fdim(3)

💖 fdim(3) — the "positive difference" girlies 💖

okay so bestie, welcome to the fdim family: that's fdim, fdimf, and fdiml — three icons, one job: positive difference. this is a page from the Library Functions Manual, and it's giving "small function, big feelings" ✨

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🌸 LIBRARY 🌸

Math library, duh — link it with -lm or she will Not Load. <em>cries in undefined reference</em> 😭

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🎀 SYNOPSIS 🎀

#include <math.h>

double fdim(double x, double y);
float fdimf(float x, float y);
long double fdiml(long double x, long double y);

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

• fdimf(), fdiml(): _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

like, yes, you need the macros for the f and l versions. it's a whole thing. 🙈

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🌈 DESCRIPTION 🌈

so these functions return the positive difference — that's literally max(x-y,0), no cap — between their arguments.

that's it. that's the function. she takes x and y, she does x-y, and if that came out negative she just… doesn't. she clamps that baby to 0. main character energy, honestly. 👑

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⭐ RETURN VALUE ⭐

• On success, these functions return the positive difference. slay. ✨
• If x or y is a NaN, a NaN is returned. it's giving "we don't know her either" 💀
• If the result overflows, a range error occurs, and the functions return HUGE_VAL, HUGE_VALF, or HUGE_VALL, respectively. big number, absolutely feral. 🔥

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😭 ERRORS 😭

need the deets on how to tell if an error even happened while calling these? go read math_error(7), she explains everything.

the following errors can occur:

• Range error: result overflow — errno is set to ERANGE. An overflow floating-point exception (FE_OVERFLOW) is raised. so yeah, she overflowed and she's loud about it. 📣

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💅 ATTRIBUTES 💅

for the whole explanation of the terms used here, see attributes(7).

(gonna bullet this out bc tables are not the vibe on a terminal okay)

• fdim(), fdimf(), fdiml() — Thread safety: MT-Safe

MT-Safe!! she's safe in threads!! we love a secure queen 🫶

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👑 STANDARDS 👑

C11, POSIX.1-2008. certified, standardised, official. periodt.

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🕰️ HISTORY 🕰️

• glibc 2.1.
• C99, POSIX.1-2001.

she's been around. she's a legacy icon. 💜

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🙈 BUGS 🙈

Before glibc 2.24 on certain architectures (e.g., x86, but not x86_64) these functions did not set errno.

so if you're on old glibc on the wrong architecture and you're checking errno… bestie she's not filling that in. not her fault, it's a whole historical situation. 😭

fr fr: don't trust errno from these on that combo. that's a real caveat, not me being dramatic (well, a little bit of me being dramatic).

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🌟 SEE ALSO 🌟

• fmax(3)

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and that's the whole page, bestie! written up in the Linux man-pages 6.20 era, dated 2026-02-08, filed under fdim(3). go forth and clamp your differences. mwah 💋