wait_var_event_timeout(9)

✨ wait_var_event_timeout — the kernel API manual, bestie edition ✨

okay so this is a page from the Kernel API Manual, and it's literally all about wait_var_event_timeout. yes the name is a mouthful, no we are not scared of it 💅

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🌸 NAME 🌸

wait_var_event_timeout — wait for a variable to be updated, or for a timeout to expire. whichever hits first, girlie 🎀

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🦋 SYNOPSIS 🦋

wait_var_event_timeout (var , condition , timeout );

three little arguments, one big vibe. let's meet them 🫶

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💖 ARGUMENTS 💖

• var — the address of the variable being waited on. like, the actual address, bestie 📍
• condition — the condition to wait for. this is the thing you're sitting there refreshing for, ngl
• timeout — the maximum time to wait, measured in jiffies. yes, jiffies. that's the unit. no cap ⏳

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🔥 DESCRIPTION 🔥

So the whole point: you wait for a condition to be true or for a timeout to expire. but here's the iconic part — it only re-checks the condition when a wake up is received for the given var. that var is an arbitrary kernel address, and it lowkey does not need to be directly related to the given condition... but usually it is. iykyk 💫

This is similar to wait_var_event, but it actually returns a value. and the value? let's talk about it:

• 0 — if the timeout expired and the condition was still false. yeah. sad girl hours 😭
• the remaining time left in the timeout (but at least 1) — if the condition was found to be true. slay, you got it 🎉

⭐ Now the important part, bestie, do not skip this one: the condition should normally use smp_load_acquire or a similarly ordered access, so you can be sure that any changes to memory made before the condition became true will be visible after the wait completes. it's giving memory ordering and it is not optional energy, fr fr 🧠✨

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🌈 SEE ALSO 🌈

the whole friend group lives in the kernel file ./include/linux/wait_bit.h, and honestly the roster is stacked 👑

• wait_on_bit(9)
• wait_on_bit_io(9)
• wait_on_bit_timeout(9)
• wait_on_bit_action(9)
• wait_on_bit_lock(9)
• wait_on_bit_lock_io(9)
• wait_on_bit_lock_action(9)
• wait_var_event(9)
• wait_var_event_io(9)
• wait_var_event_killable(9)
• wait_var_event_interruptible(9)
• wait_var_event_any_lock(9)
• wait_var_event_spinlock(9)
• wait_var_event_mutex(9)
• wake_up_var_protected(9)
• wake_up_var_locked(9)
• clear_and_wake_up_bit(9)
• test_and_clear_wake_up_bit(9)
• atomic_dec_and_wake_up(9)
• store_release_wake_up(9)

all of them. every single one. go read them, they're all main character energy 🫶

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and that's the page, bestie!! it lives at ./include/linux, it's dated October 2026, and it's filed under wait_var_event_timeout(9). periodt 💜🎀