Functions in C

factorial.c

The types of all arguments must be declared.
The return type must be declared.
If the function does not return a value, the return type is void.
The first line of the function where the arguments and return type are declared is called a function prototype.
Local (automatic variables> are declared inside the function. They must be declared before they are used. In older versions of C, they had to be declared at the top of the function.
Functions can use global variables (ones declared outside any function). This is generally frowned upon.
Functions can call themselves (this is called recursion)

Suppose function f calls g and g calls f.
This leads to a chicken and egg problem. Which function comes first?
To resolve it, use a function prototype for one of the functions at the beginning.

int f(int n);

int g(int n) {
  ...
  call to f
  ...
}

int f(int n) {
  ...
  call to g
  ...
}

In the function definition the arguments are called formal arguments.
In a function call, the arguments are called actual arguments.
All arguments are passed by value. This means that the argument is evaluated, the value is pushed onto the runtime stack and the code in the function body uses the copy that is on the stack.
Because arguments are passed by value, a function cannot change the value of a variable passed as an actual argument.
However, if a pointer is passed by value, a function can change the contents of the memory location to which the pointer points.

int swap(int *m, int *n) {
  int tmp = *m;
  *m = *n;
  *n = tmp;
}

If a and b are ints, then we can swap their values with swap(&a, &b);
When an array is passed to a function, the function cannot change the value of the array variable (which is a pointer) but it can change the contents of the array!