sysctl

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

NAME
     SYSCTL_DECL, SYSCTL_ADD_INT, SYSCTL_ADD_LONG, SYSCTL_ADD_NODE,
     SYSCTL_ADD_NODE_WITH_LABEL, SYSCTL_ADD_OPAQUE, SYSCTL_ADD_PROC,
     SYSCTL_ADD_QUAD, SYSCTL_ADD_ROOT_NODE, SYSCTL_ADD_S8, SYSCTL_ADD_S16,
     SYSCTL_ADD_S32, SYSCTL_ADD_S64, SYSCTL_ADD_STRING, SYSCTL_ADD_STRUCT,
     SYSCTL_ADD_U8, SYSCTL_ADD_U16, SYSCTL_ADD_U32, SYSCTL_ADD_U64,
     SYSCTL_ADD_UAUTO, SYSCTL_ADD_UINT, SYSCTL_ADD_ULONG, SYSCTL_ADD_UQUAD,
     SYSCTL_CHILDREN, SYSCTL_STATIC_CHILDREN, SYSCTL_NODE_CHILDREN,
     SYSCTL_PARENT, SYSCTL_INT, SYSCTL_INT_WITH_LABEL, SYSCTL_LONG,
     SYSCTL_NODE, SYSCTL_NODE_WITH_LABEL, SYSCTL_OPAQUE, SYSCTL_PROC,
     SYSCTL_QUAD, SYSCTL_ROOT_NODE, SYSCTL_S8, SYSCTL_S16, SYSCTL_S32,
     SYSCTL_S64, SYSCTL_STRING, SYSCTL_STRUCT, SYSCTL_U8, SYSCTL_U16,
     SYSCTL_U32, SYSCTL_U64, SYSCTL_UINT, SYSCTL_ULONG, SYSCTL_UQUAD — Dynamic
     and static sysctl MIB creation functions

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

     SYSCTL_DECL(name);

     struct sysctl_oid *
     SYSCTL_ADD_INT(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, int *ptr, int val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_LONG(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, long *ptr, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_NODE(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, int (*handler)(SYSCTL_HANDLER_ARGS),
         const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_NODE_WITH_LABEL(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, int (*handler)(SYSCTL_HANDLER_ARGS), const char *descr,
         const char *label);

     struct sysctl_oid *
     SYSCTL_ADD_OPAQUE(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, void *ptr, intptr_t len, const char *format,
         const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_PROC(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, void *arg1, intptr_t arg2,
         int (*handler) (SYSCTL_HANDLER_ARGS), const char *format,
         const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_QUAD(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, int64_t *ptr, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_ROOT_NODE(struct sysctl_ctx_list *ctx, int number,
         const char *name, int ctlflags, int (*handler)(SYSCTL_HANDLER_ARGS),
         const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_S8(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, int8_t *ptr, int8_t val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_S16(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, int16_t *ptr, int16_t val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_S32(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, int32_t *ptr, int32_t val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_S64(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, int64_t *ptr, int64_t val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_STRING(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, char *ptr, intptr_t len, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_STRUCT(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, void *ptr, struct_type, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_U8(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, uint8_t *ptr, uint8_t val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_U16(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, uint16_t *ptr, uint16_t val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_U32(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, uint32_t *ptr, uint32_t val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_U64(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, uint64_t *ptr, uint64_t val, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_UINT(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, unsigned int *ptr, unsigned int val,
         const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_ULONG(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, unsigned long *ptr, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_UQUAD(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, uint64_t *ptr, const char *descr);

     struct sysctl_oid *
     SYSCTL_ADD_UAUTO(struct sysctl_ctx_list *ctx,
         struct sysctl_oid_list *parent, int number, const char *name,
         int ctlflags, void *ptr, const char *descr);

     struct sysctl_oid_list *
     SYSCTL_CHILDREN(struct sysctl_oid *oidp);

     struct sysctl_oid_list *
     SYSCTL_STATIC_CHILDREN(struct sysctl_oid_list OID_NAME);

     struct sysctl_oid_list *
     SYSCTL_NODE_CHILDREN(parent, name);

     struct sysctl_oid *
     SYSCTL_PARENT(struct sysctl_oid *oid);

     SYSCTL_INT(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_INT_WITH_LABEL(parent, number, name, ctlflags, ptr, val, descr,
         label);

     SYSCTL_LONG(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_NODE(parent, number, name, ctlflags, handler, descr);

     SYSCTL_NODE_WITH_LABEL(parent, number, name, ctlflags, handler, descr,
         label);

     SYSCTL_OPAQUE(parent, number, name, ctlflags, ptr, len, format, descr);

     SYSCTL_PROC(parent, number, name, ctlflags, arg1, arg2, handler, format,
         descr);

     SYSCTL_QUAD(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_ROOT_NODE(number, name, ctlflags, handler, descr);

     SYSCTL_S8(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_S16(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_S32(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_S64(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_STRING(parent, number, name, ctlflags, arg, len, descr);

     SYSCTL_STRUCT(parent, number, name, ctlflags, ptr, struct_type, descr);

     SYSCTL_U8(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_U16(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_U32(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_U64(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_UINT(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_ULONG(parent, number, name, ctlflags, ptr, val, descr);

     SYSCTL_UQUAD(parent, number, name, ctlflags, ptr, val, descr);

DESCRIPTION
     The SYSCTL kernel interface allows dynamic or static creation of
     sysctl(8) MIB entries.  All static sysctls are automatically destroyed
     when the module which they are part of is unloaded.  Most top level
     categories are created statically and are available to all kernel code
     and its modules.

DESCRIPTION OF ARGUMENTS
     ctx       Pointer to sysctl context or NULL, if no context.  See
               sysctl_ctx_init(9) for how to create a new sysctl context.
               Programmers are strongly advised to use contexts to organize
               the dynamic OIDs which they create because when a context is
               destroyed all belonging sysctls are destroyed as well.  This
               makes the sysctl cleanup code much simpler.  Else deletion of
               all created OIDs is required at module unload.

     parent    A pointer to a struct sysctl_oid_list, which is the head of the
               parent's list of children.  This pointer is retrieved using the
               SYSCTL_STATIC_CHILDREN() macro for static sysctls and the
               SYSCTL_CHILDREN() macro for dynamic sysctls.  The
               SYSCTL_PARENT() macro can be used to get the parent of an OID.
               The macro returns NULL if there is no parent.

     number    The OID number that will be assigned to this OID.  In almost
               all cases this should be set to OID_AUTO, which will result in
               the assignment of the next available OID number.

     name      The name of the OID.  The newly created OID will contain a copy
               of the name.

     ctlflags  A bit mask of sysctl control flags.  See the section below
               describing all the control flags.

     arg1      First callback argument for procedure sysctls.

     arg2      Second callback argument for procedure sysctls.

     len       The length of the data pointed to by the ptr argument.  For
               string type OIDs a length of zero means that strlen(3) will be
               used to get the length of the string at each access to the OID.

     ptr       Pointer to sysctl variable or string data.  For sysctl values
               the pointer can be SYSCTL_NULL_XXX_PTR which means the OID is
               read-only and the returned value should be taken from the val
               argument.

     val       If the ptr argument is SYSCTL_NULL_XXX_PTR, gives the constant
               value returned by this OID.  Else this argument is not used.

     struct_type
               Name of structure type.

     handler   A pointer to the function that is responsible for handling read
               and write requests to this OID.  There are several standard
               handlers that support operations on nodes, integers, strings
               and opaque objects.  It is possible to define custom handlers
               using the SYSCTL_PROC() macro or the SYSCTL_ADD_PROC()
               function.

     format    A pointer to a string which specifies the format of the OID in
               a symbolic way.  This format is used as a hint by sysctl(8) to
               apply proper data formatting for display purposes.

               Current formats:
                     N       node
                     A       char *
                     I       int
                     IK[n]   temperature in Kelvin, multiplied by an optional
                             single digit power of ten scaling factor: 1
                             (default) gives deciKelvin, 0 gives Kelvin, 3
                             gives milliKelvin
                     IU      unsigned int
                     L       long
                     LU      unsigned long
                     Q       quad_t
                     QU      u_quad_t
                     S,TYPE  struct TYPE structures

     descr     A pointer to a textual description of the OID.

     label     A pointer to an aggregation label for this component of the
               OID.  To make it easier to export sysctl data to monitoring
               systems that support aggregations through labels (e.g.,
               Prometheus), this argument can be used to attach a label name
               to an OID.  The label acts as a hint that this component's name
               should not be part of the metric's name, but attached to the
               metric as a label instead.

               Labels should only be applied to siblings that are structurally
               similar and encode the same type of value, as aggregation is of
               no use otherwise.

CREATING ROOT NODES
     Sysctl MIBs or OIDs are created in a hierarchical tree.  The nodes at the
     bottom of the tree are called root nodes, and have no parent OID.  To
     create bottom tree nodes the SYSCTL_ROOT_NODE() macro or the
     SYSCTL_ADD_ROOT_NODE() function needs to be used.  By default all static
     sysctl node OIDs are global and need a SYSCTL_DECL() statement prior to
     their SYSCTL_NODE() definition statement, typically in a so-called header
     file.

CREATING SYSCTL STRINGS
     Zero terminated character strings sysctls are created either using the
     SYSCTL_STRING() macro or the SYSCTL_ADD_STRING() function.  If the len
     argument in zero, the string length is computed at every access to the
     OID using strlen(3).

CREATING OPAQUE SYSCTLS
     The SYSCTL_OPAQUE() or SYSCTL_STRUCT() macros or the SYSCTL_ADD_OPAQUE()
     or SYSCTL_ADD_STRUCT() functions create an OID that handle any chunk of
     data of the size specified by the len argument and data pointed to by the
     ptr argument.  When using the structure version the type is encoded as
     part of the created sysctl.

CREATING CUSTOM SYSCTLS
     The SYSCTL_PROC() macro and the SYSCTL_ADD_PROC() function create OIDs
     with the specified handler function.  The handler is responsible for
     handling all read and write requests to the OID.  This OID type is
     especially useful if the kernel data is not easily accessible, or needs
     to be processed before exporting.

CREATING A STATIC SYSCTL
     Static sysctls are declared using one of the SYSCTL_INT(),
     SYSCTL_INT_WITH_LABEL(), SYSCTL_LONG(), SYSCTL_NODE(),
     SYSCTL_NODE_WITH_LABEL(), SYSCTL_OPAQUE(), SYSCTL_PROC(), SYSCTL_QUAD(),
     SYSCTL_ROOT_NODE(), SYSCTL_S8(), SYSCTL_S16(), SYSCTL_S32(),
     SYSCTL_S64(), SYSCTL_STRING(), SYSCTL_STRUCT(), SYSCTL_U8(),
     SYSCTL_U16(), SYSCTL_U32(), SYSCTL_U64(), SYSCTL_UINT(), SYSCTL_ULONG()
     or SYSCTL_UQUAD() macros.

CREATING A DYNAMIC SYSCTL
     Dynamic nodes are created using one of the SYSCTL_ADD_INT(),
     SYSCTL_ADD_LONG(), SYSCTL_ADD_NODE(), SYSCTL_ADD_NODE_WITH_LABEL(),
     SYSCTL_ADD_OPAQUE(), SYSCTL_ADD_PROC(), SYSCTL_ADD_QUAD(),
     SYSCTL_ADD_ROOT_NODE(), SYSCTL_ADD_S8(), SYSCTL_ADD_S16(),
     SYSCTL_ADD_S32(), SYSCTL_ADD_S64(), SYSCTL_ADD_STRING(),
     SYSCTL_ADD_STRUCT(), SYSCTL_ADD_U8(), SYSCTL_ADD_U16(), SYSCTL_ADD_U32(),
     SYSCTL_ADD_U64(), SYSCTL_ADD_UAUTO(), SYSCTL_ADD_UINT(),
     SYSCTL_ADD_ULONG(), or SYSCTL_UQUAD() functions.  See
     sysctl_remove_oid(9) or sysctl_ctx_free(9) for more information on how to
     destroy a dynamically created OID.

CONTROL FLAGS
     For most of the above functions and macros, declaring a type as part of
     the access flags is not necessary — however, when declaring a sysctl
     implemented by a function, including a type in the access mask is
     required:

     CTLTYPE_NODE     This is a node intended to be a parent for other nodes.

     CTLTYPE_INT      This is a signed integer.

     CTLTYPE_STRING   This is a nul-terminated string stored in a character
                      array.

     CTLTYPE_S8       This is an 8-bit signed integer.

     CTLTYPE_S16      This is a 16-bit signed integer.

     CTLTYPE_S32      This is a 32-bit signed integer.

     CTLTYPE_S64      This is a 64-bit signed integer.

     CTLTYPE_OPAQUE   This is an opaque data structure.

     CTLTYPE_STRUCT   Alias for CTLTYPE_OPAQUE.

     CTLTYPE_U8       This is an 8-bit unsigned integer.

     CTLTYPE_U16      This is a 16-bit unsigned integer.

     CTLTYPE_U32      This is a 32-bit unsigned integer.

     CTLTYPE_U64      This is a 64-bit unsigned integer.

     CTLTYPE_UINT     This is an unsigned integer.

     CTLTYPE_LONG     This is a signed long.

     CTLTYPE_ULONG    This is an unsigned long.

     All sysctl types except for new node declarations require one of the
     following flags to be set indicating the read and write disposition of
     the sysctl:

     CTLFLAG_RD       This is a read-only sysctl.

     CTLFLAG_RDTUN    This is a read-only sysctl and tunable which is tried
                      fetched once from the system environment early during
                      module load or system boot.

     CTLFLAG_WR       This is a writable sysctl.

     CTLFLAG_RW       This sysctl is readable and writable.

     CTLFLAG_RWTUN    This is a readable and writeable sysctl and tunable
                      which is tried fetched once from the system environment
                      early during module load or system boot.

     CTLFLAG_NOFETCH  In case the node is marked as a tunable using the
                      CTLFLAG_[XX]TUN, this flag will prevent fetching the
                      initial value from the system environment.  Typically
                      this flag should only be used for very early low level
                      system setup code, and not by common drivers and
                      modules.

     CTLFLAG_MPSAFE   This sysctl(9) handler is MP safe.  Do not grab Giant
                      around calls to this handler.  This should only be used
                      for SYSCTL_PROC() entries.

     Additionally, any of the following optional flags may also be specified:

     CTLFLAG_ANYBODY  Any user or process can write to this sysctl.

     CTLFLAG_CAPRD    A process in capability mode can read from this sysctl.

     CTLFLAG_CAPWR    A process in capability mode can write to this sysctl.

     CTLFLAG_SECURE   This sysctl can be written to only if the effective
                      securelevel of the process is ≤ 0.

     CTLFLAG_PRISON   This sysctl can be written to by processes in jail(2).

     CTLFLAG_SKIP     When iterating the sysctl name space, do not list this
                      sysctl.

     CTLFLAG_TUN      Advisory flag that a system tunable also exists for this
                      variable.  The initial sysctl value is tried fetched
                      once from the system environment early during module
                      load or system boot.

     CTLFLAG_DYN      Dynamically created OIDs automatically get this flag
                      set.

     CTLFLAG_VNET     OID references a VIMAGE-enabled variable.

EXAMPLES
     Sample use of SYSCTL_DECL() to declare the security sysctl tree for use
     by new nodes:

           SYSCTL_DECL(_security);

     Examples of integer, opaque, string, and procedure sysctls follow:

           /*
            * Example of a constant integer value.  Notice that the control
            * flags are CTLFLAG_RD, the variable pointer is SYSCTL_NULL_INT_PTR,
            * and the value is declared.
            */
           SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD, SYSCTL_NULL_INT_PTR,
               sizeof(struct bio), "sizeof(struct bio)");

           /*
            * Example of a variable integer value.  Notice that the control
            * flags are CTLFLAG_RW, the variable pointer is set, and the
            * value is 0.
            */
           static int      doingcache = 1;         /* 1 => enable the cache */
           SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
               "Enable name cache");

           /*
            * Example of a variable string value.  Notice that the control
            * flags are CTLFLAG_RW, that the variable pointer and string
            * size are set.  Unlike newer sysctls, this older sysctl uses a
            * static oid number.
            */
           char kernelname[MAXPATHLEN] = "/kernel";        /* XXX bloat */
           SYSCTL_STRING(_kern, KERN_BOOTFILE, bootfile, CTLFLAG_RW,
               kernelname, sizeof(kernelname), "Name of kernel file booted");

           /*
            * Example of an opaque data type exported by sysctl.  Notice that
            * the variable pointer and size are provided, as well as a format
            * string for sysctl(8).
            */
           static l_fp pps_freq;   /* scaled frequency offset (ns/s) */
           SYSCTL_OPAQUE(_kern_ntp_pll, OID_AUTO, pps_freq, CTLFLAG_RD,
               &pps_freq, sizeof(pps_freq), "I", "");

           /*
            * Example of a procedure based sysctl exporting string
            * information.  Notice that the data type is declared, the NULL
            * variable pointer and 0 size, the function pointer, and the
            * format string for sysctl(8).
            */
           SYSCTL_PROC(_kern_timecounter, OID_AUTO, hardware, CTLTYPE_STRING |
               CTLFLAG_RW, NULL, 0, sysctl_kern_timecounter_hardware, "A",
               "");

     The following is an example of how to create a new top-level category and
     how to hook up another subtree to an existing static node.  This example
     does not use contexts, which results in tedious management of all
     intermediate oids, as they need to be freed later on:

           #include <sys/sysctl.h>
            ...
           /*
            * Need to preserve pointers to newly created subtrees,
            * to be able to free them later:
            */
           static struct sysctl_oid *root1;
           static struct sysctl_oid *root2;
           static struct sysctl_oid *oidp;
           static int a_int;
           static char *string = "dynamic sysctl";
            ...

           root1 = SYSCTL_ADD_ROOT_NODE(NULL,
                   OID_AUTO, "newtree", CTLFLAG_RW, 0, "new top level tree");
           oidp = SYSCTL_ADD_INT(NULL, SYSCTL_CHILDREN(root1),
                   OID_AUTO, "newint", CTLFLAG_RW, &a_int, 0, "new int leaf");
            ...
           root2 = SYSCTL_ADD_NODE(NULL, SYSCTL_STATIC_CHILDREN(_debug),
                   OID_AUTO, "newtree", CTLFLAG_RW, 0, "new tree under debug");
           oidp = SYSCTL_ADD_STRING(NULL, SYSCTL_CHILDREN(root2),
                   OID_AUTO, "newstring", CTLFLAG_RD, string, 0, "new string leaf");

     This example creates the following subtrees:

           debug.newtree.newstring
           newtree.newint

     Care should be taken to free all OIDs once they are no longer needed!

SYSCTL NAMING
     When adding, modifying, or removing sysctl names, it is important to be
     aware that these interfaces may be used by users, libraries,
     applications, or documentation (such as published books), and are
     implicitly published application interfaces.  As with other application
     interfaces, caution must be taken not to break existing applications, and
     to think about future use of new name spaces so as to avoid the need to
     rename or remove interfaces that might be depended on in the future.

     The semantics chosen for a new sysctl should be as clear as possible, and
     the name of the sysctl must closely reflect its semantics.  Therefore the
     sysctl name deserves a fair amount of consideration.  It should be short
     but yet representative of the sysctl meaning.  If the name consists of
     several words, they should be separated by underscore characters, as in
     compute_summary_at_mount.  Underscore characters may be omitted only if
     the name consists of not more than two words, each being not longer than
     four characters, as in bootfile.  For boolean sysctls, negative logic
     should be totally avoided.  That is, do not use names like no_foobar or
     foobar_disable.  They are confusing and lead to configuration errors.
     Use positive logic instead: foobar, foobar_enable.

     A temporary sysctl node OID that should not be relied upon must be
     designated as such by a leading underscore character in its name.  For
     example: _dirty_hack.

SEE ALSO
     sysctl(3), sysctl(8), sysctl_add_oid(9), sysctl_ctx_free(9),
     sysctl_ctx_init(9), sysctl_remove_oid(9)

HISTORY
     The sysctl(8) utility first appeared in 4.4BSD.

AUTHORS
     The sysctl implementation originally found in BSD has been extensively
     rewritten by Poul-Henning Kamp in order to add support for name lookups,
     name space iteration, and dynamic addition of MIB nodes.

     This man page was written by Robert N. M. Watson.

SECURITY CONSIDERATIONS
     When creating new sysctls, careful attention should be paid to the
     security implications of the monitoring or management interface being
     created.  Most sysctls present in the kernel are read-only or writable
     only by the superuser.  Sysctls exporting extensive information on system
     data structures and operation, especially those implemented using
     procedures, will wish to implement access control to limit the undesired
     exposure of information about other processes, network connections, etc.

     The following top level sysctl name spaces are commonly used:

     compat      Compatibility layer information.

     debug       Debugging information.  Various name spaces exist under
                 debug.

     hw          Hardware and device driver information.

     kern        Kernel behavior tuning; generally deprecated in favor of more
                 specific name spaces.

     machdep     Machine-dependent configuration parameters.

     net         Network subsystem.  Various protocols have name spaces under
                 net.

     regression  Regression test configuration and information.

     security    Security and security-policy configuration and information.

     sysctl      Reserved name space for the implementation of sysctl.

     user        Configuration settings relating to user application behavior.
                 Generally, configuring applications using kernel sysctls is
                 discouraged.

     vfs         Virtual file system configuration and information.

     vm          Virtual memory subsystem configuration and information.

                                 June 1, 2018