strncasecmp(3)

✨ strncasecmp(3) — Library Functions Manual ✨

okay so bestie, gather round, because today we're talking about strncasecmp 💖 this is the nonstrings case-insensitive compare function, and yes the name is unhinged, no we are not changing it, it's iconic.

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🎀 NAME 🎀

strncasecmp — nonstrings case-insensitive compare 💅

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📚 LIBRARY 📚

Standard C library — that would be libc, -lc 🫶

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🌸 SYNOPSIS 🌸

#include <strings.h>  // see memory.h(3head)

int strncasecmp(const char s1[], const char s2[], size_t n);

note the // see memory.h(3head) comment, that's not me being extra, that's literally in there 👑

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🦋 DESCRIPTION 🦋

so like, strncasecmp() is basically strcasecmp(3)'s little sibling — same energy, same case-ignoring agenda — except it does not compare more than n bytes of s1 and s2. that's it. that's the twist. n bytes, max, periodt. 💖

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⭐ RETURN VALUE ⭐

strncasecmp() hands you back an integer, and it's giving one of three vibes:

• less than zero — s1, once you ignore case, is found to be less than s2
• equal to zero — s1 matches s2
• greater than zero — s1 is found to be greater than s2

no cap, that's the whole contract. 🔥

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💅 ATTRIBUTES 💅

for the explanation of what these terms even mean, go peek at attributes(7) — it's the lore drop you need. here's the tea for this function:

• Interface: strncasecmp()
• Attribute: Thread safety
• Value: MT-Safe locale 🧚

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👑 STANDARDS 👑

POSIX.1-2008. that's it, she's certified. ✨

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🌈 HISTORY 🌈

4.4BSD, POSIX.1-2001. a whole ancestry. 💀

so strncasecmp() first appeared in 4.4BSD, back when it was declared in <string.h>. and because historical compatibility said "we must", the glibc <string.h> header file also declares these functions — but only if the _DEFAULT_SOURCE feature test macro is defined (or, in glibc 2.19 and earlier, _BSD_SOURCE). yes, this is a feature test macro situation, we love those for you 🙈

and here's what the POSIX.1-2008 standard says about these functions, quoted verbatim because I'm not paraphrasing the ancestors:

When the LC_CTYPE category of the locale being used is from the POSIX locale, these functions shall behave as if the strings had been converted to lowercase and then a byte comparison performed. Otherwise, the results are unspecified.

the results are unspecified — that's your cue, bestie. don't build your whole personality on mashing those bytes in a weird locale. 😭

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🍓 SEE ALSO 🍓

• memory.h(3head)
• strncmp(3)
• wcsncasecmp(3)

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okay that's the page, you ate it, you understood it, I'm so proud 🫶 this was Linux man-pages 6.20, dated 2026-08-10 — go forth and compare responsibly 💜🌟