Programming in C

134 Chapter 8 Working with Functions
Based upon this discussion, you can understand that when the value of guess 2 - x is
passed to the absoluteValue function and assigned to the formal parameter x, this
assignment has absolutely no effect on the value of x inside the squareRoot function.
Declaring Return Types and Argument Types
As mentioned previously, the C compiler assumes that a function returns a value of type
int as the default case. More specifically, when a call is made to a function, the compiler
assumes that the function returns a value of type int unless either of the following has
occurred:
1. The function has been defined in the program before the function call is encountered.
2. The value returned by the function has been declared before the function call is
encountered.
In Program 8.8, the absoluteValue function is defined before the compiler encounters
a call to this function from within the squareRoot function.The compiler knows, therefore,
that when this call is encountered, the absoluteValue function will return a value
of type float.Had the absoluteValue function been defined after the squareRoot
function, then upon encountering the call to the absoluteValue function, the compiler
would have assumed that this function returned an integer value. Most C compilers
catch this error and generate an appropriate diagnostic message.
To be able to define the absoluteValue function after the squareRoot function (or
even in another file—see Chapter 15), you must declare the type of result returned by the
absoluteValue function before the function is called.The declaration can be made inside
the squareRoot function itself, or outside of any function. In the latter case, the declaration
is usually made at the beginning of the program.
Not only is the function declaration used to declare the function’s return type, but it
is also used to tell the compiler how many arguments the function takes and what their
types are.
To declare absoluteValue as a function that returns a value of type float and that
takes a single argument, also of type float, the following declaration is used:
float absoluteValue (float);
As you can see, you just have to specify the argument type inside the parentheses, and
not its name.You can optionally specify a “dummy” name after the type if you want:
float absoluteValue (float x);
This name doesn’t have to be the same as the one used in the function definition—the
compiler ignores it anyway.
A foolproof way to write a function declaration is to simply use your text editor to
make a copy of the first line from the actual definition of the function. Remember to
place a semicolon at the end.