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4.2. The type of functions

All functions have a type: they return a value of that type whenever they are used. The reason that C doesn't have ‘procedures’, which in most other languages are simply functions without a value, is that in C it is permissible (in fact well-nigh mandatory) to discard the eventual value of most expressions. If that surprises you, think of an assignment

  a = 1;

That's a perfectly valid assignment, but don't forget that it has a value too. The value is discarded. If you want a bigger surprise, try this one:

  1;

That is an expression followed by a semicolon. It is a well formed statement according to the rules of the language; nothing wrong with it, it is just useless. A function used as a procedure is used in the same way—a value is always returned, but you don't use it:

  f(argument);

is also an expression with a discarded value.

It's all very well saying that the value returned by a function can be ignored, but the fact remains that if the function really does return a value then it's probably a programming error not to do something with it. Conversely, if no useful value is returned then it's a good idea to be able to spot anywhere that it is used by mistake. For both of those reasons, functions that don't return a useful value should be declared to be void.

Functions can return any type supported by C (except for arrays and functions), including the pointers, structures and unions which are described in later chapters. For the types that can't be returned from functions, the restrictions can often be sidestepped by using pointers instead.

All functions can be called recursively.

4.2.1. Declaring functions

Unfortunately, we are going to have to use some jargon now. This is one of the times that the use of an appropriate technical term really does reduce the amount of repetitive descriptive text that would be needed. With a bit of luck, the result is a shorter, more accurate and less confusing explanation. Here are the terms.

declaration
The point at which a name has a type associated with it.
definition
Also a declaration, but at this point some storage is reserved for the named object. The rules for what makes a declaration into a definition can be complicated, but are easy for functions: You turn a function declaration into a definition by providing a body for the function in the form of a compound statement.
formal parameters
parameters
These are the names used inside a function to refer to its arguments.
actual arguments
arguments
These are the values used as arguments when the function is actually called. In other words, the values that the formal parameters will have on entry to the function.

The terms ‘parameter’ and ‘argument’ do tend to get used as if they were interchangeable, so don't read too much into it if you see one or the other in the text below.

If you use a function before you declare it, it is implicitly declared to be ‘function returning int’. Although this will work, and was widely used in Old C, in Standard C it is bad practice—the use of undeclared functions leads to nasty problems to do with the number and type of arguments that are expected for them. All functions should be fully declared before they are used. For example, you might be intending to use a function in a private library called, say, aax1. You know that it takes no arguments and returns a double. Here is how it should be declared:

  double aax1(void);

and here is how it might be used:

main(){
      double return_v, aax1(void);
      return_v = aax1();
      exit(EXIT_SUCCESS);
}
Example 4.1

The declaration was an interesting one. It defined return_v, actually causing a variable to come into existence. It also declared aax1 without defining it; as we know, functions only become defined when a body is provided for them. Without a declaration in force, the default rules mean that aax1 would have been assumed to be int, even though it really does return a double—which means that your program will have undefined behaviour. Undefined behaviour is disastrous!

The presence of void in the argument list in the declaration shows that the function really takes no arguments. If it had been missing, the declaration would have been taken to give no information about the function's arguments. That way, compatibility with Old C is maintained at the price of the ability of the compiler to check.

To define a function you also have to provide a body for it, in the form of a compound statement. Since no function can itself contain the definition of a function, functions are all separate from each other and are only found at the outermost level of the program's structure. Here is a possible definition for the function aax1.

  double
  aax1(void) {
        /* code for function body */
        return (1.0);
  }

It is unusual for a block-structured language to prohibit you from defining functions inside other functions, but this is one of the characteristics of C. Although it isn't obvious, this helps to improve the run-time performance of C by reducing the housekeeping associated with function calls.

4.2.2. The return statement

The return statement is very important. Every function except those returning void should have at least one, each return showing what value is supposed to be returned at that point. Although it is possible to return from a function by falling through the last }, unless the function returns void an unknown value will be returned, resulting in undefined behaviour.

Here is another example function. It uses getchar to read characters from the program input and returns whatever it sees except for space, tab or newline, which it throws away.

#include <stdio.h>

int
non_space(void){
      int c;
      while ( (c=getchar ())=='\t' || c== '\n' || c==' ')
              ; /* empty statement */
      return (c);
}

Look at the way that all of the work is done by the test in the while statement, whose body was an empty statement. It is not an uncommon sight to see the semicolon of the empty statement sitting there alone and forlorn, with only a piece of comment for company and readability. Please, please, never write it like this:

  while (something);

with the semicolon hidden away at the end like that. It's too easy to miss it when you read the code, and to assume that the following statement is under the control of the while.

The type of expression returned must match the type of the function, or be capable of being converted to it as if an assignment statement were in use. For example, a function declared to return double could contain

  return (1);

and the integral value will be converted to double. It is also possible to have just return without any expression—but this is probably a programming error unless the function returns void. Following the return with an expression is not permitted if the function returns void.

4.2.3. Arguments to functions

Before the Standard, it was not possible to give any information about a function's arguments except in the definition of the function itself. The information was only used in the body of the function and was forgotten at the end. In those bad old days, it was quite possible to define a function that had three double arguments and only to pass it one int, when it was called. The program would compile normally, but simply not work properly. It was considered to be the programmer's job to check that the number and the type of arguments to a function matched correctly. As you would expect, this turned out to be a first-rate source of bugs and portability problems. Here is an example of the definition and use of a function with arguments, but omitting for the moment to declare the function fully.

#include <stdio.h>
#include <stdlib.h>
main(){
      void pmax();                    /* declaration */
      int i,j;
      for(i = -10; i <= 10; i++){
              for(j = -10; j <= 10; j++){
                      pmax(i,j);
              }
      }
      exit(EXIT_SUCCESS);
}
/*
* Function pmax.
* Returns:      void
* Prints larger of its two arguments.
*/
void
pmax(int a1, int a2){                   /* definition */
      int biggest;

      if(a1 > a2){
              biggest = a1;
      }else{
              biggest = a2;
      }

      printf("larger of %d and %d is %d\n",
              a1, a2, biggest);
}
Example 4.2

What can we learn from this? To start with, notice the careful declaration that pmax returns void. In the function definition, the matching void occurs on the line before the function name. The reason for writing it like that is purely one of style; it makes it easier to find function definitions if their names are always at the beginning of a line.

The function declaration (in main) gave no indication of any arguments to the function, yet the use of the function a couple of lines later involved two arguments. That is permitted by both the old and Standard versions of C, but must nowadays be considered to be bad practice. It is much better to include information about the arguments in the declaration too, as we will see. The old style is now an ‘obsolescent feature’ and may disappear in a later version of the Standard.

Now on to the function definition, where the body is supplied. The definition shows that the function takes two arguments, which will be known as a1 and a2 throughout the body of the function. The types of the arguments are specified too, as can be seen.

In the function definition you don't have to specify the type of each argument because they will default to int, but this is bad style. If you adopt the practice of always declaring arguments, even if they do happen to be int, it adds to a reader's confidence. It indicates that you meant to use that type, instead of getting it by accident: it wasn't simply forgotten. The definition of pmax could have been this:

  /* BAD STYLE OF FUNCTION DEFINITION */

  void
  pmax(a1, a2){
        /* and so on */

The proper way to declare and define functions is through the use of prototypes.

4.2.4. Function prototypes

The introduction of function prototypes is the biggest change of all in the Standard.

A function prototype is a function declaration or definition which includes information about the number and types of the arguments that the function takes.

Although you are allowed not to specify any information about a function's arguments in a declaration, it is purely because of backwards compatibility with Old C and should be avoided.

A declaration without any information about the arguments is not a prototype.

Here's the previous example ‘done right’.

#include <stdio.h>
#include <stdlib.h>

main(){
      void pmax(int first, int second);       /*declaration*/
      int i,j;
      for(i = -10; i <= 10; i++){
              for(j = -10; j <= 10; j++){
                      pmax(i,j);
              }
      }
      exit(EXIT_SUCCESS);
}

void
pmax(int a1, int a2){                           /*definition*/
      int biggest;

      if(a1 > a2){
              biggest = a1;
      }
      else{
              biggest = a2;
      }

      printf("largest of %d and %d is %d\n",
              a1, a2, biggest);
}
Example 4.3

This time, the declaration provides information about the function arguments, so it's a prototype. The names first and second are not an essential part of the declaration, but they are allowed to be there because it makes it easier to refer to named arguments when you're documenting the use of the function. Using them, we can describe the function simply by giving its declaration

  void pmax (int xx, int yy );

and then say that pmax prints whichever of the arguments xx or yy is the larger. Referring to arguments by their position, which is the alternative (e.g. the fifth argument), is tedious and prone to miscounting.

All the same, you can miss out the names if you want to. This declaration is entirely equivalent to the one above.

  void pmax (int,int);

All that is needed is the type names.

For a function that has no arguments the declaration is

  void f_name (void);

and a function that has one int, one double and an unspecified number of other arguments is declared this way:

  void f_name (int,double,...);

The ellipsis (...) shows that other arguments follow. That's useful because it allows functions like printf to be written. Its declaration is this:

  int printf (const char *format_string,...)

where the type of the first argument is ‘pointer to const char’; we'll discuss what that means later.

Once the compiler knows the types of a function's arguments, having seen them in a prototype, it's able to check that the use of the function conforms to the declaration.

If a function is called with arguments of the wrong type, the presence of a prototype means that the actual argument is converted to the type of the formal argument ‘as if by assignment’. Here's an example: a function is used to evaluate a square root using Newton's method of successive approximations.

#include <stdio.h>
#include <stdlib.h>
#define DELTA 0.0001
main(){
      double sq_root(double); /* prototype */
      int i;

      for(i = 1; i < 100; i++){
              printf("root of %d is %f\n", i, sq_root(i));
      }
      exit(EXIT_SUCCESS);
}

double
sq_root(double x){      /* definition */
      double curr_appx, last_appx, diff;

      last_appx = x;
      diff = DELTA+1;

      while(diff > DELTA){
              curr_appx = 0.5*(last_appx
                      + x/last_appx);
              diff = curr_appx - last_appx;
              if(diff < 0)
                      diff = -diff;
              last_appx = curr_appx;
      }
      return(curr_appx);
}
Example 4.4

The prototype tells everyone that sq_root takes a single argument of type double. The argument actually passed in the main function is an int, so it has to be converted to double first. The critical point is that if no prototype had been seen, C would assume that the programmer had meant to pass an int and an int is what would be passed. The Standard simply notes that this results in undefined behaviour, which is as understated as saying that catching rabies is unfortunate. This is a very serious error and has led to many, many problems in Old C programs.

The conversion of int to double could be done because the compiler had seen a protoytpe for the function and knew what to do about it. As you would expect, there are various rules used to decide which conversions are appropriate, so we need to look at them next.

4.2.5. Argument Conversions

When a function is called, there are a number of possible conversions that will be applied to the values supplied as arguments depending on the presence or absence of a prototype. Let's get one thing clear: although you can use these rules to work out what to do if you haven't used prototypes, it is a recipe for pain and misery in the long run. It's so easy to use prototypes that there really is no excuse for not having them, so the only time you will need to use these rules is if you are being adventurous and using functions with a variable number of arguments, using the ellipsis notation in the prototype that is explained in Chapter 9.

The rules mention the default argument promotions and compatible type. Where they are used, the default argument promotions are:

The introduction of prototypes (amongst other things) has increased the need for precision about ‘compatible types’, which was not much of an issue in Old C. The full list of rules for type compatibility is deferred until Chapter 8, because we suspect that most C programmers will never need to learn them. For the moment, we will simply work on the basis that if two types are the same, they are indisputably compatible.

The conversions are applied according to these rules (which are intended to be guidance on how to apply the Standard, not a direct quote):

  1. At the point of calling a function, if no prototype is in scope, the arguments all undergo the default argument promotions. Furthermore:
    • If the number of arguments does not agree with the number of formal parameters to the function, the behaviour is undefined.
    • If the function definition was not a definition containing a prototype, then the type of the actual arguments after promotion must be compatible with the types of the formal parameters in the definition after they too have had the promotions applied. Otherwise the behaviour is undefined.
    • If the function definition was a definition containing a prototype, and the types of the actual arguments after promotion are not compatible with the formal parameters in the prototype, then the behaviour is undefined. The behaviour is also undefined it the prototype included ellipsis (, ...).
  2. At the point of calling a function, if a prototype is in scope, the arguments are converted, as if by assignment, to the types specified in the prototype. Any arguments which fall under the variable argument list category (specified by the ... in the prototype) still undergo the default argument conversions.

    It is possible to write a program so badly that you have a prototype in scope when you call the function, but for the function definition itself not to have a prototype. Why anyone should do this is a mystery, but in this case, the function that is called must have a type that is compatible with the apparent type at the point of the call.

The order of evaluation of the arguments in the function call is explicitly not defined by the Standard.

4.2.6. Function definitions

Function prototypes allow the same text to be used for both the declaration and definition of a function. To turn a declaration:

double
some_func(int a1, float a2, long double a3);

into a definition, we provide a body for the function:

double
some_func(int a1, float a2, long double a3){
      /* body of function */
      return(1.0);
}

by replacing the semicolon at the end of the declaration with a compound statement.

In either a definition or a declaration of a function, it serves as a prototype if the parameter types are specified; both of the examples above are prototypes.

The Old C syntax for the declaration of a function's formal arguments is still supported by the Standard, although it should not be used by new programs. It looks like this, for the example above:

double
some_func(a1, a2, a3)
      int a1;
      float a2;
      long double a3;
{

      /* body of function */
      return(1.0);
}

Because no type information is provided for the parameters at the point where they are named, this form of definition does not act as a prototype. It declares only the return type of the function; nothing is remembered by the compiler about the types of the arguments at the end of the definition.

The Standard warns that support for this syntax may disappear in a later version. It will not be discussed further.

Summary

  1. Functions can be called recursively.
  2. Functions can return any type that you can declare, except for arrays and functions (you can get around that restriction to some extent by using pointers). Functions returning no value should return void.
  3. Always use function prototypes.
  4. Undefined behaviour results if you call or define a function anywhere in a program unless either
    • a prototype is always in scope for every call or definition, or
    • you are very, very careful.
  5. Assuming that you are using prototypes, the values of the arguments to a function call are converted to the types of the formal parameters exactly as if they had been assigned using the = operator.
  6. Functions taking no arguments should have a prototype with (void) as the argument specification.
  7. Functions taking a variable number of arguments must take at least one named argument; the variable arguments are indicated by ... as shown:

    int
    vfunc(int x, float y, ...);

    Chapter 9 describes how to write this sort of function.

4.2.7. Compound statements and declarations

As we have seen, functions always have a compound statement as their body. It is possible to declare new variables inside any compound statement; if any variables of the same name already exist, then the old ones are hidden by the new ones within the new compound statement. This is the same as in every other block-structured language. C restricts the declarations to the head of the compound statement (or ‘block’); once any other kind of statement has been seen in the block, declarations are no longer permitted within that block.

How can it be possible for names to be hidden? The following example shows it happening:

int a;                  /* visible from here onwards */

void func(void){
      float a;        /* a different 'a' */
      {
              char a; /* yet another 'a' */
      }
                      /* the float 'a' reappears */
}
                      /* the int 'a' reappears */
Example 4.5

A name declared inside a block hides any outer versions of the same name until the end of the block where it is declared. Inner blocks can also re-declare that name—you can do this for ever.

The scope of a name is the range in which it has meaning. Scope starts from the point at which the name is mentioned and continues from there onwards to the end of the block in which it is declared. If it is external (outside of any function) then it continues to the end of the file. If it is internal (inside a function), then it disappears at the end of the block containing it. The scope of any name can be suspended by redeclaring the name inside a block.

Using knowledge of the scope rules, you can play silly tricks like this one:

main () {}
int i;
f () {}
f2 () {}

Now f and f2 can use i, but main can't, because the declaration of the variable comes later than that of main. This is not an aspect that is used very much, but it is implicit in the way that C processes declarations. It is a source of confusion for anyone reading the file (external declarations are generally expected to precede any function definitions in a file) and should be avoided.

The Standard has changed things slightly with respect to a function's formal parameters. They are now considered to have been declared inside the first compound statement, even though textually they aren't: this goes for both the new and old ways of function definition. So, if a function has a formal parameter with the same name as something declared in the outermost compound statement, this causes an error which will be detected by the compiler.

In Old C, accidental redefinition of a function's formal parameter was a horrible and particularly difficult mistake to track down. Here is what it would look like:

/* erroneous redeclaration of arguments */

func(a, b, c){
      int a;  /* AAAAgh! */
}

The pernicious bit is the new declaration of a in the body of the function, which hides the parameter called a. Since the problem has now been eliminated we won't investigate it any further.

Footnotes

1. Stroustrup B. (1991). The C++ Programming Language 2nd edn. Reading, MA: Addison-Wesley