add_timer(9)

💖 add_timer(9) — like, the "start a timer" girlie 💖

okay so this is a page from the Kernel API Manual (add_timer(9)) and bestie, it's honestly so straightforward. we're starting a timer. that's the whole vibe. let me walk you through it 🎀

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✨ NAME ✨

add_timer — Start a timer 🕐

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✨ SYNOPSIS ✨

void add_timer (struct timer_list *timer );

like, that's it. one function, void return, one pointer. clean. iconic. 💅

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✨ ARGUMENTS ✨

• timer — The timer to be started. that's the one, that's the main character 🌟

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✨ DESCRIPTION ✨

okay so what this does: you start timer so it expires at timer->expires in the future. and timer->expires is the absolute expiry time, measured in 'jiffies' — not relative, absolute, so don't come at it with "in 5 seconds" energy, it wants the actual tick number fr fr ⏰

when the timer expires, timer->function(timer) gets invoked from soft interrupt context. not regular context. softirq. noted 👑

⚠️ before you call this, timer->expires and timer->function must both already be set. both. of them. no skipping, no "it'll be fine" — it will not be fine 🙈

and here's the little quirks, because of course there are quirks:

• if timer->function == NULL, the start operation is silently discarded. like it just... doesn't happen. no error, no fuss, no warning. just vibes and nothingness 💀
• if timer->expires is already in the past, the timer gets queued to expire at the next timer tick. close enough, we move 🦋
• this can only operate on an inactive timer. if you try to invoke it on an active timer, it gets rejected with a warning. so do a vibe check on that timer state first, bestie, seriously 🚨

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✨ SEE ALSO ✨

the whole friend group, literally all of them, no cap:

• Kernel file ./kernel/time/timer.c struct timer_base(9)
• __round_jiffies_relative(9)
• round_jiffies(9)
• round_jiffies_relative(9)
• __round_jiffies_up_relative(9)
• round_jiffies_up(9)
• round_jiffies_up_relative(9)
• timer_init_key(9)
• mod_timer_pending(9)
• mod_timer(9)
• timer_reduce(9)
• add_timer_local(9)
• add_timer_global(9)
• add_timer_on(9)
• __timer_delete(9)
• timer_delete(9)
• timer_shutdown(9)
• __try_to_del_timer_sync(9)
• timer_delete_sync_try(9)
• __timer_delete_sync(9)
• timer_delete_sync(9)
• timer_shutdown_sync(9)
• fetch_next_timer_interrupt_remote(9)
• timer_unlock_remote_bases(9)
• timer_lock_remote_bases(9)
• timer_base_is_idle(9)
• timer_expire_remote(9)
• get_next_timer_interrupt(9)
• timer_base_try_to_set_idle(9)
• timer_clear_idle(9)
• __run_timers(9)

like I said. the whole group chat. go say hi 💜

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and that's the page!! this one lives in ./kernel/time and it's dated October 2026 — so yeah, mark your calendar, that's the version and date we're repping 🗓️✨ slay, you now understand add_timer(9). periodt. 🌈🫶