fma(3)

💖 fma(3) — floating-point multiply and add 💖

okay bestie, welcome to the fma(3) page, she's a <math.h> icon and honestly? underrated.

✨ NAME ✨

fma, fmaf, fmal — floating-point multiply and add 🧮✨

📚 LIBRARY 📚

Math library (libm, link it with -lm) — yeah you gotta bring the -lm or she simply will not show up, no cap.

🌸 SYNOPSIS 🌸

#include <math.h>

double fma(double x, double y, double z);
float fmaf(float x, float y, float z);
long double fmal(long double x, long double y, long double z);

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

• fma(), fmaf(), fmal():
  • _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

like, define one of those or she's not coming out to play, iykyk 💅

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💫 DESCRIPTION 💫

These functions compute x * y + z. and here's the serving part: the result is rounded as ONE ternary operation according to the current rounding mode (see fenv(3)).

translation: she does the whole multiply-then-add as a single glow-up, one rounding, not two. that's the whole point. main character energy fr fr 👑

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🦋 RETURN VALUE 🦋

• She returns the value of x * y + z, rounded as one ternary operation. periodt.
• If x or y is a NaN, a NaN is returned. 🙈
• If x times y is an exact infinity, and z is an infinity with the opposite sign, a domain error occurs, and a NaN is returned. 💀
• If one of x or y is an infinity, the other is 0, and z is not a NaN, a domain error occurs, and a NaN is returned. 💀
• If one of x or y is an infinity, and the other is 0, and z is a NaN, a domain error occurs, and a NaN is returned. 💀
• If x times y is not an infinity times zero (or vice versa), and z is a NaN, a NaN is returned.
• If the result overflows, a range error occurs, and an infinity with the correct sign is returned.
• If the result underflows, a range error occurs, and a signed 0 is returned.

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🚨 ERRORS 🚨

Go read math_error(7) if you wanna know how to figure out whether an error actually happened when you called these functions. that's the detective work, bestie 🔍

The following errors can occur:

• Domain error: x * y + z or x * y is invalid.
  • An invalid floating-point exception (FE_INVALID) is raised.
• Range error: result overflow
  • An overflow floating-point exception (FE_OVERFLOW) is raised.
• Range error: result underflow
  • An underflow floating-point exception (FE_UNDERFLOW) is raised.

⚠️ These functions do not set errno. not even a little. don't go checking it, she said no 😭

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👑 ATTRIBUTES 👑

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

• fma(), fmaf(), fmal() — Thread safety: MT-Safe 🫶

(yes queen, she's MT-Safe, go off in all your threads simultaneously, she can handle it ✨)

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🌈 STANDARDS 🌈

C11, POSIX.1-2008. certified, official, iconic. 💅

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🎀 HISTORY 🎀

glibc 2.1. C99, POSIX.1-2001. she's been here since forever and she's been correct the whole time 🕰️✨

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🔥 SEE ALSO 🔥

• remainder(3)
• remquo(3)

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and that's the whole page, bestie!! you ate that. 💖 this little love letter comes straight from Linux man-pages 6.20, dated 2026-02-08 — mwah, go round some floats correctly 🧚🌸🦋