Updated: 2022/Sep/29

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SYSCTL(9)                  Kernel Developer's Manual                 SYSCTL(9)

     sysctl - system variable control interfaces

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

     Primary external interfaces:

     sysctl_lock(struct lwp *l, void *oldp, size_t savelen);

     sysctl_dispatch(const int *name, u_int namelen, void *oldp,
         size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
         struct lwp *l, const struct sysctlnode *rnode);

     sysctl_unlock(struct lwp *l);

     sysctl_createv(struct sysctllog **log, int cflags,
         const struct sysctlnode **rnode, const struct sysctlnode **cnode,
         int flags, int type, const char *namep, const char *desc,
         sysctlfn func, u_quad_t qv, void *newp, size_t newlen, ...);

     sysctl_destroyv(struct sysctlnode *rnode, ...);

     sysctl_free(struct sysctlnode *rnode);

     sysctl_teardown(struct sysctllog **);

     old_sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp,
         void *newp, size_t newlen, struct lwp *l);

     Core internal functions:
     sysctl_locate(struct lwp *l, const int *name, u_int namelen,
         const struct sysctlnode **rnode, int *nip);

     sysctl_lookup(const int *name, u_int namelen, void *oldp,
         size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
         struct lwp *l, const struct sysctlnode *rnode);

     sysctl_create(const int *name, u_int namelen, void *oldp,
         size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
         struct lwp *l, const struct sysctlnode *rnode);

     sysctl_destroy(const int *name, u_int namelen, void *oldp,
         size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
         struct lwp *l, const struct sysctlnode *rnode);

     sysctl_query(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
         const void *newp, size_t newlen, const int *oname, struct lwp *l,
         const struct sysctlnode *rnode);

     Simple "helper" functions:
     sysctl_needfunc(const int *name, u_int namelen, void *oldp,
         size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
         struct lwp *l, const struct sysctlnode *rnode);

     sysctl_notavail(const int *name, u_int namelen, void *oldp,
         size_t *oldlenp, const void *newp, size_t newlen, const int *oname,
         struct lwp *l, const struct sysctlnode *rnode);

     sysctl_null(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
         const void *newp, size_t newlen, const int *oname, struct lwp *l,
         const struct sysctlnode *rnode);

     The SYSCTL subsystem instruments a number of kernel tunables and other
     data structures via a simple MIB-like interface, primarily for
     consumption by userland programs, but also for use internally by the

     All operations on the SYSCTL tree must be protected by acquiring the main
     SYSCTL lock.  The only functions that can be called when the lock is not
     held are sysctl_lock(), sysctl_createv(), sysctl_destroyv(), and
     old_sysctl().  All other functions require the tree to be locked.  This
     is to prevent other users of the tree from moving nodes around during an
     add operation, or from destroying nodes or subtrees that are actively
     being used.  The lock is acquired by calling sysctl_lock() with a pointer
     to the process's lwp l (NULL may be passed to all functions as the lwp
     pointer if no lwp is appropriate, though any changes made via
     sysctl_create(), sysctl_destroy(), sysctl_lookup(), or by any helper
     function will be done with effective superuser privileges).

     The oldp and savelen arguments are a pointer to and the size of the
     memory region the caller will be using to collect data from SYSCTL.
     These may also be NULL and 0, respectively.

     The memory region will be locked via uvm_vslock() if it is a region in
     userspace.  The address and size of the region are recorded so that when
     the SYSCTL lock is to be released via sysctl_unlock(), only the lwp
     pointer l is required.

     Once the lock has been acquired, it is typical to call sysctl_dispatch()
     to handle the request.  sysctl_dispatch() will examine the contents of
     name, an array of integers at least namelen long, which is to be located
     in kernel space, in order to determine which function to call to handle
     the specific request.

     The following algorithm is used by sysctl_dispatch() to determine the
     function to call:

              Scan the tree using sysctl_locate().

              If the node returned has a "helper" function, call it.

              If the requested node was found but has no function, call

              If the node was not found and name specifies one of
               sysctl_query(), sysctl_create(), or sysctl_destroy(), call the
               appropriate function.

              If none of these options applies and no other error was yet
               recorded, return EOPNOTSUPP.
     The oldp and oldlenp arguments to sysctl_dispatch(), as with all the
     other core functions, describe an area into which the current or
     requested value may be copied.  oldp may or may not be a pointer into
     userspace (as dictated by whether l is NULL or not).  oldlenp is a
     non-NULL pointer to a size_t.  newp and newlen describe an area where the
     new value for the request may be found; newp may also be a pointer into
     userspace.  The oname argument is a non-NULL pointer to the base of the
     request currently being processed.  By simple arithmetic on name,
     namelen, and oname, one can easily determine the entire original request
     and namelen values, if needed.  The rnode value, as passed to
     sysctl_dispatch() represents the root of the tree into which the current
     request is to be dispatched.  If NULL, the main tree will be used.

     The sysctl_locate() function scans a tree for the node most specific to a
     request.  If the pointer referenced by rnode is not NULL, the tree
     indicated is searched, otherwise the main tree will be used.  The address
     of the most relevant node will be returned via rnode and the number of
     MIB entries consumed will be returned via nip, if it is not NULL.

     The sysctl_lookup() function takes the same arguments as
     sysctl_dispatch() with the caveat that the value for namelen must be zero
     in order to indicate that the node referenced by the rnode argument is
     the one to which the lookup is being applied.

     New nodes are created and destroyed by the sysctl_create() and
     sysctl_destroy() functions.  These functions take the same arguments as
     sysctl_dispatch() with the additional requirement that the namelen
     argument must be 1 and the name argument must point to an integer valued
     either CTL_CREATE or CTL_CREATESYM when creating a new node, or
     CTL_DESTROY when destroying a node.

     The newp and newlen arguments should point to a copy of the node to be
     created or destroyed.  If the create or destroy operation was successful,
     a copy of the node created or destroyed will be placed in the space
     indicated by oldp and oldlenp.  If the create operation fails because of
     a conflict with an existing node, a copy of that node will be returned

     In order to facilitate the creation and destruction of nodes from a given
     tree by kernel subsystems, the functions sysctl_createv() and
     sysctl_destroyv() are provided.  These functions take care of the
     overhead of filling in the contents of the create or destroy request,
     dealing with locking, locating the appropriate parent node, etc.

     The arguments to sysctl_createv() are used to construct the new node.  If
     the log argument is not NULL, a sysctllog structure will be allocated and
     the pointer referenced will be changed to address it.  The same log may
     be used for any number of nodes, provided they are all inserted into the
     same tree.  This allows for a series of nodes to be created and later
     removed from the tree in a single transaction (via sysctl_teardown())
     without the need for any record keeping on the caller's part.

     The cflags argument is currently unused and must be zero.  The rnode
     argument must either be NULL or a valid pointer to a reference to the
     root of the tree into which the new node must be placed.  If it is NULL,
     the main tree will be used.  It is illegal for rnode to refer to a NULL
     pointer.  If the cnode argument is not NULL, on return it will be
     adjusted to point to the address of the new node.

     The flags and type arguments are combined into the sysctl_flags field,
     and the current value for SYSCTL_VERSION is added in.  The following
     types are defined:

           CTLTYPE_NODE            A node intended to be a parent for other

           CTLTYPE_INT             A signed integer.

           CTLTYPE_STRING          A NUL-terminated string.

           CTLTYPE_QUAD            An unsigned 64-bit integer.

           CTLTYPE_STRUCT          A structure.

           CTLTYPE_BOOL            A boolean.

     The namep argument is copied into the sysctl_name field and must be less
     than SYSCTL_NAMELEN characters in length.  The string indicated by desc
     will be copied if the CTLFLAG_OWNDESC flag is set, and will be used as
     the node's description.

     Two additional remarks:

           1.   The CTLFLAG_PERMANENT flag can only be set from SYSCTL setup
                routines (see SETUP FUNCTIONS) as called by sysctl_init().

           2.   If sysctl_destroyv() attempts to delete a node that does not
                own its own description (and is not marked as permanent), but
                the deletion fails, the description will be copied and
                sysctl_destroyv() will set the CTLFLAG_OWNDESC flag.

     The func argument is the name of a "helper" function (see HELPER
     FUNCTIONS AND MACROS).  If the CTLFLAG_IMMEDIATE flag is set, the qv
     argument will be interpreted as the initial value for the new "bool",
     "int" or "quad" node.  This flag does not apply to any other type of
     node.  The newp and newlen arguments describe the data external to SYSCTL
     that is to be instrumented.  One of func, qv and the CTLFLAG_IMMEDIATE
     flag, or newp and newlen must be given for nodes that instrument data,
     otherwise an error is returned.

     The remaining arguments are a list of integers specifying the path
     through the MIB to the node being created.  The list must be terminated
     by the CTL_EOL value.  The penultimate value in the list may be
     CTL_CREATE if a dynamic MIB entry is to be made for this node.
     sysctl_createv() specifically does not support CTL_CREATESYM, since setup
     routines are expected to be able to use the in-kernel ksyms(4) interface
     to discover the location of the data to be instrumented.  If the node to
     be created matches a node that already exists, a return code of 0 is
     given, indicating success.

     When using sysctl_destroyv() to destroy a given node, the rnode argument,
     if not NULL, is taken to be the root of the tree from which the node is
     to be destroyed, otherwise the main tree is used.  The rest of the
     arguments are a list of integers specifying the path through the MIB to
     the node being destroyed.  If the node being destroyed does not exist, a
     successful return code is given.  Nodes marked with the CTLFLAG_PERMANENT
     flag cannot be destroyed.

     Helper functions are invoked with the same common argument set as
     sysctl_dispatch() except that the rnode argument will never be NULL.  It
     will be set to point to the node that corresponds most closely to the
     current request.  Helpers are forbidden from modifying the node they are
     passed; they should instead copy the structure if changes are required in
     order to effect access control or other checks.  The "helper" prototype
     and function that needs to ensure that a newly assigned value is within a
     certain range (presuming external data) would look like the following:

           static int sysctl_helper(SYSCTLFN_PROTO);

           static int
                   struct sysctlnode node;
                   int t, error;

                   t = *(int *)rnode->sysctl_data;

                   node = *rnode;
                   node.sysctl_data = &t;
                   error = sysctl_lookup(SYSCTLFN_CALL(&node));
                   if (error || newp == NULL)
                           return (error);

                   if (t < 0 || t > 20)
                           return (EINVAL);

                   *(int *)rnode->sysctl_data = t;
                   return (0);

      macros ensure that all arguments are passed properly.  The single
     argument to the SYSCTLFN_CALL macro is the pointer to the node being

     Three basic helper functions are available for use.  sysctl_needfunc()
     will emit a warning to the system console whenever it is invoked and
     provides a simplistic read-only interface to the given node.
     sysctl_notavail() will forward "queries" to sysctl_query() so that
     subtrees can be discovered, but will return EOPNOTSUPP for any other
     condition.  sysctl_null() specifically ignores any arguments given, sets
     the value indicated by oldlenp to zero, and returns success.

     Although nodes can be added to the SYSCTL tree at any time, in order to
     add nodes during the kernel bootstrap phase (and during loadable module
     initialization), a proper "setup" function must be used.  Setup functions
     are declared using the SYSCTL_SETUP macro, which takes the name of the
     function and a short string description of the function as arguments.
     (See the SYSCTL_DEBUG_SETUP kernel configuration in options(4).)

     The address of the function is added to a list of functions that
     sysctl_init() traverses during initialization.  For loadable kernel
     modules (see module(9)), the list of functions is called from the module
     loader before the module's initialization routine.  Any sysctl nodes
     created for the loadable module are removed using sysctl_teardown() after
     calling the module's termination code.

     Setup functions do not have to add nodes to the main tree, but can set up
     their own trees for emulation or other purposes.  Emulations that require
     use of a main tree but with some nodes changed to suit their own purposes
     can arrange to overlay a sparse private tree onto their main tree by
     making the e_sysctlovly member of their struct emul definition point to
     the overlaid tree.

     Setup functions should take care to create all nodes from the root down
     to the subtree they are creating, since the order in which setup
     functions are called is arbitrary (the order in which setup functions are
     called is only determined by the ordering of the object files as passed
     to the linker when the kernel is built).

     sysctl_init() is called early in the kernel bootstrap process.  It
     initializes the SYSCTL lock, calls all the registered setup functions,
     and marks the tree as permanent.

     sysctl_free() will unconditionally delete any and all nodes below the
     given node.  Its intended use is for the deletion of entire trees, not
     subtrees.  If a subtree is to be removed, sysctl_destroy() or
     sysctl_destroyv() should be used to ensure that nodes not owned by the
     sub-system being deactivated are not mistakenly destroyed.  The SYSCTL
     lock must be held when calling this function.

     sysctl_teardown() unwinds a sysctllog and deletes the nodes in the
     opposite order in which they were created.

     old_sysctl() provides an interface similar to the old SYSCTL
     implementation, with the exception that access checks on a per-node basis
     are performed if the l argument is non-NULL.  If called with a NULL
     argument, the values for newp and oldp are interpreted as kernel
     addresses, and access is performed as for the superuser.

     It is expected that nodes will be added to (or removed from) the tree
     during the following stages of a machine's lifetime:

        initialization -- when the kernel is booting
        autoconfiguration -- when devices are being probed at boot time
        "plug and play" device attachment -- when a PC-Card, USB, or other
         device is plugged in or attached
        module initialization -- when a module is being loaded
        "run-time" -- when a process creates a node via the sysctl(3)

     Nodes marked with CTLFLAG_PERMANENT can only be added to a tree during
     the first or initialization phase, and can never be removed.  The
     initialization phase terminates when the main tree's root is marked with
     the CTLFLAG_PERMANENT flag.  Once the main tree is marked in this manner,
     no nodes can be added to any tree that is marked with CTLFLAG_READONLY at
     its root, and no nodes can be added at all if the main tree's root is so

     Nodes added by device drivers, modules, and at device insertion time can
     be added to (and removed from) "read-only" parent nodes.

     Nodes created by processes can only be added to "writable" parent nodes.
     See sysctl(3) for a description of the flags that are allowed to be used
     by when creating nodes.


     The dynamic SYSCTL implementation first appeared in NetBSD 2.0.

     Andrew Brown <atatat@NetBSD.org> designed and implemented the dynamic
     SYSCTL implementation.

NetBSD 10.99                   September 6, 2022                  NetBSD 10.99