### 3.1 `isalnum`—alphanumeric character predicate

Synopsis

         #include <ctype.h>
         int isalnum(int c);

Description
`isalnum` is a macro which classifies ASCII integer values by table lookup. It is a predicate returning non-zero for alphabetic or numeric ASCII characters, and `0` for other arguments. It is defined only if c is representable as an unsigned char or if c is EOF.

You can use a compiled subroutine instead of the macro definition by undefining the macro using ``#undef isalnum`'.

Returns
`isalnum` returns non-zero if c is a letter (`a`–`z` or `A`–`Z`) or a digit (`0`–`9`).

Portability
`isalnum` is ANSI C.

No OS subroutines are required.