NAME

sysctl − get or set system information

SYNOPSIS

#include <sys/sysctl.h>
 int
sysctl(name, namelen, oldp, ∗oldlenp, ∗newp, newlen)
   int ∗name;
   u_int namelen;
   void ∗oldp;
   size_t ∗oldlenp;
   void ∗newp;
   size_t newlen

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(1) utility. 

Unless explicitly noted below, sysctl returns a consistent snapshot of the data requested.  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. 

NameNext level namesDescription
CTL_DEBUGsys/sysctl.hDebugging
CTL_FSsys/sysctl.hFile system
CTL_HWsys/sysctl.hGeneric CPU, I/O
CTL_KERNsys/sysctl.hHigh kernel limits
CTL_MACHDEPsys/sysctl.h Machine dependent
CTL_NETsys/socket.hNetworking
CTL_USERsys/sysctl.hUser-level
CTL_VMvm/vm_param.hVirtual 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_FS

There are currently no second level names for the 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 nameTypeChangeable
HW_MACHINEstringno
HW_MODELstringno
HW_NCPUintegerno
HW_BYTEORDERintegerno
HW_PHYSMEMintegerno
HW_USERMEMintegerno
HW_PAGESIZEintegerno

HW_MACHINE
The machine class.  HW_MODEL
The machine model HW_NCPU
The number of cpus.  HW_BYTEORDER
The byteorder (3412, 4321, or 1234).  HW_PHYSMEM
The bytes of physical memory.  HW_USERMEM
The bytes of non-kernel memory.  HW_PAGESIZE
The software page size. 

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 inodes, the open file entries, routing table entries, virtual memory statistics, load average history, and clock rate information. 

 Second level nameTypeChangeable
KERN_ARGMAXintegerno
KERN_BOOTTIMEstruct timevalno
KERN_CHOWN_RESTRICTEDinteger no
KERN_CLOCKRATEstruct clockinfono
KERN_FILEstruct fileno
KERN_HOSTIDlongyes
KERN_HOSTNAMEstringyes
KERN_JOB_CONTROLintegerno
KERN_MAXFILESintegerno
KERN_MAXPROCintegerno
KERN_MAXINODESintegerno
KERN_MAXTEXTSintegerno
KERN_NGROUPSintegerno
KERN_OSRELEASEstringno
KERN_OSREVintegerno
KERN_OSTYPEstringno
KERN_POSIX1integerno
KERN_PROCstruct procno
KERN_PROFnodenot applicable
KERN_SAVED_IDSintegerno
KERN_SECURELVLintegerraise only
KERN_TEXTstruct textno
KERN_VERSIONstringno
KERN_INODEstruct inodeno

KERN_ARGMAX

The maximum bytes of argument to exec(2). 

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, and the number of micro-seconds per hz tick. 

KERN_FILE

Return the entire file table as an array of extended file structures.  Each element of the array contains the kernel address of a file struct inode ∗ followed by the file itself struct file.  There can never be more than KERN_MAXFILES inodes returned. 

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 open files that may be open in the system. 

KERN_MAXPROC

The maximum number of simultaneous processes the system will allow. 

KERN_MAXINODES

The maximum number of inodes available on the system. 

KERN_MAXTEXTS

The maximum number of text structures available on the system. 

KERN_NGROUPS

The maximum number of supplemental groups. 

KERN_OSRELEASE

The system release string. 

KERN_OSREV

The system revision string. 

KERN_OSTYPE

The system type string. 

KERN_POSIX1

The version of ISO/IEC 9945 (POSIX 1003.1) with which the system attempts to comply. 

KERN_PROC

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 nameFourth level is:
KERN_PROC_ALLNone
KERN_PROC_PIDA process ID
KERN_PROC_PGRPA process group
KERN_PROC_TTYA tty device
KERN_PROC_UIDA user ID
KERN_PROC_RUIDA real user ID
KERN_PROFReturn kernel profiling information.

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 nameTypeChangeable
GPROF_STATEintegeryes
GPROF_COUNTu_short[]yes
GPROF_FROMSu_short[]yes
GPROF_TOSstruct tostructyes
GPROF_GMONPARAMstruct gmonparamno

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_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 only be lowered by process 1. 

KERN_VERSION

The system version string. 

KERN_INODE

Return the entire inode table.  Note, the inode 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 inode struct inode ∗ followed by the inode itself struct inode.  There can never be more than KERN_MAXINODES inodes returned. 

KERN_TEXT

Return the entire text table.  The returned data consists of an array whose size depends on the current number of such objects active in the system.  Each element of the array contains the kernel address of a text struct text ∗ followed by the text structure itself struct text.  There can never be more structures than returned by KERN_MAXTEXTS. 

CTL_MACHDEP

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

Second level nameTypeChangeable
CPU_CONSDEVdev_tno

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 nameTypeChangeable
PF_ROUTErouting messagesno
PF_INETinternet valuesyes

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 and sixth level names are as follows:

Fifth level nameSixth level is:
NET_RT_FLAGSrtflags
NET_RT_DUMPNone
NET_RT_IFLISTNone

PF_INET

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 nameVariable nameTypeChangeable
ipforwardingintegeryes
ipredirectintegeryes
ipttlintegeryes
icmpmaskreplintegeryes
udpchecksumintegeryes

The variables are as follows: 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. 

icmp.maskrepl

Returns 1 if ICMP network mask requests are to be answered. 

udp.checksum

Returns 1 when UDP checksums are being computed and checked.  Disabling UDP checksums is strongly discouraged. 

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 nameTypeChangeable
USER_BC_BASE_MAXintegerno
USER_BC_DIM_MAXintegerno
USER_BC_SCALE_MAXintegerno
USER_BC_STRING_MAXintegerno
USER_COLL_WEIGHTS_MAXintegerno
USER_CS_PATHstringno
USER_EXPR_NEST_MAXintegerno
USER_LINE_MAXintegerno
USER_POSIX2_CHAR_TERMintegerno
USER_POSIX2_C_BINDintegerno
USER_POSIX2_C_DEVintegerno
USER_POSIX2_FORT_DEVintegerno
USER_POSIX2_FORT_RUNintegerno
USER_POSIX2_LOCALEDEFintegerno
USER_POSIX2_SW_DEVintegerno
USER_POSIX2_UPEintegerno
USER_POSIX2_VERSIONintegerno
USER_RE_DUP_MAXintegerno
USER_STREAM_MAXintegerno
USER_TZNAME_MAXintegerno

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 POSIX 1003.2, 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 POSIX 1003.2 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 nameTypeChangeable
VM_LOADAVGstruct loadavgno
VM_METERstruct vmtotalno
VM_SWAPMAPstruct mapno
VM_COREMAPstruct mapno

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_SWAPMAP

Return the swapmap.  The size of this structure is fixed and may be determined by specifying a oldlenp initialized to zero, the kernel will fill in the size of the swapmap. 

VM_COREMAP

Same as for swapmap above except that the core allocation map is returned. 

RETURN VALUES

If the call to sysctl is successful, 0 is returned.  Otherwise −1 is returned and errno is set appropriately. 

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. 

ENOTDIR The name array specifies an intermediate rather than terminal name. 

EOPNOTSUPP 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. 

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

<sys/vmparam.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

SEE ALSO

sysctl(8)

HISTORY

The sysctl function first appeared in 4.4BSD. 

The KERN_TEXT, KERN_MAXTEXTS, VM_SWAPMAP, VM_COREMAP options are 2.11BSD specific extensions to the 4.4BSD sysctl implmentation. 

Having KERN_FILE return the address of the file structure before the actual struct file is a 2.11BSD enhancement.  The inode (vnode under 4.4) table was handled this way. 

4th Berkeley Distribution  —  January 13, 1995