isalpha(3) Library Functions Manual isalpha(3)
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, is-
print_l, ispunct_l, isspace_l, isupper_l, isxdigit_l - character clas-
sification 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 un-
signed 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 lo-
cale specified by the locale object locale. The behavior of these
functions is undefined if locale is the special locale object LC_GLOB-
AL_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 lo-
cale object locale instead of the current locale.
isalnum()
checks for an alphanumeric character; it is equivalent to (isal-
pha(c) || isdigit(c)).
isalpha()
checks for an alphabetic character; in the standard "C" locale,
it is equivalent to (isupper(c) || islower(c)). In some lo-
cales, 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" lo-
cales, these are: space, form-feed ('\f'), newline ('\n'), car-
riage 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 test-
ed class, and zero if not.
VERSIONS
isalnum_l(), isalpha_l(), isblank_l(), iscntrl_l(), isdigit_l(), is-
graph_l(), islower_l(), isprint_l(), ispunct_l(), isspace_l(), isup-
per_l(), isxdigit_l(), and isascii_l() are available since glibc 2.3.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│isalnum(), isalpha(), isascii(), isblank(), │ Thread safety │ MT-Safe │
│iscntrl(), isdigit(), isgraph(), islower(), │ │ │
│isprint(), ispunct(), isspace(), isupper(), │ │ │
│isxdigit() │ │ │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2001 specifies isalnum(), isalpha(), isblank(), iscntrl(), is-
digit(), isgraph(), islower(), isprint(), ispunct(), isspace(), isup-
per(), and isxdigit(), and also isascii() (as an XSI extension). C99
specifies all of the preceding functions, except isascii().
POSIX.1-2008 marks isascii() as obsolete, noting that it cannot be used
portably in a localized application.
POSIX.1-2008 specifies isalnum_l(), isalpha_l(), isblank_l(), iscn-
trl_l(), isdigit_l(), isgraph_l(), islower_l(), isprint_l(), is-
punct_l(), isspace_l(), isupper_l(), and isxdigit_l().
isascii_l() is a GNU extension.
NOTES
The standards require that the argument c for these functions is either
EOF or a value that is representable in the type unsigned char. 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 un-
signed char.
The details of what characters belong to which class depend on the lo-
cale. 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), isw-
graph(3), iswlower(3), iswprint(3), iswpunct(3), iswspace(3), iswup-
per(3), iswxdigit(3), newlocale(3), setlocale(3), toascii(3),
tolower(3), toupper(3), uselocale(3), ascii(7), locale(7)
Linux man-pages 6.03 2023-02-05 isalpha(3)
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