signbit(3)

💅 signbit(3) — test sign of a real floating-point number

okay bestie, welcome to signbit(3) ✨ this one lives in the Library Functions Manual and it's genuinely so underrated. it does exactly one thing and it does it flawlessly. let's get into it 🫶

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🌸 NAME 🌸

signbit — test sign of a real floating-point number

periodt. that's the whole job description. no notes. 💖

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📚 LIBRARY 📚

Math library (libm, -lm)

so yeah, you gotta link it: -lm. don't forget her, or the linker will be giving major side-eye 😭

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✨ SYNOPSIS ✨

#include <math.h>

int signbit(x);

🎀 Feature Test Macro Requirements for glibc (see feature_test_macros(7)): 🎀

• signbit():
  • _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

like, define the right macro or glibc will pretend she doesn't know you 🙈 iykyk

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💖 DESCRIPTION 💖

signbit() is a generic macro which can work on all real floating-point types. she's flexible like that 💅 she returns a nonzero value if the value of x has its sign bit set.

okay now this part is IMPORTANT, do not skip, this is the main character twist 👑

This is not the same as x < 0.0, because IEEE 754 floating point allows zero to be signed. The comparison -0.0 < 0.0 is false, but signbit(-0.0) will return a nonzero value.

like... -0.0 is out here pretending to be zero and signbit() is the only one who sees through the act. iconic behavior honestly 🔥

NaNs and infinities have a sign bit. yes, even your chaotic NaN has a sign bit. everyone's got layers bestie 🌈

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⭐ RETURN VALUE ⭐

The signbit() macro returns nonzero if the sign of x is negative; otherwise, it returns zero.

that's it. that's the tea. binary queen 👑

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💀 ERRORS 💀

No errors occur.

literally none. zero drama. the calmest function on the whole system call list 🧘‍♀️✨

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🦋 ATTRIBUTES 🦋

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

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

so she's MT-Safe — thread safe, no cap. run her from every thread you've got, she's not fazed 💪

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🌟 STANDARDS 🌟

C11, POSIX.1-2008.

certified, official, government-approved slay 📜✨

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🕰️ HISTORY 🕰️

POSIX.1-2001, C99.

This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).

she's been around since 2001 and honestly? aging like fine wine 🍷👑

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👀 SEE ALSO 👀

• copysign(3)

go read her right now, they're besties fr fr 💞

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and that's the whole page bestie!! you now officially understand signbit(3) better than 90% of people who use it 💖 remember: -0.0 is real, x < 0.0 will lie to you, and signbit() never will. go forth and check them sign bits 🌸

mwah 💋 this has been signbit(3) from Linux man-pages 6.20, dated 2026-02-08 — sourced straight from the manual, rewritten with love, zero facts harmed in the making of this page 🎀✨