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SYSCTL(3) Library Functions Manual SYSCTL(3)

sysctl — get or set system information

#include <sys/sysctl.h>

int
sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp, void *newp, size_t newlen);

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(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);

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 };

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.

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
The machine class.
The machine model
The number of cpus.
The byteorder (4,321, or 1,234).
The bytes of physical memory.
The bytes of non-kernel memory.
The software page size.

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
The maximum bytes of argument to exec(2).
A struct timeval structure is returned. This structure contains the time that the system was booted.
Return 1 if appropriate privileges are required for the chown(2) system call, otherwise 0.
A struct clockinfo structure is returned. This structure contains the clock, statistics clock and profiling clock frequencies, and the number of micro-seconds per hz tick.
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.
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 file link count.
The maximum number of open files that may be open in the system.
The maximum number of simultaneous processes the system will allow.
The maximum number of vnodes available on the system.
The maximum number of bytes in terminal canonical input line.
The minimum maximum number of bytes for which space is available in a terminal input queue.
The maximum number of bytes in a file name.
The maximum number of supplemental groups.
Return 1 if file names longer than KERN_NAME_MAX are truncated.
The system release string.
The system revision string.
The system type string.
The maximum number of bytes in a pathname.
The maximum number of bytes which will be written atomically to a pipe.
The version of ISO/IEC 9945 (POSIX 1003.1) with which the system attempts to comply.
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
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:

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.
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 only be lowered by process 1.
Returns the terminal character disabling value.
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 contains the kernel address of a vnode struct vnode * followed by the vnode itself struct vnode.

The set of variables defined is architecture dependent. Most architectures define at least the following variables.

Second level name Type Changeable
dev_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.

Second level name Type Changeable
PF_ROUTE routing messages no
PF_INET internet 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 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
Get or set various global information about the internet protocols. 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:

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 if ICMP network mask requests are to be answered.
Returns 1 when UDP checksums are being computed and checked. Disabling UDP checksums is strongly discouraged.

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
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 POSIX 1003.2, 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 POSIX 1003.2 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.

Second level name Type Changeable
VM_LOADAVG struct loadavg no
VM_METER struct vmtotal no
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.

If the call to sysctl is successful, the number of bytes copied out is returned. Otherwise -1 is returned and errno is set appropriately.

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 name array specifies an intermediate rather than terminal name.
[]
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.

<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 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

sysctl(8)

The sysctl function first appeared in 4.4BSD.

4.4BSD-Lite2 May 9, 1995 SYSCTL(3)