fwide(3)

💅 fwide(3) — the vibe check for your FILE stream's orientation

okay bestie, so the header says this is fwide(3) from the Library Functions Manual, and honestly? it's giving niche but iconic. let's get into it 🦋

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✨ NAME ✨

fwide — set and determine the orientation of a FILE stream

like, is your stream a wide-character girlie or a byte girlie? this function finds out (and sometimes decides for you 💅).

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📚 LIBRARY 📚

Standard C library (libc, -lc)

yep, she's built in. no extra drama, no extra linking. 🎀

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🌸 SYNOPSIS 🌸

#include <wchar.h>

int fwide(FILE *stream, int mode);

🧚 Feature Test Macro Requirements for glibc (peep feature_test_macros(7))

fwide():

_XOPEN_SOURCE >= 500 || _ISOC99_SOURCE
    || _POSIX_C_SOURCE >= 200112L

translation: you gotta define one of these bad boys or she simply will not appear. gatekeeping? a little. necessary? apparently. 🙈

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💖 DESCRIPTION 💖

okay so here's the tea ☕

When mode is zero, fwide() just determines the current orientation of stream. it's not changing anything, it's just looking. respectfully. 👀

• returns a positive value if stream is wide-character oriented — meaning wide-character I/O is permitted but char I/O is disallowed. so it picked a side and committed. periodt.
• returns a negative value if stream is byte oriented — meaning char I/O is permitted but wide-character I/O is disallowed.
• returns zero if stream has no orientation yet 🫥 — in this case the next I/O operation might change the orientation (to byte oriented if it's a char I/O operation, or to wide-character oriented if it's a wide-character I/O operation).

no cap: once a stream has an orientation, it cannot be changed and persists until the stream is closed. it's locked in fr fr. 🔒

When mode is nonzero, fwide() first attempts to set stream's orientation:

• to wide-character oriented if mode is greater than 0 🔥
• to byte oriented if mode is less than 0 💀

then it returns a value denoting the current orientation, as above. so it does the thing and reports back. multitasking queen. 👑

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🌈 RETURN VALUE 🌈

fwide() returns the stream's orientation, after possibly changing it.

• positive return value → wide-character oriented ✨
• negative return value → byte oriented 💅
• zero → undecided 😭

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👑 STANDARDS 👑

C11, POSIX.1-2008. certified. legit. no notes.

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🦋 HISTORY 🦋

POSIX.1-2001, C99. she's been around, she's seen things, she's still relevant. main character energy since forever.

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🍓 NOTES 🍓

• Wide-character output to a byte oriented stream can be performed through the fprintf(3) function with the %lc and %ls directives.
• Char oriented output to a wide-character oriented stream can be performed through the fwprintf(3) function with the %c and %s directives.

so basically, crossing the streams? possible, if you use the right format directives. iykyk 🧚

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⭐ SEE ALSO ⭐

• fprintf(3)
• fwprintf(3)

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and that's the whole page bestie, we ate and left no crumbs 💜 this came to you from Linux man-pages 6.20, dated 2026-02-08 — fwide(3), out. 🎀