nan(3)

✨ nan — bestie edition ✨

okay so real talk, the manual name for this one is nan (and its lil siblings nanf and nanl) and it lives in section 3 of the library functions manual 💖

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🌸 NAME 🌸

nan, nanf, nanl — they return a 'Not a Number' 💅

📚 LIBRARY 📚

Math library — that's libm, so you gotta link her with -lm bestie, don't forget or the linker WILL be dramatic 😭

🦋 SYNOPSIS 🦋

#include <math.h>

double nan(const char *tagp);
float nanf(const char *tagp);
long double nanl(const char *tagp);

Feature Test Macro Requirements for glibc

(like, peep feature_test_macros(7) if you wanna know the lore 👀)

• nan(), nanf(), nanl(): _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

fr fr you need one of those macros defined or these functions will simply not show up to the party 🙈

🎀 DESCRIPTION 🎀

These functions return a representation of a quiet NaN — and which exact one you get is determined by tagp ✨ If your implementation doesn't support quiet NaNs at all? Then these functions just return zero. that's it. that's the whole thing. 💀

The call nan("char-sequence") is literally equivalent to:

strtod("NAN(char-sequence)", NULL);

and same energy for nanf() and nanl() — they're equivalent to analogous calls to strtof(3) and strtold(3) respectively 🫶

Now, the argument tagp? It gets used in an unspecified manner. ngl that's the actual wording. On IEEE 754 systems there are, like, SO many representations of NaN, and tagp picks which one you get. On other systems it may do absolutely nothing. it's giving decorative 😭

💅 ATTRIBUTES 💅

Want the full explanation of these terms? Peep attributes(7), bestie.

• Interface: nan(), nanf(), nanl()
  • Attribute: Thread safety
  • Value: MT-Safe locale

(yes that's the whole table, it's a short king 👑)

🌈 STANDARDS 🌈

C11, POSIX.1-2008. periodt.

Also go look at IEC 559 and the appendix with recommended functions in IEEE 754/IEEE 854 if you're feeling academic 📖✨

🌟 HISTORY 🌟

• glibc 2.1. (iconic era)
• C99, POSIX.1-2001.

👀 SEE ALSO 👀

isnan(3), strtod(3), math_error(7)

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and that's the whole vibe check, bestie!! this page is from Linux man-pages 6.20, dated 2026-02-08 🎀 go forth and return your quiet NaNs, you absolute main character 💖🦋✨