Sams Teach Yourself C in 21 Days

TypevoidPointers ..............................................................................................

You’ve seen the voidkeyword used in a function declaration to specify that the function

either doesn’t take arguments or doesn’t return a value. The voidkeyword can also be

used to create a generic pointer—a pointer that can point to any type of data object. For

example, the statement

void *ptr;

declaresptras a generic pointer. ptrpoints to something; you just haven’t yet specified

what.

The most common use for type voidpointers is in declaring function parameters. You

might want to create a function that can handle different types of arguments. You can

pass it a type intone time, a type floatthe next time, and so on. By declaring that the

function takes a voidpointer as an argument, you don’t restrict it to accepting only a sin-

gle data type. If you declare the function to take a voidpointer as an argument, you can

pass the function a pointer to anything.

Here’s a simple example: You want to write a function that accepts a numeric variable as

an argument and divides it by two, returning the answer in the argument variable. Thus,

if the variable valholds the value 4, after a call to half(val)the variable valis equal to

2. Because you want to modify the argument, you pass it by reference. Because you want
   to use the function with any of C’s numeric data types, you declare the function to take a
   voidpointer:
   void half(void val);
   Now you can call the function, passing it any pointer as an argument. There’s one more
   thing you need, however. Although you can pass a voidpointer without knowing what
   data type it points to, you can’t dereference the pointer. Before the code in the function
   can do anything with the pointer, it must know the data type. You do this with a typecast,
   which is nothing more than a way of telling the program to treat this voidpointer as a
   pointer to a specific type. If pvalis a voidpointer, you typecast it as follows:
   (type )pval
   Here,typeis the appropriate data type. To tell the program that pvalis a pointer to type
   int, write
   (int )pval
   To dereference the pointer—that is, to access the intthatpvalpoints to—write
   (int *)pval

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