isalpha(3) — Linux manual page


ISALPHA(3)              Linux Programmer's Manual             ISALPHA(3)

NAME         top

       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

SYNOPSIS         top

       #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

               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
               || /* Glibc <= 2.19: */ _SVID_SOURCE

           _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:

           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 && (_SVID_SOURCE || _BSD_SOURCE)
           Before glibc 2.10:

DESCRIPTION         top

       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.

              checks for an alphanumeric character; it is equivalent to
              (isalpha(c) || isdigit(c)).

              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.

              checks whether c is a 7-bit unsigned char value that fits
              into the ASCII character set.

              checks for a blank character; that is, a space or a tab.

              checks for a control character.

              checks for a digit (0 through 9).

              checks for any printable character except space.

              checks for a lowercase character.

              checks for any printable character including space.

              checks for any printable character which is not a space or
              an alphanumeric character.

              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').

              checks for an uppercase letter.

              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         top

       The values returned are nonzero if the character c falls into the
       tested class, and zero if not.

VERSIONS         top

       isalnum_l(), isalpha_l(), isblank_l(), iscntrl_l(), isdigit_l(),
       isgraph_l(), islower_l(), isprint_l(), ispunct_l(), isspace_l(),
       isupper_l(), isxdigit_l(), and isascii_l() are available since
       glibc 2.3.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                             Attribute     Value   │
       │isalnum(), isalpha(), isascii(),      │ Thread safety │ MT-Safe │
       │isblank(), iscntrl(), isdigit(),      │               │         │
       │isgraph(), islower(), isprint(),      │               │         │
       │ispunct(), isspace(), isupper(),      │               │         │
       │isxdigit()                            │               │         │

CONFORMING TO         top

       C89 specifies isalnum(), isalpha(), iscntrl(), isdigit(),
       isgraph(), islower(), isprint(), ispunct(), isspace(), isupper(),
       and isxdigit(), but not isascii() and isblank().  POSIX.1-2001
       also specifies those functions, and also isascii() (as an XSI
       extension) and isblank().  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(),
       iscntrl_l(), isdigit_l(), isgraph_l(), islower_l(), isprint_l(),
       ispunct_l(), isspace_l(), isupper_l(), and isxdigit_l().

       isascii_l() is a GNU extension.

NOTES         top

       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 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         top

       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),

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at

GNU                            2021-03-22                     ISALPHA(3)

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