22.2 Function Declarations

To call a function, or use its name as a pointer, a function declaration for the function name must be in effect at that point in the code. The function’s definition serves as a declaration of that function for the rest of the containing scope, but to use the function in code before the definition, or from another compilation module, a separate function declaration must precede the use.

A function declaration looks like the start of a function definition. It begins with the return value type (void if none) and the function name, followed by argument declarations in parentheses (though these can sometimes be omitted). But that’s as far as the similarity goes: instead of the function body, the declaration uses a semicolon.

A declaration that specifies argument types is called a function prototype. You can include the argument names or omit them. The names, if included in the declaration, have no effect, but they may serve as documentation.

This form of prototype specifies fixed argument types:

rettype function (argtypes…);

This form says the function takes no arguments:

rettype function (void);

This form declares types for some arguments, and allows additional arguments whose types are not specified:

rettype function (argtypes…, ...);

For a parameter that’s an array of variable length, you can write its declaration with ‘*’ where the “length” of the array would normally go; for example, these are all equivalent.

double maximum (int n, int m, double a[n][m]);
double maximum (int n, int m, double a[*][*]);
double maximum (int n, int m, double a[ ][*]);
double maximum (int n, int m, double a[ ][m]);

The old-fashioned form of declaration, which is not a prototype, says nothing about the types of arguments or how many they should be:

rettype function ();

Warning: Arguments passed to a function declared without a prototype are converted with the default argument promotions (see Argument Promotions. Likewise for additional arguments whose types are unspecified.

Function declarations are usually written at the top level in a source file, but you can also put them inside code blocks. Then the function name is visible for the rest of the containing scope. For example:

void
foo (char *file_name)
{
  void save_file (char *);
  save_file (file_name);
}

If another part of the code tries to call the function save_file, this declaration won’t be in effect there. So the function will get an implicit declaration of the form extern int save_file ();. That conflicts with the explicit declaration here, and the discrepancy generates a warning.

The syntax of C traditionally allows omitting the data type in a function declaration if it specifies a storage class or a qualifier. Then the type defaults to int. For example:

static foo (double x);

defaults the return type to int. This is bad practice; if you see it, fix it.

Calling a function that is undeclared has the effect of an creating implicit declaration in the innermost containing scope, equivalent to this:

extern int function ();

This declaration says that the function returns int but leaves its argument types unspecified. If that does not accurately fit the function, then the program needs an explicit declaration of the function with argument types in order to call it correctly.

Implicit declarations are deprecated, and a function call that creates one causes a warning.