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FreeBSD 11.0 - man page for isdigit (freebsd section 3)

ISDIGIT(3)						   BSD Library Functions Manual 						ISDIGIT(3)

NAME
isdigit, isnumber -- decimal-digit character test
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <ctype.h> int isdigit(int c); int isnumber(int c); int isdigit_l(int c, locale_t loc); int isnumber_l(int c, locale_t loc);
DESCRIPTION
The isdigit() function tests for a decimal digit character. Regardless of locale, this includes the following characters only: ``0'' ``1'' ``2'' ``3'' ``4'' ``5'' ``6'' ``7'' ``8'' ``9'' The isnumber() function behaves similarly to isdigit(), but may recognize additional characters, depending on the current locale setting. The value of the argument must be representable as an unsigned char or the value of EOF. The _l-suffixed versions take an explicit locale argument, whereas the non-suffixed versions use the current global or per-thread locale.
RETURN VALUES
The isdigit() and isnumber() functions return zero if the character tests false and return non-zero if the character tests true.
COMPATIBILITY
The 4.4BSD extension of accepting arguments outside of the range of the unsigned char type in locales with large character sets is considered obsolete and may not be supported in future releases. The iswdigit() function should be used instead.
SEE ALSO
ctype(3), iswdigit(3), multibyte(3), xlocale(3), ascii(7)
STANDARDS
The isdigit() function conforms to ISO/IEC 9899:1990 (``ISO C90''). The isdigit_l() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').
HISTORY
The isnumber() function appeared in 4.4BSD.
BSD
May 4, 2007 BSD