efi_printk(9)

💖 efi_printk(9) — the Kernel API Manual page where your fave EFI stub printk lives ✨

okay so bestie, welcome to efi_printk(9) from the Kernel API Manual — yes the whole manual, and yes this function is basically the EFI stub's little messenger goblin 🧚 that sends your kernel message out into the world. lowkey iconic.

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🌸 NAME 🌸

efi_printk — print a kernel message 💌

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✨ SYNOPSIS ✨

int efi_printk (const char *fmt , ... );

like, that's the whole vibe. int in, characters out. slay.

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🎀 ARGUMENTS 🎀

• fmt — the format string 🫶 the star of the show, the main character, the thing you're actually printing.
• ... — variable arguments 💅 all the extra lil friends that get slotted into your format string. they're just vibing in the ellipsis.

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🦋 DESCRIPTION 🦋

okay so here's the tea ☕ — the first letter of the format string is what decides the logging level of the message. that's the whole trick, iykyk.

and the vibe check: if that level is less than the current EFI logging level, the message gets suppressed 😭 like, sorry bestie, not today, the filter said no.

also important and you gotta know it: the message will be truncated to 255 bytes. so don't come at it with a novel, it's giving short-form content only 🍓

no cap, 255 bytes is the limit. write like it.

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⭐ RETURN ⭐

returns the number of printed characters ✨ so you literally get receipts on how much got printed. obsessed.

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🔥 SEE ALSO 🔥

• Kernel file ./drivers/firmware/efi/libstub/printk.c — the source, go read it, it's right there 👑
• efi_char16_puts(9)
• efi_puts(9)

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so yeah, that's the page 💜 go forth, print your messages, respect the 255-byte limit, and remember that the first letter runs the whole show.

this has been ./drivers/firmware/efi/libstub (the path, the whole path) and the page is dated October 2026, from the same crew that brought you efi_printk(9) 🦋 bye bestie, mwah 💅✨