FIONREAD(2const)

💖 FIONREAD(2const) 💖

okay bestie, so this is a page from the 2const manual — that's the little corner of the Linux man-pages that documents standalone constants you pass to syscalls. And today's main characters? FIONREAD, TIOCINQ, TIOCOUTQ, TCFLSH, and TIOCSERGETLSR. Buffer counting and flushing. Riveting. Iconic. Let's go. ✨

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🌸 LIBRARY 🌸

The standard C library, aka libc, link it with -lc. You know the drill, bestie. 💅

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🦋 SYNOPSIS 🦋

#include <asm/termbits.h>  /* Definition of constants */
#include <sys/ioctl.h>

int ioctl(int fd, FIONREAD, int *argp);
int ioctl(int fd, TIOCINQ, int *argp);
int ioctl(int fd, TIOCOUTQ, int *argp);
int ioctl(int fd, TCFLSH, int arg);
int ioctl(int fd, TIOCSERGETLSR, int *argp);

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✨ DESCRIPTION ✨

so each of these is a little request you hand to ioctl(2), and here's what each one is giving:

• FIONREAD — get the number of bytes sitting in the input buffer. Just a lil headcount of what's waiting for you. 💜
• TIOCINQ — literally the same thing as FIONREAD. Same energy, different name. No cap.
• TIOCOUTQ — get the number of bytes in the output buffer. That's the stuff on its way out. 🎀
• TCFLSH — equivalent to tcflush(fd, arg).

and for what arg can actually be — the values are TCIFLUSH, TCOFLUSH, and TCIOFLUSH — go look at tcflush(3). that's where the lore lives. 📚

• TIOCSERGETLSR — get the line status register.

the status register has the TIOCSER_TEMT bit set when the output buffer is empty and the hardware transmitter is physically empty too. so like, truly, genuinely done transmitting. empty-empty. 🕳️

lowkey important caveat: this does not have to be supported by all serial tty drivers. so don't come for me if your driver just doesn't. 🙈

also tcdrain(3) does not wait and returns immediately when the TIOCSER_TEMT bit is set. no waiting around, she's already done.

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⭐ RETURN VALUE ⭐

• on success: 0 is returned. slay. 🎉
• on error: -1 is returned, and errno is set to tell you what went wrong. 💀

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🌈 SEE ALSO 🌈

• ioctl(2)
• ioctl_tty(2)
• tcflush(3)
• termios(7)

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ngl bestie, if you're mixing up FIONREAD and TIOCOUTQ, just remember: IN is what's coming to you, OUT is what's leaving. one is your inbox, one is your sent folder. periodt. 👑

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and that's the whole page, pookie 🫶 — this was served fresh from Linux man-pages 6.20, dated 2026-07-02, and yeah, the source line said FIONREAD(2const) twice because the header and footer are matching outfits. obsessed. 💅✨