SYSCTL(3)

NAME
sysctl - get or set system information

LIBRARY
libc

SYNOPSIS
#include <sys/param.h>
#include <sys/sysctl.h>
int
sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp, void *newp, size_t newlen);

DESCRIPTION
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 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 top level names are defined with a CTL_ prefix in <sys/sysctl.h>, and are as follows. The next and subsequent levels down are found in the include files listed here, and described in separate sections below.

Name Next level names Description
CTL_DEBUG sys/sysctl.h Debugging
CTL_VFS sys/mount.h Filesystem
CTL_HW sys/sysctl.h Generic CPU, I/O
CTL_KERN sys/sysctl.h High kernel limits
CTL_MACHDEP sys/sysctl.h Machine dependent
CTL_NET sys/socket.h Networking
CTL_USER sys/sysctl.h User-level
CTL_VM vm/vm_param.h Virtual memory

For example, the following retrieves the maximum number of processes allowed in the system:
    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);

CTL_DEBUG
The debugging variables vary from system to system. A debugging variable may be added or deleted without need to recompile sysctl to know about it. Each time it runs, sysctl gets the list of debugging variables from the kernel and displays their current values. The system defines twenty (struct ctldebug) variables named debug0 through debug19. They are declared as separate variables so that they can be individually initialized at the location of their associated variable. The loader prevents multiple use of the same variable by issuing errors if a variable is initialized in more than one place. For example, to export the variable dospecialcheck as a debugging variable, the following declaration would be used:
    int dospecialcheck = 1;
    struct ctldebug debug5 = { "dospecialcheck", &dospecialcheck };

CTL_VFS
A distinguished second level name, VFS_GENERIC, is used to get general information about all filesystems. One of its third level identifiers is VFS_MAXTYPENUM that gives the highest valid filesystem type number. Its other third level identifier is VFS_CONF that returns configuration information about the filesystem type given as a fourth level identifier (see getvfsbyname(3) as an example of its use). The remaining second level identifiers are the filesystem type number returned by a statfs(2) call or from VFS_CONF. The third level identifiers available for each filesystem are given in the header file that defines the mount argument structure for that filesystem.

CTL_HW
The string and integer information available for the CTL_HW level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.
Second level name Type Changeable
HW_MACHINE string no
HW_MODEL string no
HW_NCPU integer no
HW_BYTEORDER integer no
HW_PHYSMEM integer no
HW_USERMEM integer no
HW_PAGESIZE integer no
HW_MACHINE_ARCH string no

HW_MACHINE
    The machine class.

HW_MODEL
    The machine model

HW_NCPU
    The number of cpus.

HW_BYTEORDER
    The byteorder (4,321, or 1,234).

HW_PHYSMEM
    The bytes of physical memory.

HW_USERMEM
    The bytes of non-kernel memory.

HW_PAGESIZE
    The software page size.

HW_MACHINE_ARCH
    The machine cpu class.

CTL_KERN
The string and integer information available for the CTL_KERN level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value. The types of data currently available are process information, system vnodes, the open file entries, routing table entries, virtual memory statistics, load average history, and clock rate information.
Second level name Type Changeable
KERN_ARGMAX integer no
KERN_AUTONICETIME integer yes
KERN_AUTONICEVAL integer yes
KERN_BOOTTIME struct timeval no
KERN_CHOWN_RESTRICTED integer no
KERN_CLOCKRATE struct clockinfo no
KERN_DOMAINNAME string yes
KERN_FILE struct file no
KERN_FSYNC integer no
KERN_HOSTID integer yes
KERN_HOSTNAME string yes
KERN_IOV_MAX integer no
KERN_JOB_CONTROL integer no
KERN_LINK_MAX integer no
KERN_MAXFILES integer yes
KERN_MAXPARTITIONS integer no
KERN_MAXPROC integer yes
KERN_MAXVNODES integer yes
KERN_MAX_CANON integer no
KERN_MAX_INPUT integer no
KERN_MSGBUFSIZE integer no
KERN_NAME_MAX integer no
KERN_NGROUPS integer no
KERN_NO_TRUNC integer no
KERN_OSRELEASE string no
KERN_OSREV integer no
KERN_OSTYPE string no
KERN_PATH_MAX integer no
KERN_PIPE_BUF integer no
KERN_POSIX1 integer no
KERN_PROC struct proc no
KERN_PROF node not applicable
KERN_RAWPARTITION integer no
KERN_SAVED_IDS integer no
KERN_SECURELVL integer raise only
KERN_SHORTCORENAME integer yes
KERN_SYNCHRONIZED_IO integer no
KERN_SYSVMSG integer no
KERN_SYSVSEM integer no
KERN_SYSVSHM integer no
KERN_VDISABLE integer no
KERN_VERSION string no
KERN_VNODE struct vnode no

KERN_ARGMAX
    The maximum bytes of argument to execve(2).

KERN_AUTONICETIME
    The number of seconds of cpu-time a non-root process may accumulate before having its priority lowered from the default to the value of KERN_AUTONICEVAL. If set to 0, automatic lowering of priority is not performed, and if set to -1 all non-root processes are immediately lowered.

KERN_AUTONICEVAL
    The priority assigned for automatically niced processes.

KERN_BOOTTIME
    A struct timeval structure is returned. This structure contains the time that the system was booted.

KERN_CHOWN_RESTRICTED
    Return 1 if appropriate privileges are required for the chown(2) system call, otherwise 0.

KERN_CLOCKRATE
    A struct clockinfo structure is returned. This structure contains the clock, statistics clock and profiling clock frequencies, the number of micro-seconds per hz tick, and the clock skew rate.

KERN_DOMAINNAME
    Get or set the YP domain name.

KERN_FILE
    Return the entire file table. The returned data consists of a single struct filehead followed by an array of struct file, whose size depends on the current number of such objects in the system.

KERN_FSYNC
    Return 1 if the POSIX 1003.1b File Synchronization Option is available on this system, otherwise 0.

KERN_HOSTID
    Get or set the host id.

KERN_HOSTNAME
    Get or set the hostname.

KERN_IOV_MAX
    Return the maximum number of iovec structures that a process has available for use with preadv(2), pwritev(2), readv(2), recvmsg(2), sendmsg(2) and writev(2).

KERN_JOB_CONTROL
    Return 1 if job control is available on this system, otherwise 0.

KERN_LINK_MAX
    The maximum file link count.

KERN_MAXFILES
    The maximum number of open files that may be open in the system.

KERN_MAXPARTITIONS
    The maximum number of partitions allowed per disk.

KERN_MAXPROC
    The maximum number of simultaneous processes the system will allow.

KERN_MAXVNODES
    The maximum number of vnodes available on the system.

KERN_MAX_CANON
    The maximum number of bytes in terminal canonical input line.

KERN_MAX_INPUT
    The minimum maximum number of bytes for which space is available in a terminal input queue.

KERN_MSGBUFSIZE
    The maximum number of characters that the kernel message buffer can hold.

KERN_NAME_MAX
    The maximum number of bytes in a file name.

KERN_NGROUPS
    The maximum number of supplemental groups.

KERN_NO_TRUNC
    Return 1 if file names longer than KERN_NAME_MAX are truncated.

KERN_OSRELEASE
    The system release string.

KERN_OSREV
    The system revision string.

KERN_OSTYPE
    The system type string.

KERN_PATH_MAX
    The maximum number of bytes in a pathname.

KERN_PIPE_BUF
    The maximum number of bytes which will be written atomically to a pipe.

KERN_POSIX1
    The version of ISO/IEC 9945 (POSIX 1003.1) with which the system attempts to comply.

KERN_PROC
    Return the entire process table, or a subset of it. An array of struct kinfo_proc structures is returned, whose size depends on the current number of such objects in the system. The third and fourth level names are as follows:
    Third level name Fourth level is:
    KERN_PROC_ALL None
    KERN_PROC_PID A process ID
    KERN_PROC_PGRP A process group
    KERN_PROC_TTY A tty device
    KERN_PROC_UID A user ID
    KERN_PROC_RUID A real user ID

KERN_PROF
    Return profiling information about the kernel. If the kernel is not compiled for profiling, attempts to retrieve any of the KERN_PROF values will fail with EOPNOTSUPP. The third level names for the string and integer profiling information is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.
    Third level name Type Changeable
    GPROF_STATE integer yes
    GPROF_COUNT u_short[] yes
    GPROF_FROMS u_short[] yes
    GPROF_TOS struct tostruct yes
    GPROF_GMONPARAM struct gmonparam no

    The variables are as follows:

    GPROF_STATE
        Returns GMON_PROF_ON or GMON_PROF_OFF to show that profiling is running or stopped.

    GPROF_COUNT
        Array of statistical program counter counts.

    GPROF_FROMS
        Array indexed by program counter of call-from points.

    GPROF_TOS
        Array of struct tostruct describing destination of calls and their counts.

    GPROF_GMONPARAM
        Structure giving the sizes of the above arrays.

KERN_RAWPARTITION
    The raw partition of a disk (a == 0).

KERN_SAVED_IDS
    Returns 1 if saved set-group and saved set-user ID is available.

KERN_SECURELVL
    The system security level. This level may be raised by processes with appropriate privilege. It may only be lowered by process 1.

KERN_SHORTCORENAME
    Whether core dumps are named programname.core (default, value 0) or core (value 1). The default value can be changed to 1 with the kernel configuration option options SHORTCORENAME (see options(4), core(5) ).

KERN_SYNCHRONIZED_IO
    Returns 1 if the POSIX 1003.1b Synchronized I/O Option is available on this system, otherwise 0.

KERN_SYSVMSG
    Returns 1 if System V style message queue functionality is available on this system, otherwise 0.

KERN_SYSVSEM
    Returns 1 if System V style semaphore functionality is available on this system, otherwise 0.

KERN_SYSVSHM
    Returns 1 if System V style share memory functionality is available on this system, otherwise 0.

KERN_VDISABLE
    Returns the terminal character disabling value.

KERN_VERSION
    The system version string.

KERN_VNODE
    Return the entire vnode table. Note, the vnode table is not necessarily a consistent snapshot of the system. The returned data consists of an array whose size depends on the current number of such objects in the system. Each element of the array contains the kernel address of a vnode struct vnode * followed by the vnode itself struct vnode.

CTL_MACHDEP
The set of variables defined is architecture dependent. Most architectures define at least the following variables.
Second level name Type Changeable
CPU_CONSDEV dev_t no

CTL_NET
The string and integer information available for the CTL_NET level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.
Second level name Type Changeable
PF_ROUTE routing messages no
PF_INET internet values yes

PF_ROUTE
    Return the entire routing table or a subset of it. The data is returned as a sequence of routing messages (see route(4) for the header file, format and meaning). The length of each message is contained in the message header.

    The third level name is a protocol number, which is currently always 0. The fourth level name is an address family, which may be set to 0 to select all address families. The fifth and sixth level names are as follows:
    Fifth level name Sixth level is:
    NET_RT_FLAGS rtflags
    NET_RT_DUMP None
    NET_RT_IFLIST None

PF_INET
    Get or set various global information about the internet protocols. The third level name is the protocol. The fourth level name is the variable name. The currently defined protocols and names are:
    Protocol name Variable name Type Changeable
    ip forwarding integer yes
    ip redirect integer yes
    ip ttl integer yes
    ip forwsrcrt integer yes
    ip directed-broadcast integer yes
    ip allowsrcrt integer yes
    ip subnetsarelocal integer yes
    ip mtudisc integer yes
    ip anonportmin integer yes
    ip anonportmax integer yes
    ip mtudisctimeout integer yes
    icmp maskrepl integer yes
    tcp rfc1323 integer yes
    tcp sendspace integer yes
    tcp recvspace integer yes
    tcp mssdflt integer yes
    tcp syn_cache_limit integer yes
    tcp syn_bucket_limit integer yes
    tcp syn_cache_interval integer yes
    tcp init_win integer yes
    tcp mss_ifmtu integer yes
    tcp sack integer yes
    tcp win_scale integer yes
    tcp timestamps integer yes
    tcp compat_42 integer yes
    tcp cwm integer yes
    tcp cwm_burstsize integer yes
    tcp ack_on_push integer yes
    tcp keepidle integer yes
    tcp keepintvl integer yes
    tcp keepcnt integer yes
    tcp slowhz integer no
    tcp newreno integer yes
    udp checksum integer yes
    udp sendspace integer yes
    udp recvspace integer yes

    The variables are as follows:

    ip.forwarding
        Returns 1 when IP forwarding is enabled for the host, meaning that the host is acting as a router.

    ip.redirect
        Returns 1 when ICMP redirects may be sent by the host. This option is ignored unless the host is routing IP packets, and should normally be enabled on all systems.

    ip.ttl
        The maximum time-to-live (hop count) value for an IP packet sourced by the system. This value applies to normal transport protocols, not to ICMP.

    ip.forwsrcrt
        Returns 1 when forwarding of source-routed packets is enabled for the host. This value may only be changed if the kernel security level is less than 1.

    ip.directed-broadcast
        Returns 1 if directed broadcast behavior is enabled for the host.

    ip.allowsrcrt
        Returns 1 if the host accepts source routed packets.

    ip.subnetsarelocal
        Returns 1 if subnets are to be considered local addresses.

    ip.mtudisc
        Returns 1 if Path MTU Discovery is enabled.

    ip.anonportmin
        The lowest port number to use for TCP and UDP ephemeral port allocation. This cannot be set to less that 1024 or greater than 65535.

    ip.anonportmax
        The highest port number to use for TCP and UDP ephemeral port allocation. This cannot be set to less that 1024 or greater than 65535, and must be greater than ip.anonportmin.

    ip.mtudisctimeout
        Returns the number of seconds in which a route added by the Path MTU Discovery engine will time out. When the route times out, the Path MTU Discovery engine will attempt to probe a larger path MTU.

    icmp.maskrepl
        Returns 1 if ICMP network mask requests are to be answered.

    tcp.rfc1323
        Returns 1 if RFC1323 extensions to TCP are enabled.

    tcp.sendspace
        Returns the default TCP send buffer size.

    tcp.recvspace
        Returns the default TCP receive buffer size.

    tcp.mssdflt
        Returns the default maximum segment size both advertsized to the peer and to use when the peer does not advertize a maximum segment size to us during connection setup. Do not change this value unless you really know what you are doing.

    tcp.syn_cache_limit
        Returns the maximum number of entries allowed in the TCP compressed state engine.

    tcp.syn_bucket_limit
        Returns the maximum number of entries allowed per hash bucket in the TCP compressed state engine.

    tcp.syn_cache_interval
        Returns the TCP compressed state engine's timer interval.

    tcp.init_win
        Returns a value indicating the TCP initial congestion window. If this value is 0, an auto-tuning algorithm designed to use an initial window of approximately 4K bytes is in use. Otherwise, this value indicates a fixed number of packets.

    tcp.mss_ifmtu
        Returns 1 if TCP calculates the outgoing maximum segment size based on the MTU of the appropriate interface. Otherwise, it is calculated based on the greater of the MTU of the interface, and the largest (non-loopback) interface MTU on the system.

    tcp.sack
        Returns a value which determines the level of Selective Acknowledgement supported by TCP. If 2, we will transmit and receive SACK options. If 1, we will transmit SACK options, but ignore any SACK options received. If 0, SACK is disabled.

    tcp.win_scale
        If rfc1323 is enabled, a value of 1 indicates RFC1323 window scale options, for increasing the TCP window size, are enabled.

    tcp.timestamps
        If rfc1323 is enabled, a value of 1 indicates RFC1323 time stamp options, used for measuring TCP round trip times, are enabled.

    tcp.compat_42
        Returns 1 if work-arounds for bugs in the 4.2BSD TCP implementation are enabled. Use of this option is not recommended, although it may be required in order to communicate with extremely old TCP implementations.

    tcp.cwm
        Returns 1 if use of the Hughes/Touch/Heidemann Congestion Window Monitoring algorithm is enabled. This algorithm prevents line-rate bursts of packets that could otherwise occur when data begins flowing on an idle TCP connection. These line-rate bursts can contribute to network and router congestion. This can be particularly useful on World Wide Web servers which support HTTP/1.1, which has lingering connections.

    tcp.cwm_burstsize
        Returns the Congestion Window Monitoring allowed burst size, in terms of packet count.

    tcp.ack_on_push
        Returns 1 if TCP is to immediately transmit an ACK upon reception of a packet with PUSH set. This can avoid losing a round trip time in some rare situations, but has the caveat of potentially defeating TCP's delayed ACK algorithm. Use of this option is generally not recommended, but the variable exists in case your configuration really needs it.

    tcp.keepidle
        Time a connection must be idle before keepalives are sent (if keepalives are enabled for the connection). See also tcp.slowhz.

    tcp.keepintvl
        Time after a keepalive probe is sent until, in the absence of any response, another probe is sent. See also tcp.slowhz.

    tcp.keepcnt
        Number of keepalive probes sent before declaring a connection dead. If set to zero, there is no limit; keepalives will be sent until some kind of response is received from the peer.

    tcp.slowhz
        The units for tcp.keepidle and tcp.keepintvl; those variables are in ticks of a clock that ticks tcp.slowhz times per second. (That is, their values must be divided by the tcp.slowhz value to get times in seconds.)

    tcp.newreno
        Returns 1 if the use of J. Hoe's NewReno congestion control algorithm is enabled. This algorithm improves the start-up behavior of TCP connections.

    udp.checksum
        Returns 1 when UDP checksums are being computed and checked. Disabling UDP checksums is strongly discouraged.

    udp.sendspace
        Returns the default UDP send buffer size.

    udp.recvspace
        Returns the default UDP receive buffer size.

CTL_USER
The string and integer information available for the CTL_USER level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.
Second level name Type Changeable
USER_BC_BASE_MAX integer no
USER_BC_DIM_MAX integer no
USER_BC_SCALE_MAX integer no
USER_BC_STRING_MAX integer no
USER_COLL_WEIGHTS_MAX integer no
USER_CS_PATH string no
USER_EXPR_NEST_MAX integer no
USER_LINE_MAX integer no
USER_POSIX2_CHAR_TERM integer no
USER_POSIX2_C_BIND integer no
USER_POSIX2_C_DEV integer no
USER_POSIX2_FORT_DEV integer no
USER_POSIX2_FORT_RUN integer no
USER_POSIX2_LOCALEDEF integer no
USER_POSIX2_SW_DEV integer no
USER_POSIX2_UPE integer no
USER_POSIX2_VERSION integer no
USER_RE_DUP_MAX integer no
USER_STREAM_MAX integer no
USER_TZNAME_MAX integer no

USER_BC_BASE_MAX
    The maximum ibase/obase values in the bc(1) utility.

USER_BC_DIM_MAX
    The maximum array size in the bc(1) utility.

USER_BC_SCALE_MAX
    The maximum scale value in the bc(1) utility.

USER_BC_STRING_MAX
    The maximum string length in the bc(1) utility.

USER_COLL_WEIGHTS_MAX
    The maximum number of weights that can be assigned to any entry of the LC_COLLATE order keyword in the locale definition file.

USER_CS_PATH
    Return a value for the PATH environment variable that finds all the standard utilities.

USER_EXPR_NEST_MAX
    The maximum number of expressions that can be nested within parenthesis by the expr(1) utility.

USER_LINE_MAX
    The maximum length in bytes of a text-processing utility's input line.

USER_POSIX2_CHAR_TERM
    Return 1 if the system supports at least one terminal type capable of all operations described in POSIX 1003.2, otherwise 0.

USER_POSIX2_C_BIND
    Return 1 if the system's C-language development facilities support the C-Language Bindings Option, otherwise 0.

USER_POSIX2_C_DEV
    Return 1 if the system supports the C-Language Development Utilities Option, otherwise 0.

USER_POSIX2_FORT_DEV
    Return 1 if the system supports the FORTRAN Development Utilities Option, otherwise 0.

USER_POSIX2_FORT_RUN
    Return 1 if the system supports the FORTRAN Runtime Utilities Option, otherwise 0.

USER_POSIX2_LOCALEDEF
    Return 1 if the system supports the creation of locales, otherwise 0.

USER_POSIX2_SW_DEV
    Return 1 if the system supports the Software Development Utilities Option, otherwise 0.

USER_POSIX2_UPE
    Return 1 if the system supports the User Portability Utilities Option, otherwise 0.

USER_POSIX2_VERSION
    The version of POSIX 1003.2 with which the system attempts to comply.

USER_RE_DUP_MAX
    The maximum number of repeated occurrences of a regular expression permitted when using interval notation.

USER_STREAM_MAX
    The minimum maximum number of streams that a process may have open at any one time.

USER_TZNAME_MAX
    The minimum maximum number of types supported for the name of a timezone.

CTL_VM
The string and integer information available for the CTL_VM level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.
Second level name Type Changeable
VM_LOADAVG struct loadavg no
VM_METER struct vmtotal no

VM_LOADAVG
    Return the load average history. The returned data consists of a struct loadavg.

VM_METER
    Return the system wide virtual memory statistics. The returned data consists of a struct vmtotal.

CTL_DDB
The integer information available for the CTL_DDB level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.
Second level name Type Changeable
DBCTL_RADIX integer yes
DBCTL_MAXOFF integer yes
DBCTL_LINES integer yes
DBCTL_TABSTOPS integer yes
DBCTL_ONPANIC integer yes

DBCTL_RADIX
    The input and output radix.

DBCTL_MAXOFF
    The maximum symbol offset.

DBCTL_LINES
    Number of display lines.

DBCTL_TABSTOPS
    Tab width.

DBCTL_ONPANIC
    If non-zero, DDB will be entered when the kernel panics.

These MIB nodes are also available as variables from within the DDB. See ddb(4) for more details.

RETURN VALUES
If the call to sysctl is successful, the number of bytes copied out is returned. Otherwise -1 is returned and errno is set appropriately.

ERRORS
The following errors may be reported:

[EFAULT]
    The buffer name, oldp, newp, or length pointer oldlenp contains an invalid address.

[EINVAL]
    The name array is less than two or greater than CTL_MAXNAME.

[EINVAL]
    A non-null newp is given and its specified length in newlen is too large or too small.

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

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

[EOPNOTSUPP]
    The name array specifies a value that is unknown.

[EPERM]
    An attempt is made to set a read-only value.

[EPERM]
    A process without appropriate privilege attempts to set a value.

[EPERM]
    An attempt to change a value protected by the current kernel security level is made.

FILES
<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
<vm/vm_param.h>
    definitions for second level virtual memory identifiers
<netinet/in.h>
    definitions for third level Internet identifiers and fourth level IP identifiers
<netinet/icmp_var.h>
    definitions for fourth level ICMP identifiers
<netinet/tcp_var.h>
    definitions for fourth level TCP identifiers
<netinet/udp_var.h>
    definitions for fourth level UDP identifiers

SEE ALSO
sysctl(8)

HISTORY
The sysctl function first appeared in 4.4BSD.