OSD(9)							   BSD Kernel Developer's Manual						    OSD(9)

NAME

     osd, osd_register, osd_deregister, osd_set, osd_get, osd_del, osd_call, osd_exit -- Object Specific Data

SYNOPSIS

     #include <sys/osd.h>

     typedef void
     (*osd_destructor_t)(void *value);

     typedef int
     (*osd_method_t)(void *obj, void *data);

     int
     osd_register(u_int type, osd_destructor_t destructor, osd_method_t *methods);

     void
     osd_deregister(u_int type, u_int slot);

     int
     osd_set(u_int type, struct osd *osd, u_int slot, void *value);

     void *
     osd_get(u_int type, struct osd *osd, u_int slot);

     void
     osd_del(u_int type, struct osd *osd, u_int slot);

     int
     osd_call(u_int type, u_int method, void *obj, void *data);

     void
     osd_exit(u_int type, struct osd *osd);

DESCRIPTION

     The osd framework provides a mechanism to dynamically associate arbitrary data at run-time with any kernel data structure which has been
     suitably modified for use with osd.  The one-off modification required involves embedding a struct osd inside the kernel data structure.

     An additional benefit is that after the initial change to a structure is made, all subsequent use of osd with the structure involves no
     changes to the structure's layout.  By extension, if the data structure is part of the ABI, osd provides a way of extending the structure in
     an ABI preserving manner.

     The details of the embedded struct osd are not relevant to consumers of the osd framework and should not be manipulated directly.

     Data associated with a structure is referenced by the osd framework using a type/slot identifier pair.  Types are statically defined in
     <sys/osd.h> and provide a high-level grouping for slots to be registered under.  Slot identifiers are dynamically assigned by the framework
     when a data type is registered using osd_register() and remains valid until a corresponding call to osd_deregister().

   Functions
     The osd_register() function registers a type/slot identifier pair with the osd framework for use with a new data type.  The function may
     sleep and therefore cannot be called from a non-sleepable context.  The type argument specifies which high-level type grouping from
     <sys/osd.h> the slot identifier should be allocated under.  The destructor argument specifies an optional osd_destructor_t function pointer
     that will be called for objects of the type being registered which are later destroyed by the osd_del() function.	NULL may be passed if no
     destructor is required.  The methods argument specifies an optional array of osd_method_t function pointers which can be later invoked by the
     osd_call() function.  NULL may be passed if no methods are required.  The methods argument is currently only useful with the OSD_JAIL type
     identifier.

     The osd_deregister() function deregisters a previously registered type/slot identifier pair.  The function may sleep and therefore cannot be
     called from a non-sleepable context.  The type argument specifies which high-level type grouping from <sys/osd.h> the slot identifier is
     allocated under.  The slot argument specifies the slot identifier which is being deregistered and should be the value that was returned by
     osd_register() when the data type was registered.

     The osd_set() function associates a data object pointer with a kernel data structure's struct osd member.	The type argument specifies which
     high-level type grouping from <sys/osd.h> the slot identifier is allocated under.	The osd argument is a pointer to the kernel data struc-
     ture's struct osd which will have the value pointer associated with it.  The slot argument specifies the slot identifier to assign the value
     pointer to.  The value argument points to a data object to associate with osd.

     The osd_get() function returns the data pointer associated with a kernel data structure's struct osd member from the specified type/slot
     identifier pair.  The type argument specifies which high-level type grouping from <sys/osd.h> the slot identifier is allocated under.  The
     osd argument is a pointer to the kernel data structure's struct osd to retrieve the data pointer from.  The slot argument specifies the slot
     identifier to retrieve the data pointer from.

     The osd_del() function removes the data pointer associated with a kernel data structure's struct osd member from the specified type/slot
     identifier pair.  The type argument specifies which high-level type grouping from <sys/osd.h> the slot identifier is allocated under.  The
     osd argument is a pointer to the kernel data structure's struct osd to remove the data pointer from.  The slot argument specifies the slot
     identifier to remove the data pointer from.  If an osd_destructor_t function pointer was specified at registration time, the destructor func-
     tion will be called and passed the data pointer for the type/slot identifier pair which is being deleted.

     The osd_call() function calls the specified osd_method_t function pointer for all currently registered slots of a given type on the specified
     obj and data pointers.  The function may sleep and therefore cannot be called from a non-sleepable context.  The type argument specifies
     which high-level type grouping from <sys/osd.h> to call the method for.  The method argument specifies the index into the osd_method_t array
     that was passed to osd_register().  The obj and data arguments are passed to the method function pointer of each slot.

     The osd_exit() function removes all data object pointers from all currently registered slots for a given type for the specified kernel data
     structure's struct osd member.  The type argument specifies which high-level type grouping from <sys/osd.h> to remove data pointers from.
     The osd argument is a pointer to the kernel data structure's struct osd to remove all data object pointers for all currently registered slots
     from.

IMPLEMENTATION NOTES

     osd uses a two dimensional matrix (array of arrays) as the data structure to manage the external data associated with a kernel data struc-
     ture's struct osd member.	The type identifier is used as the index into the outer array, and the slot identifier is used as the index into
     the inner array. To set or retrieve a data pointer for a given type/slot identifier pair, osd_set() and osd_get() perform the equivalent of
     array[type][slot], which is both constant time and fast.

     If osd_set() is called on a struct osd for the first time, the array for storing data pointers is dynamically allocated using malloc(9) with
     M_NOWAIT to a size appropriate for the slot identifier being set.	If a subsequent call to osd_set() attempts to set a slot identifier which
     is numerically larger than the slot used in the previous osd_set() call, realloc(9) is used to grow the array to the appropriate size such
     that the slot identifier can be used.  To maximise the efficiency of any code which calls osd_set() sequentially on a number of different
     slot identifiers (e.g. during an initialisation phase) one should loop through the slot identifiers in descending order from highest to low-
     est.  This will result in only a single malloc(9) call to create an array of the largest slot size and all subsequent calls to osd_set() will
     proceed without any realloc(9) calls.

     The osd API is geared towards slot identifiers storing pointers to the same underlying data structure type for a given osd type identifier.
     This is not a requirement, and khelp(9) for example stores completely different data types in slots under the OSD_KHELP type identifier.

   Locking
     osd internally uses a mix of mutex(9), rmlock(9) and sx(9) locks to protect its internal data structures and state.

     Responsibility for synchronising access to a kernel data structure's struct osd member is left to the subsystem that uses the data structure
     and calls the osd API.

     osd_get() only acquires an rmlock in read mode, therefore making it safe to use in the majority of contexts within the kernel including most
     fast paths.

RETURN VALUES

     osd_register() returns the slot identifier for the newly registered data type.

     osd_set() returns zero on success or ENOMEM if the specified type/slot identifier pair triggered an internal realloc(9) which failed.

     osd_get() returns the data pointer for the specified type/slot identifier pair, or NULL if the slot has not been initialised yet.

     osd_call() returns zero if no method is run or the method for each slot runs successfully.  If a method for a slot returns non-zero,
     osd_call() terminates prematurely and returns the method's error to the caller.

SEE ALSO

     khelp(9)

HISTORY

     The Object Specific Data (OSD) facility first appeared in FreeBSD 8.0.

AUTHORS

     The osd facility was written by Pawel Jakub Dawidek <pjd@FreeBSD.org>.

     This manual page was written by Lawrence Stewart <lstewart@FreeBSD.org>.

BSD
								  January 5, 2011							       BSD