--- name: "sysctl" section: "3" platform: "linux" package: "freebsd-manpages" version: "6.0-5" release: "Debian 4.0 (Etch)" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/sysctl/3" --- # sysctl(3) get or set system information ## NAME **sysctl**, **sysctlbyname**, **sysctlnametomib** – get or set system information ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** **#include \** *int*\ **sysctl**(*int \*name*, *u\_int namelen*, *void \*oldp*, *size\_t \*oldlenp*, *void \*newp*, *size\_t newlen*); *int*\ **sysctlbyname**(*const char \*name*, *void \*oldp*, *size\_t \*oldlenp*, *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)](/sysctl/8.md) 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)](/getvfsbyname/3.md) as an example of its use). The remaining second level identifiers are the file system type number returned by a [statfs(2)](/statfs/2.md) 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\_FLOATINGPOINT | integer | no | | HW\_MACHINE\_ARCH | string | 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. - HW\_FLOATINGPOINT Nonzero if the floating point support is in hardware. - HW\_MACHINE\_ARCH The machine dependent architecture type. ### 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 file | no | | KERN\_HOSTID | integer | 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 | struct proc | no | | 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 vnode | no | - KERN\_ARGMAX The maximum bytes of argument to [execve(2)](/execve/2.md). - 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 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\_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 *M**mm**R**xx*, 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 pairs of *struct* *proc* followed by corresponding *struct* *eproc* structures is returned, whose size depends on the current number of such objects in the system. | | | | --- | --- | | 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 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 | - 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 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 i386 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)](/route/4.md) 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 | 0 or if\_index | | NET\_RT\_IFMALIST | 0 or if\_index | - 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 | - 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)](/inet6/4.md). For variables net.inet6.\*.ipsec6, please refer to [ipsec(4)](/ipsec/4.md). ### 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)](/bc/1.md) utility. - USER\_BC\_DIM\_MAX The maximum array size in the [bc(1)](/bc/1.md) utility. - USER\_BC\_SCALE\_MAX The maximum scale value in the [bc(1)](/bc/1.md) utility. - USER\_BC\_STRING\_MAX The maximum string length in the [bc(1)](/bc/1.md) 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)](/expr/1.md) 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\_METER | struct vmtotal | no | | VM\_PAGEOUT\_ALGORITHM | integer | yes | | VM\_SWAPPING\_ENABLED | integer | maybe | | VM\_V\_CACHE\_MAX | integer | yes | | VM\_V\_CACHE\_MIN | integer | yes | | 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\_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\_CACHE\_MAX Maximum desired size of the cache queue. - VM\_V\_CACHE\_MIN Minimum desired size of the cache queue. If the cache queue size falls very far below this value, the pageout daemon is awakened. - 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. ## RETURN VALUES The **sysctl()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ## FILES - In sys/sysctl.h definitions for top level identifiers, second level kernel and hardware identifiers, and user level identifiers - In sys/socket.h definitions for second level network identifiers - In sys/gmon.h definitions for third level profiling identifiers - In vm/vm\_param.h definitions for second level virtual memory identifiers - In netinet/in.h definitions for third level IPv4/IPv6 identifiers and fourth level IPv4/v6 identifiers - In netinet/icmp\_var.h definitions for fourth level ICMP identifiers - In netinet/icmp6.h definitions for fourth level ICMPv6 identifiers - In 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 [sysconf(3)](/sysconf/3.md), [sysctl(8)](/sysctl/8.md) ## HISTORY The **sysctl**() function first appeared in 4.4BSD.