printk(9)

💖 printk(9) — Kernel API Manual 💖

okay bestie, so the header situation: this is printk(9) from the Kernel API Manual, and no I'm not copying that line verbatim, we're folding it in cute-style, you're welcome 🫶

---

✨ NAME ✨

printk — print a kernel message 💬

---

🌸 SYNOPSIS 🌸

printk (fmt , ... );

---

🎀 ARGUMENTS 🎀

• fmt — the format string, the blueprint of your message, the main character of the call ✨
• ... — variable arguments, aka the vibes you feed into the format string, as many as you need 💅

---

🦋 DESCRIPTION 🦋

This is printk. That's it, that's the whole intro, iconic behavior honestly. It can be called from any context — any. context. We want it to work, and we mean it fr fr 🔥

If printk indexing is enabled, _printk is called from printk_index_wrap. Otherwise, printk is simply #defined to _printk. That's the whole routing situation, no cap.

Now the juicy part: we try to grab the console_lock. If we succeed, it's easy — we log the output and call the console drivers. Slay, done, no drama 💅

But if we fail to get the semaphore? Not me panicking — we just place the output into the log buffer and return. The current holder of the console_sem will notice the new output in console_unlock, and will send it to the consoles before releasing the lock. Teamwork, bestie 🫶

⚠️ lowkey important caveat: one effect of this deferred printing is that code which calls printk and then changes console_loglevel may break. This is because console_loglevel is inspected when the actual printing occurs. So like… the timing is giving delayed-reaction and your loglevel change might arrive too late 😭

See also: printf(3)

See the vsnprintf documentation for format string extensions over C99. yes really, go read it, it's the lore 🌈

---

👑 SEE ALSO 👑

Kernel file ./include/linux/printk.h, printk_cpu_sync_get_irqsave(9), printk_cpu_sync_put_irqrestore(9), pr_fmt(9), pr_emerg(9), pr_alert(9), pr_crit(9), pr_err(9), pr_warn(9), pr_notice(9), pr_info(9), pr_cont(9), pr_devel(9), pr_debug(9), print_hex_dump_bytes(9) — a whole friend group of logging besties, go say hi 💜

---

and that's the whole page!! 🎀 sign-off time: this little beauty lives under ./include/linux, dated October 2026, and it's printk(9) forever and always ⭐ bye bestie, go log something 💖