NMDM(4)

📦 version 12.2, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking

💖 NMDM(4) 💖

okay bestie, gather round, because today we're talking about nmdm — the nullmodem terminal driver, straight out of the Kernel Interfaces Manual. yes it's a kernel driver, yes it's giving unbothered, and yes you're gonna understand it. periodt. 🎀

✨ NAME ✨

nmdm — nullmodem terminal driver 🦋

✨ SYNOPSIS ✨

so like, you wanna compile this driver into the kernel? iconic. drop this line into your kernel configuration file:

device nmdm

or, alternatively, if you're more of a "load it as a module at boot time" kind of girlie, put this in loader.conf(5) instead:

nmdm_load="YES"

pick your lane, both slay. 💅

✨ DESCRIPTION ✨

okay so here's the whole vibe: the nmdm driver hands you two tty(4) devices, and it wires them together with a virtual “null modem” cable. that's it. that's the whole thing. a fake cable. no cap. 🔥

now, the ✨interesting✨ part. if either of those two tty devices has the CDSR_OFLOW bit set in their line discipline — that's the “stty dsrflow” thing btw, iykyk — then the nmdm device will emulate the speed that's configured in the termios(4) settings. speed match, but make it virtual. 💫

and the way that this speed emulation works? it runs independently in the two directions, and it's controlled by the slower end's termios settings — specifically c_ispeed, c_ospeed, CS5 ... CS8, CSTOPB and PARENB. slowest one sets the tempo, fr fr. the way the slower end is literally the main character. 👑

✨ FILES ✨

  • /dev/nmdmN[AB] — nullmodem device nodes. where the A node has a matching B node 🫶

also! the nmdm driver implements “on-demand device creation”, which is exactly as cute as it sounds: you just access a given instance in /dev and boom, it creates it. no setup, no ceremony, it just shows up for you. ✨

✨ DIAGNOSTICS ✨

None. 💀

like genuinely. nothing. no drama, no errors, no nothing. it's that chill.

✨ SEE ALSO ✨

✨ HISTORY ✨

the nmdm driver first appeared in FreeBSD 4.4. a legend from day one, honestly. 🌈

and that's the whole page bestie!! this one came from the Debian side of things, dated January 23, 2018 — so like, a little vintage, but she still ate. 💖 go forth and null-modem responsibly. mwah. 💅✨🎀

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