strcasestr(3)

✨ strcasestr — string case-insensitive search substring ✨

okay bestie so listen 💖 this one is a <string.h> girlie from the Standard C library (libc, -lc) and she is SO underrated. no cap.

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🌸 SYNOPSIS 🌸

#define _GNU_SOURCE  // See feature_test_macros(7)
#include <string.h>

char *strcasestr(const char *haystack, const char *needle);

heads up queen 👑 you literally cannot use this without that #define _GNU_SOURCE line first. it's giving gatekept, but that's just how GNU rolls. go read feature_test_macros(7) if you wanna know why, iykyk.

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💅 DESCRIPTION 💅

So strcasestr() is basically strstr()'s cooler twin 🫶 she finds your *needle* inside your *haystack*… except she ignores the case of BOTH arguments. both!! not one, both. iconic behaviour honestly.

capital letters, lowercase letters, she does not care, she is unbothered, she is thriving 🦋

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🎀 RETURN VALUE 🎀

bestie i'm not gonna lie to you, this section just points you at strstr(3). go check that one, it's got the whole story ✨

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⭐ ATTRIBUTES ⭐

if you wanna know what these words even mean, attributes(7) has the lore 📖

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

so yes she's thread safe, but read the fine print: it's MT-Safe locale — locale enters the chat 🌀

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🌈 STANDARDS 🌈

GNU. that's it. that's the whole section. she's a GNU original and she said what she said 💜

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🧚 HISTORY 🧚

GNU. again. same vibe. GNU from the start, no notes 💀

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🍓 SEE ALSO 🍓

• string(3)
• strstr(3)

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and that's the whole page bestie!! stay case-insensitive, stay unbothered, and if anyone asks — this was served fresh from Linux man-pages 6.20, dated 2026-05-16 🌸 periodt 💖✨