SYSCTL(3)

NAME
sysctl, sysctlbyname, sysctlnametomib - get or set system information

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/sysctl.h>
int
sysctl(const int *name, u_int namelen, void *oldp, size_t *oldlenp, const void *newp, size_t newlen);
int
sysctlbyname(const char *name, void *oldp, size_t *oldlenp, const void *newp, size_t newlen);
int
sysctlnametomib(const char *name, int *mibp, size_t *sizep);

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 sysctlbyname() function accepts an ASCII representation of the name and internally looks up the integer name vector. Apart from that, it behaves the same as the standard sysctl() function.

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 and after a call that returns with the error code ENOMEM. 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 accepts an ASCII representation of the name, looks up the integer name vector, and returns the numeric representation in the mib array pointed to by mibp. The number of elements in the mib array is given by the location specified by sizep before the call, and that location gives the number of entries copied after a successful call. The resulting mib and size may be used in subsequent sysctl() calls to get the data associated with the requested ASCII name. This interface is intended for use by applications that want to repeatedly request the same variable (the sysctl() function runs in about a third the time as the same request made via the sysctlbyname() function). The sysctlnametomib() function is also useful for fetching mib prefixes and then adding a final component. For example, to fetch process information for processes with pid's less than 100:

    int i, mib[4];
    size_t len;
    struct kinfo_proc kp;
    /* Fill out the first three components of the mib */
    len = 4;
    sysctlnametomib("kern.proc.pid", mib, &len);
    /* Fetch and print entries for pid's < 100 */
    for (i = 0; i < 100; i++) {
    	mib[3] = i;
    	len = sizeof(kp);
    	if (sysctl(mib, 4, &kp, &len, NULL, 0) == -1)
    		perror("sysctl");
    	else if (len > 0)
    		printkproc(&kp);
    }

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_FLOATINGPT | integer | no
HW_MACHINE_ARCH | string | no
HW_MACHINE_PLATFORM | string | no
HW_SENSORS | node | not applicable

HW_MACHINE
    The machine class.

HW_MODEL
    The machine model

HW_NCPU
    The number of cpus.

HW_BYTEORDER
    The byteorder (4321, or 1234).

HW_PHYSMEM
    The bytes of physical memory.

HW_USERMEM
    The bytes of non-kernel memory.

HW_PAGESIZE
    The software page size.

HW_FLOATINGPT
    Nonzero if the floating point support is in hardware.

HW_MACHINE_ARCH
    The machine dependent architecture type.

HW_MACHINE_PLATFORM
    The platform architecture type.

HW_SENSORS
    Third level comprises an array of struct sensordev structures containing information about devices that may attach hardware monitoring sensors.

    Third, fourth and fifth levels together comprise an array of struct sensor structures containing snapshot readings of hardware monitoring sensors. In such usage, third level indicates the numerical representation of the sensor device name to which the sensor is attached (device's xname and number shall be matched with the help of struct sensordev structure above), fourth level indicates sensor type and fifth level is an ordinal sensor number (unique to the specified sensor type on the specified sensor device).

    The sensordev and sensor structures and sensor_type enumeration are defined in <sys/sensors.h>.

  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_BOOTFILE | string | yes
KERN_BOOTTIME | struct timespec | no
KERN_CLOCKRATE | struct clockinfo | no
KERN_FILE | struct kinfo_file | no
KERN_HOSTID | integer | yes
KERN_HOSTNAME | string | yes
KERN_JOB_CONTROL | integer | no
KERN_MAXFILES | integer | yes
KERN_MAXFILESPERPROC | integer | yes
KERN_MAXPOSIXLOCKSPERUID | integer | yes
KERN_MAXPROC | integer | no
KERN_MAXPROCPERUID | integer | yes
KERN_MAXVNODES | integer | yes
KERN_NGROUPS | integer | no
KERN_NISDOMAINNAME | string | yes
KERN_OSRELDATE | integer | no
KERN_OSRELEASE | string | no
KERN_OSREV | integer | no
KERN_OSTYPE | string | no
KERN_POSIX1 | integer | no
KERN_PROC | struct kinfo_proc | no
KERN_SAVED_IDS | integer | no
KERN_SECURELVL | integer | raise only
KERN_VERSION | string | no
KERN_VNODE | struct vnode | no

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

KERN_BOOTFILE
    The full pathname of the file from which the kernel was loaded.

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

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 skew rate.

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

KERN_HOSTID
    Get or set the host id.

KERN_HOSTNAME
    Get or set the hostname.

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

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

KERN_MAXFILESPERPROC
    The maximum number of files that may be open for a single process. This limit only applies to processes with an effective uid of nonzero at the time of the open request. Files that have already been opened are not affected if the limit or the effective uid is changed.

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

KERN_MAXPROCPERUID
    The maximum number of concurrent processes the system will allow for a single effective uid. This limit only applies to processes with an effective uid of nonzero at the time of a fork request. Processes that have already been started are not affected if the limit is changed.

KERN_MAXVNODES
    The maximum number of vnodes available on the system.

KERN_NGROUPS
    The maximum number of supplemental groups.

KERN_NISDOMAINNAME
    The name of the current YP/NIS domain.

KERN_OSRELDATE
    The same as KERN_OSREV.

KERN_OSRELEASE
    The system release string.

KERN_OSREV
    The system revision number, in base-10 format Mmmmpp, representing the following components:
    Code | Component
    M | major
    mmm | minor
    pp | patch

    This is __DragonFly_version from <sys/param.h>.

    The minor component is an even-number for release and odd-number for development branches.

KERN_OSTYPE
    The system type string.

KERN_POSIX1
    The version of p1003.1 with which the system attempts to comply.

KERN_PROC
    Return selected information about specific running processes.

    For the following names, an array of struct kinfo_proc structures is returned, whose size depends on the current number of such objects in the system. Adding the flag KERN_PROC_FLAG_LWP to the third level name signals that information about all light weight processes of the selected processes should be returned.
    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

    For the following names, a NUL-terminated string is returned.
    Third level name | Fourth level is:
    KERN_PROC_ARGS | A process ID
    KERN_PROC_CWD | A process ID
    KERN_PROC_PATHNAME | A process ID

    The variables are as follows:

    KERN_PROC_ARGS
        Returns the command line argument array of a process, in a flattened form, i.e. NUL-terminated arguments follow each other. A process can set its own process title by changing this value.

    KERN_PROC_CWD
        Returns the current working directory of a process.

    KERN_PROC_PATHNAME
        Returns the path of a process' text file. A process ID of -1 implies the current process.

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 not be lowered.

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. The following variables are defined for the x86_64 architecture.
Second level name | Type | Changeable
CPU_CONSDEV | dev_t | no
CPU_ADJKERNTZ | int | yes
CPU_DISRTCSET | int | yes
CPU_BOOTINFO | struct bootinfo | no
CPU_WALLCLOCK | int | yes

  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 | IPv4 values | yes
PF_INET6 | IPv6 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 IPv4 (Internet Protocol version 4). The third level name is the protocol. The fourth level name is the variable name. The currently defined protocols and names are:
    Protocol | Variable | Type | Changeable
    icmp | bmcastecho | integer | yes
    icmp | maskrepl | integer | yes
    ip | forwarding | integer | yes
    ip | redirect | integer | yes
    ip | ttl | integer | yes
    udp | checksum | integer | yes

    The variables are as follows:

    icmp.bmcastecho
        Returns 1 if an ICMP echo request to a broadcast or multicast address is to be answered.

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

    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.

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

PF_INET6
    Get or set various global information about IPv6 (Internet Protocol version 6). The third level name is the protocol. The fourth level name is the variable name.

    For variables net.inet6.*, please refer to inet6(4).

  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 p1003.2, 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 p1003.2 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_PAGEOUT_ALGORITHM | integer | yes
VM_SWAPPING_ENABLED | integer | maybe
VM_V_FREE_MIN | integer | yes
VM_V_FREE_RESERVED | integer | yes
VM_V_FREE_TARGET | integer | yes
VM_V_INACTIVE_TARGET | integer | yes
VM_V_PAGEOUT_FREE_MIN | integer | yes
VM_V_PAGING_WAIT | integer | yes
VM_V_PAGING_START | integer | yes
VM_V_PAGING_TARGET1 | integer | yes
VM_V_PAGING_TARGET2 | integer | yes

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.

VM_PAGEOUT_ALGORITHM
    0 if the statistics-based page management algorithm is in use or 1 if the near-LRU algorithm is in use.

VM_SWAPPING_ENABLED
    1 if process swapping is enabled or 0 if disabled. This variable is permanently set to 0 if the kernel was built with swapping disabled.

VM_V_FREE_MIN
    Minimum amount of memory (cache memory plus free memory) required to be available before a process waiting on memory will be awakened.

VM_V_FREE_RESERVED
    Processes will awaken the pageout daemon and wait for memory if the number of free and cached pages drops below this value.

VM_V_FREE_TARGET
    The total amount of free memory (including cache memory) that the pageout daemon tries to maintain.

VM_V_INACTIVE_TARGET
    The desired number of inactive pages that the pageout daemon should achieve when it runs. Inactive pages can be quickly inserted into process address space when needed.

VM_V_PAGEOUT_FREE_MIN
    If the amount of free and cache memory falls below this value, the pageout daemon will enter "memory conserving mode" to avoid deadlock.

VM_V_PAGING_WAIT
    If the amount of free and cache memory falls below this value, most userland processes will begin short stalls on VM allocations and page faults, and return to normal operation once the amount of free and cache memory goes above this value (that is, as soon as possible).

VM_V_PAGING_START
    The pageout daemon is activated when the amount of free and cache memory falls below this value. The daemon will free memory up until the amount of free and cache memory reaches VM_V_PAGING_TARGET1, and then continue to free memory up more slowly until the amount of free and cache memory reaches VM_V_PAGING_TARGET2.

RETURN VALUES

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 IPv4/IPv6 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/udp_var.h>
    definitions for fourth level UDP identifiers

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.

[EISDIR]
    The name array specifies a terminal name, but the actual name is not terminal.

[ENOENT]
    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.

SEE ALSO
sysconf(3), sysctl(8)

HISTORY
The sysctl() function first appeared in 4.4BSD.