TIOCMSET(2const)

✨ TIOCMSET ✨ (that's the page name bestie, from the (2const) section 💅)

okay so like. this one is titled TIOCMSET, and honestly the whole squad is here: TIOCMGET, TIOCMSET, TIOCMBIC, TIOCMBIS, TIOCMIWAIT, TIOCGICOUNT — they're all about modem control. yep. modem. control. the little wires in your serial port that talk to each other. it's giving vintage tech but she still ate 🌸

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📚 LIBRARY

Standard C library — that's libc, linked with -lc. your bestie libc has been carrying this whole thing, no cap 🫶

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🌟 SYNOPSIS

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

int ioctl(int fd, TIOCMGET, int *argp);
int ioctl(int fd, TIOCMSET, const int *argp);
int ioctl(int fd, TIOCMBIC, const int *argp);
int ioctl(int fd, TIOCMBIS, const int *argp);
int ioctl(int fd, TIOCMIWAIT, int arg);
int ioctl(int fd, TIOCGICOUNT, struct serial_icounter_struct *argp);

#include <linux/serial.h>

struct serial_icounter_struct;

like yes, it's ioctl six different ways, they're all serving their own little purpose 💜

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💖 DESCRIPTION

🎀 TIOCMGET

Grab the status of the modem bits. just. read the vibe check of your modem bits, that's it.

🎀 TIOCMSET

Set the status of the modem bits. you're the main character now, you decide what the bits are doing.

🎀 TIOCMBIC

Clear the indicated modem bits. girl, clear it. set those specific bits to off.

🎀 TIOCMBIS

Set the indicated modem bits. opposite energy, we're turning them ON now ✨

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The following bits are the ones these ioctls use — memorize them, they're iconic:

• TIOCM_LE — DSR (data set ready/line enable)
• TIOCM_DTR — DTR (data terminal ready)
• TIOCM_RTS — RTS (request to send)
• TIOCM_ST — Secondary TXD (transmit)
• TIOCM_SR — Secondary RXD (receive)
• TIOCM_CTS — CTS (clear to send)
• TIOCM_CAR — DCD (data carrier detect)
• TIOCM_CD — see TIOCM_CAR
• TIOCM_RNG — RNG (ring)
• TIOCM_RI — see TIOCM_RNG
• TIOCM_DSR — DSR (data set ready)

🎀 TIOCMIWAIT

Wait for any of the 4 modem bits (DCD, RI, DSR, CTS) to change. the way that you just sit there and wait for something to happen… relatable honestly 😭

The bits of interest are specified as a bit mask in arg, by ORing together any of the bit values, TIOCM_RNG, TIOCM_DSR, TIOCM_CD, and TIOCM_CTS. The caller should use TIOCGICOUNT to see which bit has changed. iykyk — you don't get told who moved, you just know someone did.

🎀 TIOCGICOUNT

Get counts of input serial line interrupts (DCD, RI, DSR, CTS). The counts are written to the serial_icounter_struct structure pointed to by argp. receipts, basically. we keep track of everything here 💅

Note: both 1->0 and 0->1 transitions are counted, except for RI, where only 0->1 transitions are counted. bestie RI is different. RI always is. 🙈

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✅ RETURN VALUE

On success, 0 is returned. slay. On error, -1 is returned, and errno is set to indicate the error. so like, check errno if things went sideways 💀

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🧚 EXAMPLES

Check the condition of DTR on the serial port. that's it, that's the whole mission:

#include <asm/termbits.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <unistd.h>

int
main(void)
{
    int fd, serial;

    fd = open("/dev/ttyS0", O_RDONLY);
    ioctl(fd, TIOCMGET, &serial);
    if (serial & TIOCM_DTR)
        puts("TIOCM_DTR is set");
    else
        puts("TIOCM_DTR is not set");
    close(fd);
}

see how simple? open /dev/ttyS0, ask, check the bit, close. chef's kiss 👑

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👀 SEE ALSO

• ioctl(2)
• ioctl_tty(2)

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okay that's the page!! ily, go forth and control those modem bits 💖 this little manual page is from Linux man-pages 6.20, dated 2026-02-08 — and it's the TIOCMSET(2const) page, so like, tag yourself, I'm TIOCMSET 🌈💜