SYSCTL_ADD_OID(9)

💅 SYSCTL_ADD_OID(9) — Kernel Developer's Manual, but make it a whole vibe 💅

🎀 NAME 🎀

sysctl_add_oid, sysctl_move_oid, sysctl_remove_oid, sysctl_remove_name — runtime sysctl tree manipulation. Yep bestie, we’re doing live tree surgery on sysctl OIDs ✨

🌸 SYNOPSIS 🌸

#include <sys/types.h>
#include <sys/sysctl.h>

struct sysctl_oid *
sysctl_add_oid(struct sysctl_ctx_list *ctx,
    struct sysctl_oid_list *parent, int number, const char *name,
    int kind, void *arg1, intmax_t arg2,
    int (*handler) (SYSCTL_HANDLER_ARGS), const char *format,
    const char *descr, const char *label);

int
sysctl_move_oid(struct sysctl_oid *oidp,
    struct sysctl_oid_list *parent);

int
sysctl_remove_oid(struct sysctl_oid *oidp, int del, int recurse);

int
sysctl_remove_name(struct sysctl_oid *oidp, const char *name, int del,
    int recurse);

🦋 DESCRIPTION 🦋

Okay so these functions are the interface for creating and deleting sysctl OIDs at runtime, like when a module is out here living its best life. The wrapper macros defined by sysctl(9) are recommended when creating new OIDs, and sysctl_add_oid() should not be called directly from the code. Not me emphasizing that, it’s literally in the rules 😌

Dynamic OIDs of type CTLTYPE_NODE are reusable so that several code sections can create and delete them, but in reality they are allocated and freed based on their reference count. As a consequence, it is possible for two or more code sections to create partially overlapping trees that they both can use. It is not possible to create overlapping leaves, nor to create different child types with the same name and parent. That’s the boundary, bestie 🚧

The sysctl_add_oid() function creates a raw OID of any type and connects it to its parent node, if any. If the OID is successfully created, the function returns a pointer to it; else it returns NULL. Many of the arguments for sysctl_add_oid() are common to the wrapper macros defined by sysctl(9).

The sysctl_move_oid() function reparents an existing OID. The OID is assigned a new number as if it had been created with number set to OID_AUTO. Glow-up via relocation, honestly 💫

The sysctl_remove_oid() function removes a dynamically created OID from the tree and optionally freeing its resources. It takes the following arguments:

• oidp — A pointer to the dynamic OID to be removed. If the OID is not dynamic, or the pointer is NULL, the function returns EINVAL.
• del — If non-zero, sysctl_remove_oid() will try to free the OID’s resources when the reference count of the OID becomes zero. However, if del is set to 0, the routine will only deregister the OID from the tree, without freeing its resources. This behaviour is useful when the caller expects to rollback (possibly partially failed) deletion of many OIDs later.
• recurse — If non-zero, attempt to remove the node and all its children. If recurse is set to 0, any attempt to remove a node that contains any children will result in a ENOTEMPTY error.

⚠️ WARNING: use recursive deletion with extreme caution! Normally it should not be needed if contexts are used. Contexts take care of tracking inter-dependencies between users of the tree. However, in some extreme cases it might be necessary to remove part of the subtree no matter how it was created, in order to free some other resources. Be aware, though, that this may result in a system panic(9) if other code sections continue to use removed subtrees. 💀

The sysctl_remove_name() function looks up the child node matching the name argument and then invokes the sysctl_remove_oid() function on that node, passing along the del and recurse arguments. If a node having the specified name does not exist, an error code of ENOENT is returned. Else the error code from sysctl_remove_oid() is returned.

In most cases the programmer should use contexts, as described in sysctl_ctx_init(9), to keep track of created OIDs, and to delete them later in orderly fashion. Organised queen behaviour 👑

🌈 SEE ALSO 🌈

• sysctl(8)
• sysctl(9)
• sysctl_ctx_free(9)
• sysctl_ctx_init(9)

⭐ HISTORY ⭐

These functions first appeared in FreeBSD 4.2. Vintage but still serving ✨

💖 AUTHORS 💖

Andrzej Bialecki <abial@FreeBSD.org>

🙈 BUGS 🙈

Sharing nodes between many code sections causes interdependencies that sometimes may lock the resources. For example, if module A hooks up a subtree to an OID created by module B, module B will be unable to delete that OID. These issues are handled properly by sysctl contexts.

Many operations on the tree involve traversing linked lists. For this reason, OID creation and removal is relatively costly. Lowkey expensive, but that’s the tradeoff 🫠

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🎀 That’s the whole page, bestie. It’s from Debian, dated December 13, 2016, and the manual name is SYSCTL_ADD_OID(9). Mwah 💋