Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

SYSCTL(3)                  Library Functions Manual                  SYSCTL(3)

     sysctl, sysctlbyname, sysctlgetmibinfo, sysctlnametomib, asysctl,
     asysctlbyname - get or set system information

     Standard C Library (libc, -lc)

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

     sysctl(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
         const void *newp, size_t newlen);

     sysctlbyname(const char *sname, void *oldp, size_t *oldlenp,
         const void *newp, size_t newlen);

     sysctlgetmibinfo(const char *sname, int *name, u_int *namelenp,
         char *cname, size_t *csz, struct sysctlnode **rnode, int v);

     sysctlnametomib(const char *sname, int *name, size_t *namelenp);

     void *
     asysctl(const int *name, size_t namelen, size_t *len);

     void *
     asysctlbyname(const char *sname, size_t *len);

     The sysctl function retrieves system information and allows processes
     with appropriate privileges to set system information.  The information
     available from sysctl consists of integers, strings, and tables.
     Information may be retrieved and set from the command interface using the
     sysctl(8) utility.

     Unless explicitly noted below, sysctl returns a consistent snapshot of
     the data requested.  Consistency is obtained by locking the destination
     buffer into memory so that the data may be copied out without blocking.
     Calls to sysctl are serialized to avoid deadlock.

     The state is described using a ``Management Information Base'' (MIB)
     style name, listed in name, which is a namelen length array of integers.

     The sysctlbyname() function accepts a string representation of a MIB
     entry and internally maps it to the appropriate numeric MIB
     representation.  Its semantics are otherwise no different from sysctl().

     The information is copied into the buffer specified by oldp.  The size of
     the buffer is given by the location specified by oldlenp before the call,
     and that location gives the amount of data copied after a successful
     call.  If the amount of data available is greater than the size of the
     buffer supplied, the call supplies as much data as fits in the buffer
     provided and returns with the error code ENOMEM.  If the old value is not
     desired, oldp and oldlenp should be set to NULL.

     The size of the available data can be determined by calling sysctl with a
     NULL parameter for oldp.  The size of the available data will be returned
     in the location pointed to by oldlenp.  For some operations, the amount
     of space may change often.  For these operations, the system attempts to
     round up so that the returned size is large enough for a call to return
     the data shortly thereafter.

     To set a new value, newp is set to point to a buffer of length newlen
     from which the requested value is to be taken.  If a new value is not to
     be set, newp should be set to NULL and newlen set to 0.

     The sysctlnametomib() function can be used to map the string
     representation of a MIB entry to the numeric version.  The name argument
     should point to an array of integers large enough to hold the MIB, and
     namelenp should indicate the number of integer slots available.
     Following a successful translation, the size_t indicated by namelenp will
     be changed to show the number of slots consumed.

     The sysctlgetmibinfo() function performs name translation similar to
     sysctlnametomib(), but also canonicalizes the name (or returns the first
     erroneous token from the string being parsed) into the space indicated by
     cname and csz.  csz should indicate the size of the buffer pointed to by
     cname and on return, will indicate the size of the returned string
     including the trailing `nul' character.

     The rnode and v arguments to sysctlgetmibinfo() are used to provide a
     tree for it to parse into, and to get back either a pointer to, or a copy
     of, the terminal node.  If rnode is NULL, sysctlgetmibinfo() uses its own
     internal tree for parsing, and checks it against the kernel at each call,
     to make sure that the name-to-number mapping is kept up to date.  The v
     argument is ignored in this case.  If rnode is not NULL but the pointer
     it references is, on a successful return, rnode will be adjusted to point
     to a copy of the terminal node.  The v argument indicates which version
     of the sysctl node structure the caller wants.  The application must
     later free() this copy.  If neither rnode nor the pointer it references
     are NULL, the pointer is used as the address of a tree over which the
     parsing is done.  In this last case, the tree is not checked against the
     kernel, no refreshing of the mappings is performed, and the value given
     by v must agree with the version indicated by the tree.  It is
     recommended that applications always use SYSCTL_VERSION as the value for
     v, as defined in the include file sys/sysctl.h.

     The numeric and text names of sysctl variables are described in
     sysctl(7).  The numeric names are defined as preprocessor macros.  The
     top level names are defined with a CTL_ prefix in <sys/sysctl.h>.  The
     next and subsequent levels down have different prefixes for each subtree.

     For example, the following retrieves the maximum number of processes
     allowed in the system - the kern.maxproc variable:
           int mib[2], maxproc;
           size_t len;

           mib[0] = CTL_KERN;
           mib[1] = KERN_MAXPROC;
           len = sizeof(maxproc);
           sysctl(mib, 2, &maxproc, &len, NULL, 0);

     To retrieve the standard search path for the system utilities -
           int mib[2];
           size_t len;
           char *p;

           mib[0] = CTL_USER;
           mib[1] = USER_CS_PATH;
           sysctl(mib, 2, NULL, &len, NULL, 0);
           p = malloc(len);
           sysctl(mib, 2, p, &len, NULL, 0);

     The asysctl() and asysctlbyname() functions are wrappers for sysctl() and
     sysctlbyname().  They return memory allocated with malloc(3) and resize
     the buffer in a loop until all data fits.

     Several meta-identifiers are provided to perform operations on the sysctl
     tree itself, or support alternate means of accessing the data
     instrumented by the sysctl tree.

     Name                Description
     CTL_QUERY           Retrieve a mapping of names to numbers below a given
     CTL_CREATE          Create a new node
     CTL_CREATESYM       Create a new node by its kernel symbol
     CTL_DESTROY         Destroy a node
     CTL_DESCRIBE        Retrieve node descriptions

     The core interface to all of these meta-functions is the structure that
     the kernel uses to describe the tree internally, as defined in
     <sys/sysctl.h> as:

     struct sysctlnode {
             uint32_t sysctl_flags;          /* flags and type */
             int32_t sysctl_num;             /* mib number */
             char sysctl_name[SYSCTL_NAMELEN]; /* node name */
             uint32_t sysctl_ver;        /* node's version vs. rest of tree */
             uint32_t __rsvd;
             union {
                     struct {
                             uint32_t suc_csize; /* size of child node array */
                             uint32_t suc_clen; /* number of valid children */
                             struct sysctlnode* suc_child; /* array of child nodes */
                     } scu_child;
                     struct {
                             void *sud_data; /* pointer to external data */
                             size_t sud_offset; /* offset to data */
                     } scu_data;
                     int32_t scu_alias;      /* node this node refers to */
                     int32_t scu_idata;      /* immediate "int" data */
                     u_quad_t scu_qdata;     /* immediate "u_quad_t" data */
             } sysctl_un;
             size_t _sysctl_size;            /* size of instrumented data */
             sysctlfn _sysctl_func;          /* access helper function */
             struct sysctlnode *sysctl_parent; /* parent of this node */
             const char *sysctl_desc;        /* description of node */

     #define sysctl_csize    sysctl_un.scu_child.suc_csize
     #define sysctl_clen     sysctl_un.scu_child.suc_clen
     #define sysctl_child    sysctl_un.scu_child.suc_child
     #define sysctl_data     sysctl_un.scu_data.sud_data
     #define sysctl_offset   sysctl_un.scu_data.sud_offset
     #define sysctl_alias    sysctl_un.scu_alias
     #define sysctl_idata    sysctl_un.scu_idata
     #define sysctl_qdata    sysctl_un.scu_qdata

     Querying the tree to discover the name to number mapping permits dynamic
     discovery of all the data that the tree currently has instrumented.  For
     example, to discover all the nodes below the CTL_VFS node:

           struct sysctlnode query, vfs[128];
           int mib[2];
           size_t len;

           mib[0] = CTL_VFS;
           mib[1] = CTL_QUERY;
           memset(&query, 0, sizeof(query));
           query.sysctl_flags = SYSCTL_VERSION;
           len = sizeof(vfs);
           sysctl(mib, 2, &vfs[0], &len, &query, sizeof(query));

     Note that a reference to an empty node with sysctl_flags set to
     SYSCTL_VERSION is passed to sysctl in order to indicate the version that
     the program is using.  All dynamic operations passing nodes into sysctl
     require that the version be explicitly specified.

     Creation and destruction of nodes works by constructing part of a new
     node description (or a description of the existing node) and invoking
     CTL_CREATE (or CTL_CREATESYM) or CTL_DESTROY at the parent of the new
     node, with a pointer to the new node passed via the new and newlen
     arguments.  If valid values for old and oldlenp are passed, a copy of the
     new node once in the tree will be returned.  If the create operation
     fails because a node with the same name or MIB number exists, a copy of
     the conflicting node will be returned.

     The minimum requirements for creating a node are setting the sysctl_flags
     to indicate the new node's type, sysctl_num to either the new node's
     number (or CTL_CREATE or CTL_CREATESYM if a dynamically allocated MIB
     number is acceptable), sysctl_size to the size of the data to be
     instrumented (which must agree with the given type), and sysctl_name must
     be set to the new node's name.  Nodes that are not of type "node" must
     also have some description of the data to be instrumented, which will
     vary depending on what is to be instrumented.

     If existing kernel data is to be covered by this new node, its address
     should be given in sysctl_data or, if CTL_CREATESYM is used, sysctl_data
     should be set to a string containing its name from the kernel's symbol
     table.  If new data is to be instrumented and an initial value is
     available, the new integer or quad type data should be placed into either
     sysctl_idata or sysctl_qdata, respectively, along with the
     CTLFLAG_IMMEDIATE flag being set, or sysctl_data should be set to point
     to a copy of the new data, and the CTLFLAG_OWNDATA flag must be set.
     This latter method is the only way that new string and struct type nodes
     can be initialized.  Invalid kernel addresses are accepted, but any
     attempt to access those nodes will return an error.

     The sysctl_csize, sysctl_clen, sysctl_child, sysctl_parent, and
     sysctl_alias members are used by the kernel to link the tree together and
     must be NULL or 0.  Nodes created in this manner cannot have helper
     functions, so sysctl_func must also be NULL.  If the sysctl_ver member is
     non-zero, it must match either the version of the parent or the version
     at the root of the MIB or an error is returned.  This can be used to
     ensure that nodes are only added or removed from a known state of the
     tree.  Note: It may not be possible to determine the version at the root
     of the tree.

     This example creates a new subtree and adds a node to it that controls
     the audiodebug kernel variable, thereby making it tunable at at any time,
     without needing to use ddb(4) or kvm(3) to alter the kernel's memory

           struct sysctlnode node;
           int mib[2];
           size_t len;

           mib[0] = CTL_CREATE;            /* create at top-level */
           len = sizeof(node);
           memset(&node, 0, len);
           snprintf(node.sysctl_name, sizeof(node.sysctl_name), "local");
           node.sysctl_num = CTL_CREATE;   /* request dynamic MIB number */
           sysctl(&mib[0], 1, &node, &len, &node, len);

           mib[0] = node.sysctl_num;       /* use new MIB number */
           mib[1] = CTL_CREATESYM;         /* create at second level */
           len = sizeof(node);
           memset(&node, 0, len);
           snprintf(node.sysctl_name, sizeof(node.sysctl_name), "audiodebug");
           node.sysctl_num = CTL_CREATE;
           node.sysctl_data = "audiodebug"; /* kernel symbol to be used */
           sysctl(&mib[0], 2, NULL, NULL, &node, len);

     The process for deleting nodes is similar, but less data needs to be
     supplied.  Only the sysctl_num field needs to be filled in; almost all
     other fields must be left blank.  The sysctl_name and/or sysctl_ver
     fields can be filled in with the name and version of the existing node as
     additional checks on what will be deleted.  If all the given data fail to
     match any node, nothing will be deleted.  If valid values for old and
     oldlenp are supplied and a node is deleted, a copy of what was in the MIB
     tree will be returned.

     This sample code shows the deletion of the two nodes created in the above

           int mib[2];

           len = sizeof(node);
           memset(&node, 0, len);
           node.sysctl_flags = SYSCTL_VERSION;

           mib[0] = 3214;                  /* assumed number for "local" */
           mib[1] = CTL_DESTROY;
           node.sysctl_num = 3215;         /* assumed number for "audiodebug" */
           sysctl(&mib[0], 2, NULL, NULL, &node, len);

           mib[0] = CTL_DESTROY;
           node.sysctl_num = 3214;         /* now deleting "local" */
           sysctl(&mib[0], 1, NULL, NULL, &node, len);

     Descriptions of each of the nodes can also be retrieved, if they are
     available.  Descriptions can be retrieved in bulk at each level or on a
     per-node basis.  The layout of the buffer into which the descriptions are
     returned is a series of variable length structures, each of which
     describes its own size.  The length indicated includes the terminating
     `nul' character.  Nodes that have no description or where the description
     is not available are indicated by an empty string.  The descr_ver will
     match the sysctl_ver value for a given node, so that descriptions for
     nodes whose number have been recycled can be detected and ignored or

     struct sysctldesc {
             int32_t         descr_num;      /* mib number of node */
             uint32_t        descr_ver;      /* version of node */
             uint32_t        descr_len;      /* length of description string */
             char            descr_str[1];   /* not really 1...see above */

     The NEXT_DESCR() macro can be used to skip to the next description in the
     retrieved list.

           struct sysctlnode desc;
           struct sysctldesc *d;
           char buf[1024];
           int mib[2];
           size_t len;

           /* retrieve kern-level descriptions */
           mib[0] = CTL_KERN;
           mib[1] = CTL_DESCRIBE;
           d = (struct sysctldesc *)&buf[0];
           len = sizeof(buf);
           sysctl(mib, 2, d, &len, NULL, 0);
           while ((caddr_t)d < (caddr_t)&buf[len]) {
                   printf("node %d: %.*s\n", d->descr_num, d->descr_len,
                   d = NEXT_DESCR(d);

           /* retrieve description for kern.securelevel */
           memset(&desc, 0, sizeof(desc));
           desc.sysctl_flags = SYSCTL_VERSION;
           desc.sysctl_num = KERN_SECURELEVEL;
           d = (struct sysctldesc *)&buf[0];
           len = sizeof(buf);
           sysctl(mib, 2, d, &len, &desc, sizeof(desc));
           printf("kern.securelevel: %.*s\n", d->descr_len, d->descr_str);

     Descriptions can also be set as follows, subject to the following rules:

        The kernel securelevel is at zero or lower
        The caller has super-user privileges
        The node does not currently have a description
        The node is not marked as "permanent"

           struct sysctlnode desc;
           int mib[2];

           /* presuming the given top-level node was just added... */
           mib[0] = 3214; /* mib numbers taken from previous examples */
           mib[1] = CTL_DESCRIBE;
           memset(&desc, 0, sizeof(desc));
           desc.sysctl_flags = SYSCTL_VERSION;
           desc.sysctl_num = 3215;
           desc.sysctl_desc = "audio debug control knob";
           sysctl(mib, 2, NULL, NULL, &desc, sizeof(desc));

     Upon successfully setting a description, the new description will be
     returned in the space indicated by the oldp and oldlenp arguments.

     The sysctl_flags field in the struct sysctlnode contains the sysctl
     version, node type information, and a number of flags.  The macros
     SYSCTL_VERS(), SYSCTL_TYPE(), and SYSCTL_FLAGS() can be used to access
     the different fields.  Valid flags are:

     Name                    Description
     CTLFLAG_READONLY        Node is read-only
     CTLFLAG_READWRITE       Node is writable by the superuser
     CTLFLAG_ANYWRITE        Node is writable by anyone
     CTLFLAG_PRIVATE         Node is readable only by the superuser
     CTLFLAG_PERMANENT       Node cannot be removed (cannot be set by
     CTLFLAG_OWNDATA         Node owns data and does not instrument existing
     CTLFLAG_IMMEDIATE       Node contains instrumented data and does not
                             instrument existing data
     CTLFLAG_HEX             Node's contents should be displayed in a
                             hexadecimal form
     CTLFLAG_ROOT            Node is the root of a tree (cannot be set at any
     CTLFLAG_ANYNUMBER       Node matches any MIB number (cannot be set by
     CTLFLAG_HIDDEN          Node not displayed by default
     CTLFLAG_ALIAS           Node refers to a sibling node (cannot be set by
     CTLFLAG_OWNDESC         Node owns its own description string space

     If the call to sysctl is successful, 0 is returned.  Otherwise -1 is
     returned and errno is set appropriately.

     <sys/sysctl.h>         definitions for top level identifiers, second
                            level kernel and hardware identifiers, and user
                            level identifiers
     <sys/socket.h>         definitions for second level network identifiers
     <sys/gmon.h>           definitions for third level profiling identifiers
     <uvm/uvm_param.h>      definitions for second level virtual memory
     <netinet/in.h>         definitions for third level IPv4/v6 identifiers
                            and fourth level IPv4/v6 identifiers
     <netinet/icmp_var.h>   definitions for fourth level ICMP identifiers
     <netinet/icmp6.h>      definitions for fourth level ICMPv6 identifiers
     <netinet/tcp_var.h>    definitions for fourth level TCP identifiers
     <netinet/udp_var.h>    definitions for fourth level UDP identifiers
     <netinet6/udp6_var.h>  definitions for fourth level IPv6 UDP identifiers
     <netipsec/ipsec.h>     definitions for fourth level IPsec identifiers
     <netipsec/key_var.h>   definitions for third level PF_KEY identifiers
     <machine/cpu.h>        definitions for second level machdep identifiers

     The following errors may be reported:

     [EFAULT]           The buffer name, oldp, newp, or length pointer oldlenp
                        contains an invalid address, or the requested value is
                        temporarily unavailable.

     [EINVAL]           The name array is zero or greater than CTL_MAXNAME; or
                        a non-null newp is given and its specified length in
                        newlen is too large or too small, or the given value
                        is not acceptable for the given node.

     [EISDIR]           The name array specifies an intermediate rather than
                        terminal name.

     [ENOENT]           The name array specifies a node that does not exist in
                        the tree; or an attempt was made to destroy a node
                        that does not exist, or to create or destroy a node
                        below a node that does not exist.

     [ENOMEM]           The length pointed to by oldlenp is too short to hold
                        the requested value.

     [ENOTDIR]          The name array specifies a node below a node that
                        addresses data.

     [ENOTEMPTY]        An attempt was made to destroy a node that still has

     [EOPNOTSUPP]       The name array specifies a value that is unknown or a
                        meta-operation was attempted that the requested node
                        does not support.

     [EPERM]            An attempt is made to set a read-only value; or a
                        process without appropriate privilege attempts to set
                        a value or to create or destroy a node; or an attempt
                        to change a value protected by the current kernel
                        security level is made.

     sysctl(7), sysctl(8), secmodel_securelevel(9)

     The sysctl function first appeared in 4.4BSD.

NetBSD 10.99                  September 14, 2019                  NetBSD 10.99