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3.6 isdigit, isdigit_l—decimal digit predicate

Synopsis 

     #include <ctype.h>
     int isdigit(int c);
     
     #include <ctype.h>
     int isdigit_l(int c, locale_t locale);

Description
isdigit is a macro which classifies singlebyte charset values by table lookup. It is a predicate returning non-zero for decimal digits, and 0 for other characters. It is defined only if c is representable as an unsigned char or if c is EOF.

isdigit_l is like isdigit but performs the check based on the locale specified by the locale object locale. If locale is LC_GLOBAL_LOCALE or not a valid locale object, the behaviour is undefined.

You can use a compiled subroutine instead of the macro definition by undefining the macro using `#undef isdigit' or `#undef isdigit_l'.
Returns
isdigit, isdigit_l return non-zero if c is a decimal digit (0–9).
Portability
isdigit is ANSI C. isdigit_l is POSIX-1.2008.

No supporting OS subroutines are required.