Provided by: freebsd-manpages_12.2-1_all 
NAME
sysctl, sysctlbyname, sysctlnametomib — get or set system information
LIBRARY
Standard C Library (libc, -lc)
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 the NULL argument 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> 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
A distinguished second level name, VFS_GENERIC, is used to get general information about all file
systems. One of its third level identifiers is VFS_MAXTYPENUM that gives the highest valid file system
type number. Its other third level identifier is VFS_CONF that returns configuration information about
the file system type given as a fourth level identifier (see getvfsbyname(3) as an example of its use).
The remaining second level identifiers are the file system type number returned by a statfs(2) call or
from VFS_CONF. The third level identifiers available for each file system are given in the header file
that defines the mount argument structure for that 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_FLOATINGPT integer no
HW_MACHINE_ARCH string no
HW_REALMEM integer no
HW_AVAILPAGES integer no
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
Amount of physical memory (in bytes), minus the amount used by the kernel, pre-loaded modules,
and (on x86) the dcons buffer.
HW_USERMEM
Amount of memory (in bytes) which is not wired.
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_REALMEM
Amount of memory (in bytes) reported by the firmware. That value is sometimes not sane; in that
case, the kernel reports the max memory address instead.
HW_AVAILPAGES
The same value as HW_PHYSMEM, measured in pages rather than bytes.
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 timeval no
KERN_CLOCKRATE struct clockinfo no
KERN_FILE struct xfile no
KERN_HOSTID integer yes
KERN_HOSTUUID string yes
KERN_HOSTNAME string yes
KERN_JOB_CONTROL integer no
KERN_MAXFILES integer yes
KERN_MAXFILESPERPROC 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 node not applicable
KERN_PROF node not applicable
KERN_QUANTUM integer yes
KERN_SAVED_IDS integer no
KERN_SECURELVL integer raise only
KERN_UPDATEINTERVAL integer no
KERN_VERSION string no
KERN_VNODE struct xvnode 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 timeval 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 xfile, whose size
depends on the current number of such objects in the system.
KERN_HOSTID
Get or set the host ID.
KERN_HOSTUUID
Get or set the host's universally unique identifier (UUID).
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 kernel release version in the format MmmRxx, where M is the major version, mm is the two
digit minor version, R is 0 if release branch, otherwise 1, and xx is updated when the available
APIs change.
The userland release version is available from <osreldate.h>; parse this file if you need to get
the release version of the currently installed userland.
KERN_OSRELEASE
The system release string.
KERN_OSREV
The system revision string.
KERN_OSTYPE
The system type string.
KERN_POSIX1
The version of IEEE Std 1003.1 (“POSIX.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.
Third Level Name Fourth Level
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
If the third level name is KERN_PROC_ARGS then the command line argument array is returned in a
flattened form, i.e., zero-terminated arguments follow each other. The total size of array is
returned. It is also possible for a process to set its own process title this way. If the third
level name is KERN_PROC_PATHNAME, the path of the process' text file is stored. For
KERN_PROC_PATHNAME, a process ID of -1 implies the current process.
Third Level Name Fourth Level
KERN_PROC_ARGS A process ID
KERN_PROC_PATHNAME A process 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 ENOENT. 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_QUANTUM
The maximum period of time, in microseconds, for which a process is allowed to run without being
preempted if other processes are in the run queue.
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 consists of a struct xvnode.
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, sixth, and
seventh level names are as follows:
Fifth level Sixth Level Seventh Level
NET_RT_FLAGS rtflags None
NET_RT_DUMP None None or fib number
NET_RT_IFLIST 0 or if_index None
NET_RT_IFMALIST 0 or if_index None
NET_RT_IFLISTL 0 or if_index None
The NET_RT_IFMALIST name returns information about multicast group memberships on all interfaces
if 0 is specified, or for the interface specified by if_index.
The NET_RT_IFLISTL is like NET_RT_IFLIST, just returning message header structs with additional
fields allowing the interface to be extended without breaking binary compatibility. The
NET_RT_IFLISTL uses 'l' versions of the message header structures: struct if_msghdrl and struct
ifa_msghdrl.
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.
For variables net.inet.*.ipsec, please refer to ipsec(4).
PF_INET6
Get or set various global information about the 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). For variables net.inet6.*.ipsec6, please
refer to ipsec(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
IEEE Std 1003.2 (“POSIX.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 IEEE Std 1003.2 (“POSIX.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_TOTAL struct vmtotal no
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_OVERCOMMIT integer yes
VM_LOADAVG
Return the load average history. The returned data consists of a struct loadavg.
VM_TOTAL
Return the system wide virtual memory statistics. The returned data consists of a struct
vmtotal.
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_OVERCOMMIT
Overcommit behaviour, as described in tuning(7).
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global
variable errno is set to indicate the error.
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.
[ENOMEM] The smaller of either the length pointed to by oldlenp or the estimated size of the
returned data exceeds the system limit on locked memory.
[ENOMEM] Locking the buffer oldp, or a portion of the buffer if the estimated size of the data
to be returned is smaller, would cause the process to exceed its per-process locked
memory limit.
[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
confstr(3), kvm(3), sysconf(3), sysctl(8)
HISTORY
The sysctl() function first appeared in 4.4BSD.
Debian March 22, 2019 SYSCTL(3)