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void                     The Void Function Return Type

    The void keyword is overloaded--it has three separate and unrelated
    meanings, depending on the context in which it appears.  It can be
    used to type a function; as a function prototype; and to define a
    generic pointer type.

  ----------------------------- Function Type ------------------------------

    The void type applied to functions signifies that the named function
    has no return value. It can also be applied as a cast, in which case
    it causes the actual return value to be ignored (which is the default
    behavior anyway).

      Notes:    A syntax error is generated if you attempt to use the
                value returned from a function declared as void, or if
                you try to return a value from a function defined as
                having a void type.

                If a function's return value is not used, it is ignored,
                so the void cast above is superfluous.  It simply
                documents the default behavior.

                C does not require a non-void function to have a return
                value.  If a non-void function does have a return value,
                C does not require that the value to be used.

  -------------------------------- Example ---------------------------------

           void clear_screen();           /* function declaration */
           void clear_screen()            /* function definition */
           {
           ...
           }

           (void) printf("Hello there.\n");


  -------------------------- Function Prototypes ---------------------------

    Used as the lone formal argument list specifier in a function
    declaration, `void' indicates that the function has no arguments.

      Notes:    If this declaration is in scope when getfpvalue is
                called, the compiler can check that no arguments are
                passed in the call.

  -------------------------------- Example ---------------------------------

                   [external] double getfpvalue(void);


  ------------------------ Generic Pointer Type ----------------------------

    In certain cases, it is useful to have a generic pointer type. This
    is done using the type void *.  A void pointer is large enough to
    represent a pointer to any data type.  A data pointer can be moved
    into and out of a void pointer without loss of information.

      Notes:    You cannot dereference an object pointed to by a void
                pointer,  since the compiler does not know the size or
                representation of the object being pointed to--it only
                knows its address. You must first assign it to some other
                data pointer type.

                While a pointer to type T can safely be copied into a
                void pointer and back again without loss of information,
                do not store a pointer to T in a void pointer and read it
                out as a type other than T. In the example below, a char
                pointer is copied to pv, which is then copied to a double
                pointer. Since char and double pointers may have
                different attributes, this may generate an error.

  -------------------------------- Example ---------------------------------

           extern int test(void *);
           void *pv;
           char *pc;
           double *pd;

           pv = pc;
           pc = pv;                       /* OK */

           pv = pc;
           pd = pv;                       /* potential problem */

           test(pv);                      test(pc);