__setfpucw(3)

💖 __setfpucw — the i386 FPU control word girlie 💖

okay so real talk, bestie: we're in section 3 of the Library Functions Manual and this page is lowkey a museum exhibit. it's giving vintage. it's giving "removed in glibc 2.1". but you asked, so we're gonna learn it properly. 🫶

---

✨ NAME ✨

__setfpucw — set the FPU control word on the i386 architecture (and yes, she is obsolete, keep reading, we'll get to that 😭)

---

📚 LIBRARY 📚

Standard C library (libc, -lc) — the classic, the icon, the one you already have linked.

---

🎀 SYNOPSIS 🎀

#include <i386/fpu_control.h>

[[deprecated]] void __setfpucw(unsigned short control_word);

see that [[deprecated]] right there? that's the compiler literally side-eyeing you. respect the label. 🙈

---

🌸 DESCRIPTION 🌸

so what does she do? __setfpucw() transfers control_word straight to the registers of the FPU (floating-point unit) on the i386 architecture. 💅

back in the day this was the way to control:

• floating-point precision
• floating-point rounding
• floating-point exceptions

that's it, that's the whole job. one unsigned short and you're controlling how your floats behave. main character energy honestly. ✨

---

⭐ STANDARDS ⭐

GNU. that's the whole standard section. GNU. periodt. 👑

---

💀 HISTORY 💀

Removed in glibc 2.1.

girl. she's gone. if you're on anything remotely modern, this function does not exist. do not come for her, she has left the building. 😭

---

🧚 NOTES 🧚

okay so, since __setfpucw() packed her bags, C99 came through with a whole new crew of functions, prototypes chilling in <fenv.h>. here's the full roster, no cap:

controlling FPU rounding modes:

• fegetround(3)
• fesetround(3)

the floating-point environment:

• fegetenv(3)
• feholdexcept(3)
• fesetenv(3)
• feupdateenv(3)

FPU exception handling:

• feclearexcept(3)
• fegetexceptflag(3)
• feraiseexcept(3)
• fesetexceptflag(3)
• fetestexcept(3)

and if you still genuinely need to poke the FPU control word directly — like, direct access, no middleman — the _FPU_GETCW and _FPU_SETCW macros from <fpu_control.h> are right there for you. 💜 (note: <fpu_control.h>, not the <i386/fpu_control.h> from the synopsis. that distinction? iconic.)

---

🔥 EXAMPLES 🔥

__setfpucw(0x1372)

what does that magic number do? it sets the FPU control word on the i386 architecture to:

• extended precision
• rounding to nearest
• exceptions on overflow, zero divide and NaN

three for one, ate that. no crumbs. 👑

---

🌈 SEE ALSO 🌈

• feclearexcept(3)
• <fpu_control.h>

---

that's the whole page, bestie! 🦋 source: Linux man-pages 6.20, dated 2026-02-08 — go forth, respect the deprecated tag, and use <fenv.h> like a modern girlie. mwah. 💖✨🎀