libc.info: isdigit

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`isdigit'--decimal digit predicate

*Synopsis*
     #include <ctype.h>
     int isdigit(int C);
   *Description*
`isdigit' is a macro which classifies ASCII integer values by table
lookup.  It is a predicate returning non-zero for decimal digits, and 0
for other characters.  It is defined only when `isascii'(C) is true or
C is EOF.
   You can use a compiled subroutine instead of the macro definition by
undefining the macro using ``#undef isdigit''.
*Returns*
`isdigit' returns non-zero if C is a decimal digit (`0'-`9').
*Portability*
`isdigit' is ANSI C.
   No supporting OS subroutines are required.