__register_sysctl_table(9)

📦 version 7.3.1, in nixpkgs unstable

💖 __register_sysctl_table — from the Kernel API Manual, bestie 💖

okay so this is <pink>__register_sysctl_table(9)</pink>, straight out of the Linux Kernel API Manual, and it's literally the function that puts a leaf sysctl table on the tree. yes, leaf. like a leaf on a tree. the tree is the sysctl tree. keep up, i believe in you 🍓

✨ SYNOPSIS ✨

struct ctl_table_header * __register_sysctl_table (struct ctl_table_set
*set , const char *path , const struct ctl_table *table , size_t
table_size );

look at that signature. it's giving "takes four things, returns a header pointer." main character energy fr 🫶

🎀 ARGUMENTS 🎀

  • set — the sysctl tree you're registering on, i.e. which tree you're hanging this bad boy off of.
  • path — the path to the directory that the sysctl table lives in.
  • table — the top-level table structure. <red>listen very closely</red> — this table should not be free'd after registration, so please please please do not put it on the stack. it can either be a global, or dynamically allocated by the caller and then free'd later after sysctl unregistration. that's the whole vibe 💅
  • table_size — the number of elements in table. count them. yes, all of them.

🌸 DESCRIPTION 🌸

so what does it do? it registers a sysctl table hierarchy. simple as. and table should be a filled-in ctl_table array — like, actually filled in, not half-done, we're not doing that today 🙈

🦋 the struct ctl_table members, explained 🦋

the members of the struct ctl_table structure are used as follows:

  • procname — the name of the sysctl file under /proc/sys. set it to NULL if you don't want to enter a sysctl file.
  • data — a pointer to data for use by proc_handler.
  • maxlen — the maximum size in bytes of the data.
  • mode — the file permissions for the /proc/sys file.
  • type — defines the target type (described in the struct definition).
  • proc_handler — the text handler routine (described below).

💜 extra1, extra2 💜

extra1, extra2 — extra pointers usable by the proc handler routines. and here's a little XXX note straight from the source, <yellow>verbatim</yellow>:

XXX: we should eventually modify these to use long min / max [0] [0] https://lkml.kernel.org/87zgpte9o4.fsfemail.froward.int.ebiederm.org

so yeah, that's a known someday-maybe. noted. iykyk 💀

🧚 leaf nodes & handlers 🧚

leaf nodes in the sysctl tree get represented by a single file under /proc. <red>non-leaf nodes are not allowed.</red> not a suggestion, bestie, it's the rule.

there must be a proc_handler routine for any terminal nodes. luckily there are several default handlers available that cover the common cases — slay:

  • proc_dostring
  • proc_dointvec
  • proc_dointvec_jiffies
  • proc_dointvec_userhz_jiffies
  • proc_dointvec_minmax
  • proc_doulongvec_ms_jiffies_minmax
  • proc_doulongvec_minmax

it's the handler's job to read the input buffer from user memory and process it, and the handler should return 0 on success. zero means success. we love a clean return 🌈

⭐ return value ⭐

this routine returns NULL on a failure to register, and a pointer to the table header on success. so like — check it. NULL = it did not ate. pointer = it ate. 👑

🔥 SEE ALSO 🔥

kernel file ./fs/proc/proc_sysctl.c, plus these icons:

💖 and that's the page!! the kernel file lives at ./fs/proc, this one's dated October 2026, and it is of course __register_sysctl_table(9). go forth and register your leaves responsibly, bestie 🎀✨🦋
📦 where this page comes from