### 4.2 `isalpha`, `isalpha_l`—alphabetic character predicate ¶

Synopsis

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

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

Description
`isalpha` is a macro which classifies singlebyte charset values by table lookup. It is a predicate returning non-zero when c represents an alphabetic ASCII character, and 0 otherwise. It is defined only if c is representable as an unsigned char or if c is EOF.

`isalpha_l` is like `isalpha` 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 isalpha`’ or ‘`#undef isalpha_l`’.

Returns
`isalpha`, `isalpha_l` return non-zero if c is a letter.

Portability
`isalpha` is ANSI C. `isalpha_l` is POSIX-1.2008.

No supporting OS subroutines are required.