SYSCTL(3)

NAME
sysctl - get or set system information

SYNOPSIS
#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/sysctl.h File system
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
There are currently no second level names for the file system.

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
    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.

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_BOOTTIME struct timeval no
KERN_CHOWN_RESTRICTED integer no
KERN_CLOCKRATE struct clockinfo no
KERN_FILE struct file no
KERN_HOSTID integer yes
KERN_HOSTNAME string yes
KERN_JOB_CONTROL integer no
KERN_LINK_MAX integer no
KERN_MAXFILES integer yes
KERN_MAXPROC integer yes
KERN_MAXVNODES integer yes
KERN_MAX_CANON integer no
KERN_MAX_INPUT 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_SAVED_IDS integer no
KERN_SECURELVL integer raise only
KERN_VDISABLE integer no
KERN_VERSION string no
KERN_VNODE struct vnode no

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

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, and the number of micro-seconds per hz tick.

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_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_LINK_MAX
    The maximum file link count.

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

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_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_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_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
CPU_ADJKERNTZ int yes
CPU_DISRTCSET 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 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
    icmp maskrepl integer yes
    udp checksum 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.

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

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

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.

RETURN VALUES
If the call to sysctl is successful, 0 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.

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/udp_var.h>
    definitions for fourth level UDP identifiers

SEE ALSO
sysctl(8)

HISTORY
The sysctl function first appeared in 4.4BSD.