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SYSCTL(3) Library Functions Manual SYSCTL(3)

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
<sys/sysctl.h> Debugging
<sys/mount.h> File system
<sys/sysctl.h> Generic CPU, I/O
<sys/sysctl.h> High kernel limits
<sys/sysctl.h> Machine dependent
<sys/socket.h> Networking
<sys/sysctl.h> User-level
<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
string no
string no
integer no
integer no
integer no
integer no
integer no
integer no
string no
integer no
The machine class.
The machine model
The number of cpus.
The byteorder (4321 or 1234).
The bytes of physical memory.
The bytes of non-kernel memory.
The software page size.
Nonzero if the floating point support is in hardware.
The machine dependent architecture type.
The bytes of real memory.

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
integer no
string yes
struct timeval no
struct clockinfo no
struct xfile no
integer yes
string yes
string yes
integer no
integer yes
integer yes
integer no
integer yes
integer yes
integer no
string yes
integer no
string no
integer no
string no
integer no
node not applicable
node not applicable
integer yes
integer no
integer raise only
integer no
string no
struct xvnode no
The maximum bytes of argument to execve(2).
The full pathname of the file from which the kernel was loaded.
A struct timeval structure is returned. This structure contains the time that the system was booted.
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.
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.
Get or set the host ID.
Get or set the host's universally unique identifier (UUID).
Get or set the hostname.
Return 1 if job control is available on this system, otherwise 0.
The maximum number of files that may be open in the system.
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.
The maximum number of concurrent processes the system will allow.
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.
The maximum number of vnodes available on the system.
The maximum number of supplemental groups.
The name of the current YP/NIS domain.
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.

The system release string.
The system revision string.
The system type string.
The version of IEEE Std 1003.1 (“POSIX.1”) with which the system attempts to comply.
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
None
A process ID
A process group
A tty device
A user ID
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
A process ID
A process ID
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
integer yes
u_short[] yes
u_short[] yes
struct tostruct yes
struct gmonparam no

The variables are as follows:

Returns GMON_PROF_ON or GMON_PROF_OFF to show that profiling is running or stopped.
Array of statistical program counter counts.
Array indexed by program counter of call-from points.
Array of struct tostruct describing destination of calls and their counts.
Structure giving the sizes of the above arrays.
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.
Returns 1 if saved set-group and saved set-user ID is available.
The system security level. This level may be raised by processes with appropriate privilege. It may not be lowered.
The system version string.
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
routing messages no
IPv4 values yes
IPv6 values yes
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
rtflags None
None None or fib number
0 or if_index None
0 or if_index None
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.

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:

Returns 1 if an ICMP echo request to a broadcast or multicast address is to be answered.
Returns 1 if ICMP network mask requests are to be answered.
Returns 1 when IP forwarding is enabled for the host, meaning that the host is acting as a router.
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.
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.
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).

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
integer no
integer no
integer no
integer no
integer no
string no
integer no
integer no
integer no
integer no
integer no
integer no
integer no
integer no
integer no
integer no
integer no
integer no
integer no
integer no
The maximum ibase/obase values in the bc(1) utility.
The maximum array size in the bc(1) utility.
The maximum scale value in the bc(1) utility.
The maximum string length in the bc(1) utility.
The maximum number of weights that can be assigned to any entry of the LC_COLLATE order keyword in the locale definition file.
Return a value for the PATH environment variable that finds all the standard utilities.
The maximum number of expressions that can be nested within parenthesis by the expr(1) utility.
The maximum length in bytes of a text-processing utility's input line.
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.
Return 1 if the system's C-language development facilities support the C-Language Bindings Option, otherwise 0.
Return 1 if the system supports the C-Language Development Utilities Option, otherwise 0.
Return 1 if the system supports the FORTRAN Development Utilities Option, otherwise 0.
Return 1 if the system supports the FORTRAN Runtime Utilities Option, otherwise 0.
Return 1 if the system supports the creation of locales, otherwise 0.
Return 1 if the system supports the Software Development Utilities Option, otherwise 0.
Return 1 if the system supports the User Portability Utilities Option, otherwise 0.
The version of IEEE Std 1003.2 (“POSIX.2”) with which the system attempts to comply.
The maximum number of repeated occurrences of a regular expression permitted when using interval notation.
The minimum maximum number of streams that a process may have open at any one time.
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
struct loadavg no
struct vmtotal no
integer maybe
integer yes
integer yes
integer yes
integer yes
integer yes
integer yes
Return the load average history. The returned data consists of a struct loadavg.
Return the system wide virtual memory statistics. The returned data consists of a struct vmtotal.
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.
Minimum amount of memory (cache memory plus free memory) required to be available before a process waiting on memory will be awakened.
Processes will awaken the pageout daemon and wait for memory if the number of free and cached pages drops below this value.
The total amount of free memory (including cache memory) that the pageout daemon tries to maintain.
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.
If the amount of free and cache memory falls below this value, the pageout daemon will enter "memory conserving mode" to avoid deadlock.
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:
[]
The buffer name, oldp, newp, or length pointer oldlenp contains an invalid address.
[]
The name array is less than two or greater than CTL_MAXNAME.
[]
A non-null newp is given and its specified length in newlen is too large or too small.
[]
The length pointed to by oldlenp is too short to hold the requested value.
[]
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.
[]
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.
[]
The name array specifies an intermediate rather than terminal name.
[]
The name array specifies a terminal name, but the actual name is not terminal.
[]
The name array specifies a value that is unknown.
[]
An attempt is made to set a read-only value.
[]
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.
October 22, 2017 Linux 4.9.0-9-amd64