22.1.1 Function Parameter Variables

A function parameter variable is a local variable (see Local Variables) used within the function to store the value passed as an argument in a call to the function. Usually we say “function parameter” or “parameter” for short, not mentioning the fact that it’s a variable.

We declare these variables in the beginning of the function definition, in the parameter list. For example,

fib (int n)

has a parameter list with one function parameter n, which has type int.

Function parameter declarations differ from ordinary variable declarations in several ways:

• Inside the function definition header, commas separate parameter declarations, and each parameter needs a complete declaration including the type. For instance, if a function foo has two int parameters, write this:

  foo (int a, int b)
  

  You can’t share the common int between the two declarations:

  foo (int a, b)      /* Invalid! */
  

• A function parameter variable is initialized to whatever value is passed in the function call, so its declaration cannot specify an initial value.
• Writing an array type in a function parameter declaration has the effect of declaring it as a pointer. The size specified for the array has no effect at all, and we normally omit the size. Thus,

  foo (int a[5])
  foo (int a[])
  foo (int *a)
  

  are equivalent.

• The scope of the parameter variables is the entire function body, notwithstanding the fact that they are written in the function header, which is just outside the function body.

If a function has no parameters, it would be most natural for the list of parameters in its definition to be empty. But that, in C, has a special meaning for historical reasons: “Do not check that calls to this function have the right number of arguments.” Thus,

int
foo ()
{
  return 5;
}

int
bar (int x)
{
  return foo (x);
}

would not report a compilation error in passing x as an argument to foo. By contrast,

int
foo (void)
{
  return 5;
}

int
bar (int x)
{
  return foo (x);
}

would report an error because foo is supposed to receive no arguments.