FDIM(3P)

✨ FDIM(3P) — POSIX Programmer's Manual ✨

okay bestie, welcome to fdim(3P), straight out of the POSIX Programmer's Manual 💅 (yes, the manual name is literally in the title, we're not playing). Let's get into it.

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🌸 PROLOG 🌸

This manual page is part of the POSIX Programmer's Manual. Heads up though, fr fr: the Linux implementation of this interface may differ (so go consult the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface may not be implemented on Linux at all. Vibe check accordingly. 🫶

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👑 NAME 👑

fdim, fdimf, fdiml — compute positive difference between two floating-point numbers

the way these three are just siblings with the same job... iconic. 💖

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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);

look at them. three little functions. double, float, long double. slay. ✨

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🦋 DESCRIPTION 🦋

okay so first thing: the functionality described on this reference page is aligned with the ISO C standard. Any conflict between what's written here and the ISO C standard is unintentional — nobody's trying to start drama. This volume of POSIX.1‐2017 defers to the ISO C standard, periodt.

So what do these functions actually do? They determine the positive difference between their arguments:

• if x is greater than y, then x-y is returned 💖
• if x is less than or equal to y, then +0 is returned 🎀

that's it. that's the whole personality. and honestly? we respect it.

Now, an application that wishes to check for error situations should do the whole ritual: set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. Then on return:

• if errno is non-zero, or
• if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero,

…then bestie, an error has occurred. 😭 no cap.

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💜 RETURN VALUE 💜

Upon successful completion, these functions shall return the positive difference value. easy. 😌

If x-y is positive and overflows, a range error shall occur, and fdim(), fdimf(), and fdiml() shall return the value of the macro HUGE_VAL, HUGE_VALF, and HUGE_VALL, respectively. it's giving "we tried our best and then hit the ceiling". 💀

If the correct value would cause underflow, a range error may occur, and fdim(), fdimf(), and fdiml() shall return the correct value, or (if the IEC 60559 Floating-Point option is not supported) an implementation-defined value no greater in magnitude than DBL_MIN, FLT_MIN, and LDBL_MIN, respectively.

If x or y is NaN, a NaN shall be returned. NaN in, NaN out, that's the law. 🙈

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🔥 ERRORS 🔥

The fdim() function shall fail if:

• Range Error — The result overflows.
  • If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the overflow floating-point exception shall be raised.

The fdim() function may fail if:

• Range Error — The result underflows.
  • If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the underflow floating-point exception shall be raised.

The following sections are informative. (translation: they're the bonus content, not the legally-binding main character energy 🫶)

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🌈 EXAMPLES 🌈

None.

not a single example. the audacity. we move. 💅

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🌸 APPLICATION USAGE 🌸

On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero. they're not a couple, they're just roommates, but one of them has to pay rent. 🧚

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⭐ RATIONALE ⭐

None.

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🦋 FUTURE DIRECTIONS 🦋

None.

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👑 SEE ALSO 👑

• feclearexcept()
• fetestexcept()
• fmax()
• fmin()
• Section 4.20, Treatment of Error Conditions for Mathematical Functions, <math.h>

go read those too, they're the friend group. 💖

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💅 COPYRIGHT 💅

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page bestie!! brought to you by IEEE/The Open Group, dated 2017, filed under FDIM(3P) — stay hydrated, check your errno, and never let your floats underflow without a range error. mwah 💋✨🫶