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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.