SYSCTL(3) BSD Library Functions Manual 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. Infor‐
mation may be retrieved and set from the command interface using the
sysctl(1) 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 initial‐
ized at the location of their associated variable. The loader prevents
multiple use of the same variable by issuing errors if a variable is ini‐
tialized 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
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 exec(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 con‐
tains the clock, statistics clock and profiling clock frequen‐
cies, 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 pro‐
filing 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 nec‐
essarily 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 architec‐
tures 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 proto‐
cols. 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 nor‐
mal 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 discour‐
aged.
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 val‐
ues 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 defini‐
tion file.
USER_CS_PATH
Return a value for the PATH
environment variable that
finds all the standard utili‐
ties.
USER_EXPR_NEST_MAX
The maximum number of expres‐
sions that can be nested
within parenthesis by the
expr(1) utility.
USER_LINE_MAX
The maximum length in bytes
of a text-processing util‐
ity's input line.
USER_POSIX2_CHAR_TERM
Return 1 if the system sup‐
ports at least one terminal
type capable of all opera‐
tions described in POSIX
1003.2, otherwise 0.
USER_POSIX2_C_BIND
Return 1 if the system's C-
language development facili‐
ties support the C-Language
Bindings Option, otherwise 0.
USER_POSIX2_C_DEV
Return 1 if the system sup‐
ports the C-Language Develop‐
ment Utilities Option, other‐
wise 0.
USER_POSIX2_FORT_DEV
Return 1 if the system sup‐
ports the FORTRAN Development
Utilities Option, otherwise
0.
USER_POSIX2_FORT_RUN
Return 1 if the system sup‐
ports the FORTRAN Runtime
Utilities Option, otherwise
0.
USER_POSIX2_LOCALEDEF
Return 1 if the system sup‐
ports the creation of
locales, otherwise 0.
USER_POSIX2_SW_DEV
Return 1 if the system sup‐
ports the Software Develop‐
ment Utilities Option, other‐
wise 0.
USER_POSIX2_UPE
Return 1 if the system sup‐
ports 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, 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.
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 identi‐
fiers
<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 ALSOsysctl(8)HISTORY
The sysctl function first appeared in 4.4BSD.
BSD April 19, 2024 BSD
