SYSCTL(3) Library Functions Manual SYSCTL(3)

sysctl, sysctlbyname, sysctlnametomibget or set system information

Standard C Library (libc, -lc)

#include <sys/types.h>
#include <sys/sysctl.h>

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);

The () 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, () 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 () 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 () 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 () 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 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.

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 sys/resources.h Virtual memory (struct loadavg)

For example, the following retrieves the maximum number of processes allowed in the system:

int maxproc;
size_t len = sizeof(maxproc);
sysctlbyname("kern.maxproc", &maxproc, &len, NULL, 0);

To retrieve the standard search path for the system utilities:

char *p;
size_t len;
sysctlbyname("user.cs_path", NULL, &len, NULL, 0);
p = malloc(len);
sysctlbyname("user.cs_path", p, &len, NULL, 0);

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.

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.

Type Changeable
hw.activecpu int32_t no
hw.byteorder int32_t no
hw.cacheconfig uint64_t[] no
hw.cachelinesize int64_t no
hw.cachesize uint64_t[] no
hw.cpu64bit_capable int32_t no
hw.cpufamily uint32_t no
hw.cpufrequency int64_t no
hw.cpufrequency_max int64_t no
hw.cpufrequency_min int64_t no
hw.cpusubtype int32_t no
hw.cputhreadtype int32_t no
hw.cputype int32_t no
hw.l1dcachesize int64_t no
hw.l1icachesize int64_t no
hw.l2cachesize int64_t no
hw.l3cachesize int64_t no
hw.logicalcpu int32_t no
hw.logicalcpu_max int32_t no
hw.machine char[] no
hw.memsize int64_t no
hw.model char[] no
hw.ncpu int32_t no
hw.packages int32_t no
hw.pagesize int64_t no
hw.physicalcpu int32_t no
hw.physicalcpu_max int32_t no
hw.tbfrequency int64_t no
The byte order (4321 or 1234).
The machine model.
The number of cpus. This attribute is deprecated and it is recommended that hw.logicalcpu, hw.logicalcpu_max, hw.physicalcpu, or hw.physicalcpu_max be used instead.
The number of logical processors available in the current power management mode.
The maximum number of logical processors that could be available this boot.
The number of physical processors available in the current power management mode.
The maximum number of physical processors that could be available this boot.
The software page size in bytes.

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.

Type Changeable
kern.argmax int32_t no
kern.bootargs char[] no
kern.boottime struct timeval no
kern.check_openevt int32_t yes
kern.clockrate struct clockinfo no
kern.coredump int32_t yes
kern.corefile char[] yes
kern.flush_cache_on_write int32_t yes
kern.hostid int32_t yes
kern.hostname char[] yes
kern.job_control int32_t no
kern.maxfiles int32_t yes
kern.maxfilesperproc int32_t yes
kern.maxnbuf int32_t yes
kern.maxproc int32_t yes
kern.maxprocperuid int32_t yes
kern.maxvnodes int32_t yes
kern.msgbuf int32_t yes
kern.nbuf int32_t no
kern.netboot int32_t no
kern.ngroups int32_t no
kern.nisdomainname char[] yes
kern.num_files int32_t no
kern.num_tasks int32_t no
kern.num_taskthreads int32_t no
kern.num_threads int32_t no
kern.num_vnodes int32_t no
kern.nx int32_t yes
kern.osrelease char[] no
kern.osrevision int32_t no
kern.ostype char[] no
kern.osversion char[] yes
kern.posix1version int32_t no
kern.procname char[] yes
kern.safeboot int32_t no
kern.saved_ids int32_t no
kern.secure_kernel int32_t no
kern.securelevel int32_t yes
kern.singleuser int32_t no
kern.sleeptime struct timeval no
kern.slide int32_t no
kern.stack_depth_max int32_t no
kern.stack_size int32_t no
kern.sugid_coredump int32_t yes
kern.sugid_scripts int32_t yes
kern.symfile char[] no
kern.usrstack int32_t no
kern.usrstack64 int64_t no
kern.uuid char[] no
kern.version char[] no
kern.waketime struct timeval no
The maximum bytes of argument to execve(2).
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.
Get or set the host id.
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 system release string.
The system revision number.
The system type string.
The version of IEEE Std 1003.1 (“POSIX.1”) with which the system attempts to comply.
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.

The set of variables defined is architecture dependent. The following variables are defined for the i386 architecture.

Type Changeable
machdep.cpu.address_bits.physical int32_t no
machdep.cpu.address_bits.virtual int32_t no
machdep.cpu.brand int32_t no
machdep.cpu.brand_string char[] no
machdep.cpu.cache.L2_associativity int32_t no
machdep.cpu.cache.linesize int32_t no
machdep.cpu.cache.size int32_t no
machdep.cpu.core_count int32_t no
machdep.cpu.cores_per_package int32_t no
machdep.cpu.extfamily int32_t no
machdep.cpu.extfeature_bits int64_t no
machdep.cpu.extfeatures char[] no
machdep.cpu.extmodel int32_t no
machdep.cpu.family int32_t no
machdep.cpu.feature_bits int64_t no
machdep.cpu.features char[] no
machdep.cpu.leaf7_feature_bits uint32_t no
machdep.cpu.leaf7_features char[] no
machdep.cpu.logical_per_package int32_t no
machdep.cpu.max_basic uint32_t no
machdep.cpu.max_ext uint32_t no
machdep.cpu.microcode_version int32_t no
machdep.cpu.model int32_t no
machdep.cpu.processor_flag int32_t no
machdep.cpu.signature int32_t no
machdep.cpu.stepping int32_t no
machdep.cpu.thread_count int32_t no
machdep.cpu.tlb.data.large int32_t no
machdep.cpu.tlb.data.large_level1 int32_t no
machdep.cpu.tlb.data.small int32_t no
machdep.cpu.tlb.data.small_level1 int32_t no
machdep.cpu.tlb.inst.large int32_t no
machdep.cpu.tlb.inst.small int32_t no
machdep.cpu.tlb.shared int32_t no
machdep.cpu.ucupdate int32_t yes
machdep.cpu.vendor char[] no
machdep.cpu.xsave.extended_state uint32_t no
machdep.tsc.deep_idle_rebase uint32_t yes
machdep.tsc.frequency int64_t no
machdep.tsc.nanotime.generation uint32_t no
machdep.tsc.nanotime.shift uint32_t no

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.

Type Changeable
PF_ROUTE routing messages no
PF_INET IPv4 values yes
PF_INET6 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:

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.

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:
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).

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.

Type Changeable
user.bc_base_max int32_t no
user.bc_dim_max int32_t no
user.bc_scale_max int32_t no
user.bc_string_max int32_t no
user.coll_weights_max int32_t no
user.cs_path char[] no
user.expr_nest_max int32_t no
user.line_max int32_t no
user.posix2_c_bind int32_t no
user.posix2_c_dev int32_t no
user.posix2_char_term int32_t no
user.posix2_fort_dev int32_t no
user.posix2_fort_run int32_t no
user.posix2_localedef int32_t no
user.posix2_sw_dev int32_t no
user.posix2_upe int32_t no
user.posix2_version int32_t no
user.re_dup_max int32_t no
user.stream_max int32_t no
user.tzname_max int32_t 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'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 at least one terminal type capable of all operations described in IEEE Std 1003.2 (“POSIX.2”), 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.

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.

Type Changeable
vm.loadavg struct loadavg no
vm.swapusage struct xsw_usage no
Return the load average history. The returned data consists of a struct loadavg.

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.

<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
<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

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.

confstr(3), sysconf(3), sysctl(8)

The sysctl() function first appeared in 4.4BSD.

May 17, 2013 macOS 14.3