nextup(3)

💖 nextup(3) — the bestie manual 💖

okay so bestie, welcome to nextup(3) from the Library Functions Manual 🎀 this is your girl explaining how to grab the very next floating-point number in whichever direction you want. no cap, these six functions are lowkey iconic.

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✨ NAME ✨

nextup, nextupf, nextupl, nextdown, nextdownf, nextdownl — return next floating-point number toward positive/negative infinity

like, they literally just scootch you one representable float over. one tiny step. that's it. that's the whole job. 💅

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📚 LIBRARY 📚

Math library (libm, -lm)

so when you compile, don't forget the -lm at the end or the linker is gonna ghost you fr fr 😭

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🌸 SYNOPSIS 🌸

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

double nextup(double x);
float nextupf(float x);
long double nextupl(long double x);

double nextdown(double x);
float nextdownf(float x);
long double nextdownl(long double x);

six functions, one vibe check: three go up, three go down, and they come in double, float, and long double flavors 💖 also head's up, you need that _GNU_SOURCE define, and you gotta check out feature_test_macros(7) about it ⭐

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🦋 DESCRIPTION 🦋

nextup(), nextupf(), and nextupl() return the next representable floating-point number greater than x.

translation: the tiniest possible step to the right. like the very next door neighbor float. no stairs skipped, no cap. 🪜

If x is the smallest representable negative number in the corresponding type, these functions return -0. If x is +0 or -0, the returned value is the smallest representable positive number of the corresponding type.

(yes bestie, they really do hand you a -0 in that one specific case, that's not a typo, that's the standard being extra 🙈)

If x is positive infinity, the returned value is positive infinity. If x is negative infinity, the returned value is the largest representable finite negative number of the corresponding type.

so +∞ just stays +∞ like it's too famous to move ✨ but −∞ actually takes a step and lands on the biggest finite negative number. main character energy honestly 👑

If x is NaN, the returned value is NaN.

The value returned by nextdown(x) is -nextup(-x), and similarly for the other types.

obsessed with this btw — nextdown is literally just nextup in a mirror 🪞 that's the whole trick, that's the tea, iykyk 💅

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💫 RETURN VALUE 💫

See DESCRIPTION.

bestie the manpage really said "go reread the section above" and honestly? valid. it's all up there fr fr 🫶

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🎀 ATTRIBUTES 🎀

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

• Interface: nextup(), nextupf(), nextupl(), nextdown(), nextdownf(), nextdownl()
  • Attribute: Thread safety
  • Value: MT-Safe

MT-Safe means these are safe to call from multiple threads at the same time, no drama, no fighting, they all get along 🌈 slay

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⭐ STANDARDS ⭐

These functions are described in IEEE Std 754-2008 - Standard for Floating-Point Arithmetic and ISO/IEC TS 18661.

yes she's certified. standards-approved. a whole legitimate baddie of a function 💙

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🌈 HISTORY 🌈

glibc 2.24.

(she's not ancient, but she's been around long enough to know things 💜)

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👑 SEE ALSO 👑

nearbyint(3), nextafter(3)

if you liked nextup, you're gonna wanna meet her cousins. nextafter(3) is the one who lets you pick the direction, and nearbyint(3) rounds like a pro 🎯

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okay that's the whole page bestie, you're gonna do amazing, go forth and increment some floats 💖✨🎀

this gem came from Linux man-pages 6.20, dated 2026-02-08 — mwah, take care of her 🫶💜