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isalpha(3) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ATTRIBUTES \|STANDARDS \|HISTORY \|CAVEATS \|SEE ALSO \|COLOPHON

isalpha(3)               Library Functions Manualisalpha(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

LIBRARY top

       Standard C library (libc,-lc)

SYNOPSIS top

#include <ctype.h>int isalnum(intc);int isalpha(intc);int iscntrl(intc);int isdigit(intc);int isgraph(intc);int islower(intc);int isprint(intc);int ispunct(intc);int isspace(intc);int isupper(intc);int isxdigit(intc);int isascii(intc);int isblank(intc);int isalnum_l(intc, locale_tlocale);int isalpha_l(intc, locale_tlocale);int isblank_l(intc, locale_tlocale);int iscntrl_l(intc, locale_tlocale);int isdigit_l(intc, locale_tlocale);int isgraph_l(intc, locale_tlocale);int islower_l(intc, locale_tlocale);int isprint_l(intc, locale_tlocale);int ispunct_l(intc, locale_tlocale);int isspace_l(intc, locale_tlocale);int isupper_l(intc, locale_tlocale);int isxdigit_l(intc, locale_tlocale);int isascii_l(intc, locale_tlocale);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):isascii():           _XOPEN_SOURCE               || /* glibc >= 2.19: */ _DEFAULT_SOURCE               || /* glibc <= 2.19: */ _SVID_SOURCEisblank():           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112Lisalnum_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_SOURCEisascii_l():           Since glibc 2.10:               _XOPEN_SOURCE >= 700 && (_SVID_SOURCE || _BSD_SOURCE)           Before glibc 2.10:               _GNU_SOURCE

DESCRIPTION top

       These  functions  check whetherc, which must have the value of anunsigned char  orEOF,  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 objectlocale.   The  behavior  of       these  functions  is  undefined  iflocale  is the special locale       objectLC_GLOBAL_LOCALE(seeduplocale(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 objectlocale 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 whichisalpha() is true—letters which are neither uppercase nor              lowercase.isascii()              checks whetherc is a 7-bitunsigned 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 of0 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 characterc falls into the       tested class, and zero if not.

ATTRIBUTES top

       For an explanation of the terms used in this section, seeattributes(7).       ┌──────────────────────────────────────┬───────────────┬─────────┐       │Interface│Attribute│Value│       ├──────────────────────────────────────┼───────────────┼─────────┤       │isalnum(),isalpha(),isascii(),     │ Thread safety │ MT-Safe │       │isblank(),iscntrl(),isdigit(),     │               │         │       │isgraph(),islower(),isprint(),     │               │         │       │ispunct(),isspace(),isupper(),     │               │         │       │isxdigit()                           │               │         │       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

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 top

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 top

       The standards require that the argumentc for these functions is       eitherEOFor a value that is representable in the typeunsignedchar; otherwise, the behavior is undefined.  If the argumentc is       of typechar, it must be cast tounsigned char, as in the       following example:           char c;           ...           res = toupper((unsigned char) c);       This is necessary becausechar may be the equivalent ofsignedchar, in which case a byte where the top bit is set would be sign       extended when converting toint, yielding a value that is outside       the range ofunsigned 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 defaultClocale.

COLOPHON top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  If you discover any rendering problems in this HTML       version of the page, or you believe there is a better or more up-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-05-17isalpha(3)

Pages that refer to this page:bash(1), PR_SET_VMA(2const), getopt(3), iswalnum(3), iswalpha(3), iswblank(3), iswcntrl(3), iswdigit(3), iswgraph(3), iswlower(3), iswprint(3), iswpunct(3), iswspace(3), iswupper(3), iswxdigit(3), localeconv(3), setlocale(3), sscanf(3), strtod(3), strtol(3), strtoul(3), toascii(3), toupper(3), wctype(3), bpf-helpers(7), locale(7), lsof(8)