isalpha(3)

💖 isalpha(3) — the character classification girlies 💖

okay bestie, sit down, because this is the page for the entire ctype.h classification squad. The manual name is isalpha but she's literally the spokesmodel for a whole friend group. This is a Library Functions Manual page, and it's giving "we check what KIND of character you are" energy. No cap. 🦋

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✨ 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 👑

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📚 LIBRARY 📚

Standard C library (libc, -lc). The classic. The blueprint. 💅

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🎀 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

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🌸 DESCRIPTION 🌸

So here's the tea: these functions check whether c — which must have the value of an unsigned char or EOF, and yes that matters, we'll get to it — falls into a certain character class according to the specified locale. The functions without the "_l" suffix do the check based on the current locale.

The functions with the "_l" suffix do the check based on the locale specified by the locale object locale. And heads up, this part is important: the behavior of these functions is undefined if locale is the special locale object LC_GLOBAL_LOCALE (peep duplocale(3)) or if it isn't a valid locale object handle. So don't do that. 🙈

The list below explains how the functions without the "_l" suffix work; the "_l" versions differ only in using the locale object locale instead of the current locale.

• isalnum() — checks for an alphanumeric character. It's literally equivalent to (isalpha(c) || isdigit(c)). Two for one. Slay. ✨
• isalpha() — checks for an alphabetic character. In the standard "C" locale it's equivalent to (isupper(c) || islower(c)). But in some locales there may be additional characters for which isalpha() is true — letters that are neither uppercase nor lowercase. Gender is a spectrum and so is casing, apparently. 💜
• isascii() — checks whether c is a 7-bit unsigned char value that fits into the ASCII character set. Small, humble, only 128 vibes.
• isblank() — checks for a blank character; that is, a space or a tab. Just those two. That's the whole list, bestie.
• iscntrl() — checks for a control character. The unhinged ones.
• isdigit() — checks for a digit (0 through 9).
• isgraph() — checks for any printable character except space. So like, visible, but no room for personal space. no cap.
• islower() — checks for a lowercase character.
• isprint() — checks for any printable character including space. Space finally gets included, we love an inclusive queen 🫶
• ispunct() — checks for any printable character that is not a space or an alphanumeric character. The punctuation girlies: ! ? ; you know the vibes.
• 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'). Six icons. periodt.
• 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. Yes, both cases, it's an ally. 🌈

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💫 RETURN VALUE 💫

The values returned are nonzero if the character c falls into the tested class, and zero if not. That's it. Binary. No complicated situationship. 💅

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🔥 ATTRIBUTES 🔥

For an explanation of the terms used in this section, see attributes(7).

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

(yeah the original had a whole table, it's the same info, enjoy the bullet-point remix 🎀)

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⭐ STANDARDS ⭐

• isalnum()
• isalpha()
• iscntrl()
• isdigit()
• isgraph()
• islower()
• isprint()
• ispunct()
• isspace()
• isupper()
• isxdigit()
• isblank()

➡️ those all be 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()

➡️ those be POSIX.1-2008.

• isascii_l()

➡️ that one's GNU. Glibc original flavor, iykyk.

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🕰️ HISTORY 🕰️

• isalnum()
• isalpha()
• iscntrl()
• isdigit()
• isgraph()
• islower()
• isprint()
• ispunct()
• isspace()
• isupper()
• isxdigit()

➡️ C89, POSIX.1-2001. Ancient. Elders. Respect them.

• 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. So she's retired, but she still shows up to the function calls. 💀

• 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.

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👑 CAVEATS 👑

🚨 THE BIG ONE. 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. Not "kinda weird". Undefined. As in, anything can happen, and it will happen to you specifically. 😭

If the argument c is of type char, it must be cast to unsigned char, like this:

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's outside the range of unsigned char. So the cast isn't ✨aesthetic✨, it's load-bearing. 🫶

Also, 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. She's uppercase in spirit. 💜

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🌈 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)

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and that's the page, bestie!! 🎀 this came to you from Linux man-pages 6.20, dated 2026-02-08, and she's isalpha(3) in the Library Functions Manual. go forth and cast your chars to unsigned char. mwah 💋💖✨