Provided by: manpages-dev_6.15-1_all 
NAME
isalnum, isalpha, isascii, isblank, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace,
isupper, isxdigit, isalnum_l, isalpha_l, isascii_l, isblank_l, iscntrl_l, isdigit_l, isgraph_l,
islower_l, isprint_l, ispunct_l, isspace_l, isupper_l, isxdigit_l - character classification functions
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <ctype.h>
int isalnum(int c);
int isalpha(int c);
int iscntrl(int c);
int isdigit(int c);
int isgraph(int c);
int islower(int c);
int isprint(int c);
int ispunct(int c);
int isspace(int c);
int isupper(int c);
int isxdigit(int c);
int isascii(int c);
int isblank(int c);
int isalnum_l(int c, locale_t locale);
int isalpha_l(int c, locale_t locale);
int isblank_l(int c, locale_t locale);
int iscntrl_l(int c, locale_t locale);
int isdigit_l(int c, locale_t locale);
int isgraph_l(int c, locale_t locale);
int islower_l(int c, locale_t locale);
int isprint_l(int c, locale_t locale);
int ispunct_l(int c, locale_t locale);
int isspace_l(int c, locale_t locale);
int isupper_l(int c, locale_t locale);
int isxdigit_l(int c, locale_t locale);
int isascii_l(int c, locale_t locale);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
isascii():
_XOPEN_SOURCE
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE
isblank():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
isalnum_l(), isalpha_l(), isblank_l(), iscntrl_l(), isdigit_l(), isgraph_l(), islower_l(), isprint_l(),
ispunct_l(), isspace_l(), isupper_l(), isxdigit_l():
Since glibc 2.10:
_XOPEN_SOURCE >= 700
Before glibc 2.10:
_GNU_SOURCE
isascii_l():
Since glibc 2.10:
_XOPEN_SOURCE >= 700 && (_SVID_SOURCE || _BSD_SOURCE)
Before glibc 2.10:
_GNU_SOURCE
DESCRIPTION
These functions check whether c, which must have the value of an unsigned char or EOF, falls into a
certain character class according to the specified locale. The functions without the "_l" suffix perform
the check based on the current locale.
The functions with the "_l" suffix perform the check based on the locale specified by the locale object
locale. The behavior of these functions is undefined if locale is the special locale object
LC_GLOBAL_LOCALE (see duplocale(3)) or is not a valid locale object handle.
The list below explains the operation of the functions without the "_l" suffix; the functions with the
"_l" suffix differ only in using the locale object locale instead of the current locale.
isalnum()
checks for an alphanumeric character; it is equivalent to (isalpha(c) || isdigit(c)).
isalpha()
checks for an alphabetic character; in the standard "C" locale, it is equivalent to
(isupper(c) || islower(c)). In some locales, there may be additional characters for which
isalpha() is true—letters which are neither uppercase nor lowercase.
isascii()
checks whether c is a 7-bit unsigned char value that fits into the ASCII character set.
isblank()
checks for a blank character; that is, a space or a tab.
iscntrl()
checks for a control character.
isdigit()
checks for a digit (0 through 9).
isgraph()
checks for any printable character except space.
islower()
checks for a lowercase character.
isprint()
checks for any printable character including space.
ispunct()
checks for any printable character which is not a space or an alphanumeric character.
isspace()
checks for white-space characters. In the "C" and "POSIX" locales, these are: space, form-feed
('\f'), newline ('\n'), carriage return ('\r'), horizontal tab ('\t'), and vertical tab ('\v').
isupper()
checks for an uppercase letter.
isxdigit()
checks for hexadecimal digits, that is, one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F.
RETURN VALUE
The values returned are nonzero if the character c falls into the tested class, and zero if not.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│ isalnum(), isalpha(), isascii(), isblank(), iscntrl(), isdigit(), │ Thread safety │ MT-Safe │
│ isgraph(), islower(), isprint(), ispunct(), isspace(), isupper(), │ │ │
│ isxdigit() │ │ │
└─────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
isalnum()
isalpha()
iscntrl()
isdigit()
isgraph()
islower()
isprint()
ispunct()
isspace()
isupper()
isxdigit()
isblank()
C11, POSIX.1-2008.
isascii()
isalnum_l()
isalpha_l()
isblank_l()
iscntrl_l()
isdigit_l()
isgraph_l()
islower_l()
isprint_l()
ispunct_l()
isspace_l()
isupper_l()
isxdigit_l()
POSIX.1-2008.
isascii_l()
GNU.
HISTORY
isalnum()
isalpha()
iscntrl()
isdigit()
isgraph()
islower()
isprint()
ispunct()
isspace()
isupper()
isxdigit()
C89, POSIX.1-2001.
isblank()
C99, POSIX.1-2001.
isascii()
POSIX.1-2001 (XSI).
POSIX.1-2008 marks it as obsolete, noting that it cannot be used portably in a localized
application.
isalnum_l()
isalpha_l()
isblank_l()
iscntrl_l()
isdigit_l()
isgraph_l()
islower_l()
isprint_l()
ispunct_l()
isspace_l()
isupper_l()
isxdigit_l()
glibc 2.3. POSIX.1-2008.
isascii_l()
glibc 2.3.
CAVEATS
The standards require that the argument c for these functions is either EOF or a value that is
representable in the type unsigned char; otherwise, the behavior is undefined. If the argument c is of
type char, it must be cast to unsigned char, as in the following example:
char c;
...
res = toupper((unsigned char) c);
This is necessary because char may be the equivalent of signed char, in which case a byte where the top
bit is set would be sign extended when converting to int, yielding a value that is outside the range of
unsigned char.
The details of what characters belong to which class depend on the locale. For example, isupper() will
not recognize an A-umlaut (Ä) as an uppercase letter in the default C locale.
SEE ALSO
iswalnum(3), iswalpha(3), iswblank(3), iswcntrl(3), iswdigit(3), iswgraph(3), iswlower(3), iswprint(3),
iswpunct(3), iswspace(3), iswupper(3), iswxdigit(3), newlocale(3), setlocale(3), toascii(3), tolower(3),
toupper(3), uselocale(3), ascii(7), locale(7)
Linux man-pages 6.15 2025-05-17 isalpha(3)