The void data type was introduced to make C syntactically
consistent. The main reason for void is to declare functions
that have no return value. The word "void" is therefore used in the
sense of "empty" rather than that of "invalid".
C functions are considered by the compiler to return type int
unless otherwise specified. Although the data returned by a function
can legally be ignored by the function calling it, the void data
type was introduced by the ANSI standard so that C compilers can issue
warnings when an integer value is not returned by a function that is
supposed to return one. If you want to write a function that does not
return a value, simply declare it void. A function
declared void has no return value and simply returns with the
Variables can be declared void as well as functions:
A variable that is itself declared void (such as
my_variable above) is useless; it cannot be assigned a value,
cannot be cast to another type, in fact, cannot be used in any way.
Void pointers (type void *) are a different case, however. A
void pointer is a generic pointer; any pointer can be cast to a
void pointer and back without any loss of information. Any type of
pointer can be assigned to (or compared with) a void pointer,
without casting the pointer explicitly.
Finally, a function call can be cast to void in order to
explicitly discard a return value. For example, printf returns a
value, but it is seldom used. Nevertheless, the two lines of code that
follow are equivalent:
(void) printf ("Hullo!\n");
There is no good reason to prefer the second line to the first, however, so
using the more concise form is preferred.