ioctl_pipe(2)

💖 ioctl_pipe — the manual, but make it a group chat 💖

okay bestie, so this is the ioctl_pipe(2) page from the System Calls Manual section. it's short, it's sweet, it's giving "niche kernel knowledge", and we're gonna devour it. 🍓

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✨ NAME ✨

ioctl_pipe — literally just the ioctl() operations for the general notification mechanism. that's the whole vibe. 🎀

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🦋 SYNOPSIS 🦋

no cap, you're gonna need these headers first:

#include <linux/watch_queue.h>  /* Definition of IOC_WATCH_QUEUE_* */
#include <sys/ioctl.h>

int ioctl(int pipefd, IOC_WATCH_QUEUE_SET_SIZE, int size);
int ioctl(int pipefd, IOC_WATCH_QUEUE_SET_FILTER,
          struct watch_notification_filter *filter);

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🌸 DESCRIPTION 🌸

so here's the tea ☕ — these ioctl(2) operations exist so you can set up your general notification queue parameters. and where does this notification queue live? on top of a pipe(2) that you opened with the O_NOTIFICATION_PIPE flag. iconic behaviour, honestly.

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💅 IOC_WATCH_QUEUE_SET_SIZE (since Linux 5.8)

this one preallocates the pipe buffer memory so it can fit size notification messages. 💾

heads up bestie: right now, size must be between 1 and 512. no in-between nonsense, no going feral with big numbers. 512 is the ceiling, periodt.

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🫶 IOC_WATCH_QUEUE_SET_FILTER (since Linux 5.8)

a watch queue filter can limit the events that get received — like a bouncer for your kernel events, only letting the ones you care about through. 🚪✨

filters get passed in a struct watch_notification_filter, and each individual filter inside is described by a struct watch_notification_type_filter. two structs, bestie, one job. 🧚

struct watch_notification_filter {
        __u32   nr_filters;
        __u32   __reserved;
        struct watch_notification_type_filter filters[];
};

struct watch_notification_type_filter {
        __u32   type;
        __u32   info_filter;
        __u32   info_mask;
        __u32   subtype_filter[8];
};

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👑 SEE ALSO 👑

• pipe(2)
• ioctl(2)

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and that's the whole page, bestie!! 💜 lowkey a masterpiece of minimalism, highkey essential if you're messing with notification queues. go read the SEE ALSO's, they're the supporting cast and they ate. 💅

this has been Linux man-pages 6.20, dated 2026-02-08 — mwah, sleep well, drink water, and always close your file descriptors. 🎀✨