usb_ifnum_to_if(9)

📦 version 7.3.1, in nixpkgs unstable

💅 usb_ifnum_to_if — the Kernel API Manual, bestie edition 🦋

Okay so this is usb_ifnum_to_if(9) from the Kernel API Manual, and honestly? It's giving "small function, big responsibilities." Let's get into it ✨

🌸 NAME 🌸

usb_ifnum_to_if — get the interface object with a given interface number. That's the whole vibe. You give it a number, it gives you the interface. Iconic behavior, no notes 💖

✨ SYNOPSIS ✨

struct usb_interface * usb_ifnum_to_if (const struct usb_device *dev ,
unsigned ifnum );

Like, that's the whole signature. Look at it. So clean. So demure 🎀

🫶 ARGUMENTS 🫶

  • dev — the device whose current configuration is being considered. Like, the device we're snooping on. Respectfully 👀
  • ifnum — the desired interface. The one you're hunting for, bestie. The main character of this search 🔥

💜 DESCRIPTION 💜

So here's what actually happens: this function walks the device descriptor for the currently active configuration to find the interface object that has that particular interface number. It's literally a little scavenger hunt through the descriptor 🧚‍♀️

Now, important caveat time, because this is where people get caught out 🚨 — configuration descriptors are not required to assign interface numbers sequentially. So you cannot, and I mean cannot, assume that the first interface in that descriptor corresponds to interface zero. That assumption? Wrong. Incorrect. Do not pass go 💀

This routine exists specifically to help device drivers avoid exactly that kind of mistake. It's the friend who stops you from texting your ex, except the ex is a wrong interface number. Grateful for it, truly 🙏

But — and this is a big but — you should make sure that you do the right thing with any alternate settings available for this interface. Don't just grab the interface and dip. Handle the altsettings properly, okay? 💅

🚨 Do NOT call this function unless you are bound to one of the interfaces on this device, or you have locked the device! This is not a "eh, probably fine" situation. It's a hard requirement. Like, fr fr. Respect the locking rules or the kernel will be giving you problems 😭

⭐ RETURN ⭐

Returns a pointer to the interface that has ifnum as its interface number — if it found one, bestie! 🎉 If not? NULL. That's it. That's the deal. Check your return values, always 🫶

🦋 SEE ALSO 🦋

Kernel file ./drivers/usb/core/usb.c and this whole little friend group of functions, all invited to the group chat:

🌈 Sign-off 🌈

This lil page lives in ./drivers/usb/core, dated October 2026, and it's page number 9 of its manual (usb_ifnum_to_if(9)). Anyway bestie, go forth and look up your interfaces correctly 💖💅✨

📦 where this page comes from