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ISDIGIT(3) Library Functions Manual ISDIGIT(3)

isdigit, isdigit_l —
decimal-digit single-byte character test

#include <ctype.h>

int
isdigit(int c);

int
isdigit_l(int c, locale_t locale);

The isdigit() and isdigit_l() functions test for any decimal-digit character. The complete list of decimal digits is 0 and 1–9, in any locale.

These functions return zero if the character tests false or non-zero if the character tests true.

On systems supporting non-ASCII single-byte character encodings, different c arguments may correspond to the digits, and the results of isdigit() may depend on the LC_CTYPE locale(1).

isalnum(3), isalpha(3), isascii(3), isblank(3), iscntrl(3), isgraph(3), islower(3), isprint(3), ispunct(3), isspace(3), isupper(3), iswdigit(3), isxdigit(3), stdio(3), toascii(3), tolower(3), toupper(3), ascii(7)

The isdigit() function conforms to ANSI X3.159-1989 (“ANSI C89”), and isdigit_l() to IEEE Std 1003.1-2008 (“POSIX.1”).

The isdigit() function first appeared in Version 7 AT&T UNIX, and isdigit_l() has been available since OpenBSD 6.2.

The argument c must be EOF or representable as an unsigned char; otherwise, the result is undefined.
September 5, 2017 OpenBSD-current