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SYSCTL(9)              FreeBSD Kernel Developer's Manual             SYSCTL(9)

     Static sysctl declaration functions

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


     SYSCTL_INT(parent, nbr, name, access, ptr, val, descr);

     SYSCTL_LONG(parent, nbr, name, access, ptr, val, descr);

     SYSCTL_NODE(parent, nbr, name, access, handler, descr);

     SYSCTL_OPAQUE(parent, nbr, name, access, ptr, len, fmt, descr);

     SYSCTL_PROC(parent, nbr, name, access, ptr, arg, handler, fmt, descr);

     SYSCTL_STRING(parent, nbr, name, access, arg, len, descr);

     SYSCTL_STRUCT(parent, nbr, name, access, ptr, type, descr);

     SYSCTL_UINT(parent, nbr, name, access, ptr, val, descr);

     SYSCTL_ULONG(parent, nbr, name, access, ptr, val, descr);

     The SYSCTL_DECL kernel interfaces allow code to statically declare
     sysctl(8) MIB entries, which will be initialized when the kernel module
     containing the declaration is initialized.  When the module is unloaded,
     the sysctl will be automatically destroyed.

     Sysctl nodes are created in a hierarchical tree, with all static nodes
     being represented by named C data structures; in order to create a new
     node under an existing node in the tree, the structure representing the
     desired parent node must be declared in the current context using

     New nodes are declared using one of SYSCTL_INT, SYSCTL_LONG, SYSCTL_NODE,
     and SYSCTL_ULONG.  Each macro accepts a parent name, as declared using
     SYSCTL_DECL, an OID number, typically OID_AUTO, a node name, a set of
     control and access flags, and a description.  Depending on the macro, a
     pointer to a variable supporting the MIB entry, a size, a value, and a
     function pointer implementing the MIB entry may also be present.

     For most of the above macros, declaring a type as part of the access
     flags is not necessary -- however, when declaring a sysctl implemented by
     a function, including a type in the access mask is required:

     CTLTYPE_NODE    This is a node intended to be a parent for other nodes.

     CTLTYPE_INT     This is a signed integer.

     CTLTYPE_STRING  This is a nul-terminated string stored in a character

     CTLTYPE_QUAD    This is a 64-bit signed integer.

     CTLTYPE_OPAQUE  This is an opaque data structure.


     CTLTYPE_UINT    This is an unsigned integer.

     CTLTYPE_LONG    This is a signed long.

     CTLTYPE_ULONG   This is an unsigned long.

     All sysctl types except for new node declarations require one or more
     flags to be set indicating the read and write disposition of the sysctl:

     CTLFLAG_RD       This is a read-only sysctl.  It Dv CTLFLAG_WR This is a
                      writable sysctl.

     CTLFLAG_RW       This sysctl is readable and writable.

     CTLFLAG_ANYBODY  Any user or process can write to this sysctl.

     CTLFLAG_SECURE   This sysctl can be written to only if the effective
                      securelevel of the process is <= 0.

     CTLFLAG_PRISON   This sysctl can be written to by processes in jail(2).

     CTLFLAG_SKIP     When iterating the sysctl name space, do not list this

     CTLFLAG_TUN      Also declare a system tunable with the same name to ini-
                      tialize this variable.

     CTLFLAG_RDTUN    Also declare a system tunable with the same name to ini-
                      talize this variable; however, the run-time variable is

     When creating new sysctls, careful attention should be paid to the secu-
     rity implications of the monitoring or management interface being cre-
     ated.  Most sysctls present in the kernel are read-only or writable only
     by the superuser.  Sysctls exporting extensive information on system data
     structures and operation, especially those implemented using procedures,
     will wish to implement access control to limit the undesired exposure of
     information about other processes, network connections, etc.

     The following top level sysctl name spaces are commonly used:

     compat      Compatibility layer information.

     debug       Debugging information.  Various name spaces exist under

     hw          Hardware and device driver information.

     kern        Kernel behavior tuning; generally deprecated in favor of more
                 specific name spaces.

     machdep     Machine-dependent configuration parameters.

     net         Network subsystem.  Various protocols have name spaces under

     regression  Regression test configuration and information.

     security    Security and security-policy configuration and information.

     sysctl      Reserved name space for the implementation of sysctl.

     user        Configuration settings relating to user application behavior.
                 Generally, configuring applications using kernel sysctls is

     vfs         Virtual file system configuration and information.

     vm          Virtual memory subsystem configuration and information.

     Sample use of SYSCTL_DECL to declare the "security" sysctl tree for use
     by new nodes:


     Examples of integer, opaque, string, and procedure sysctls follow:

            * Example of a constant integer value.  Notice that the control
            * flags are CTLFLAG_RD, the variable pointer is NULL, and the
            * value is declared.
           SYSCTL_INT(_debug_sizeof, OID_AUTO, bio, CTLFLAG_RD, NULL,
               sizeof(struct bio), "sizeof(struct bio)");

            * Example of a variable integer value.  Notice that the control
            * flags are CTLFLAG_RW, the variable pointer is set, and the
            * value is 0.
           static int      doingcache = 1;         /* 1 => enable the cache */
           SYSCTL_INT(_debug, OID_AUTO, vfscache, CTLFLAG_RW, &doingcache, 0,
               "Enable name cache");

            * Example of a variable string value.  Notice that the control
            * flags are CTLFLAG_RW, that the variable pointer and string
            * size are set.  Unlike newer sysctls, this older sysctl uses a
            * static oid number.
           char kernelname[MAXPATHLEN] = "/kernel";        /* XXX bloat */
           SYSCTL_STRING(_kern, KERN_BOOTFILE, bootfile, CTLFLAG_RW,
               kernelname, sizeof(kernelname), "Name of kernel file booted");

            * Example of an opaque data type exported by sysctl.  Notice that
            * the variable pointer and size are provided, as well as a format
            * string for sysctl(8).
           static l_fp pps_freq;   /* scaled frequence offset (ns/s) */
           SYSCTL_OPAQUE(_kern_ntp_pll, OID_AUTO, pps_freq, CTLFLAG_RD,
               &pps_freq, sizeof(pps_freq), "I", "");

            * Example of a procedure based sysctl exporting string
            * information.  Notice that the data type is declared, the NULL
            * variable pointer and 0 size, the function pointer, and the
            * format string for sysctl(8).
           SYSCTL_PROC(_kern_timecounter, OID_AUTO, hardware, CTLTYPE_STRING |
               CTLFLAG_RW, NULL, 0, sysctl_kern_timecounter_hardware, "A",

     When adding, modifying, or removing sysctl names, it is important to be
     aware that these interfaces may be used by users, libraries, applica-
     tions, or documentation (such as published books), and are implicitly
     published application interfaces.  As with other application interfaces,
     caution must be taken not to break existing applications, and to think
     about future use of new name spaces so as to avoid the need to rename or
     remove interfaces that might be depended on in the future.

     sysctl(8), sysctl_add_oid(9), sysctl_ctx_free(9), sysctl_ctx_init(9),

     sysctl(8) first appeared in 4.4BSD.

     The sysctl implementation originally found in BSD has been extensively
     rewritten by Poul-Henning Kamp in order to add support for name lookups,
     name space iteration, and dynamic addition of MIB nodes.

     This man page was written by
     Robert N. M. Watson.

FreeBSD 6.2                     April 28, 2006                     FreeBSD 6.2


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