = *a + 10; } ... y = 5; inc_by_10(&(y)); ... and achieve the same effect. What languages like Visual Basic do for you is to automatically take the address of the variable you specify as the argument and pass that value to the function in the argument. Then they implicitly dereference the address every time you use the argument in the function body, just like we did explicitly in the example above. That is all there is to pass by reference arguments, the trick is to remember that they are actually pointers to variables.