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


khelp, khelp_init_osd, khelp_destroy_osd, khelp_get_id, khelp_get_osd, khelp_add_hhook, khelp_remove_hhook, KHELP_DECLARE_MOD, KHELP_DECLARE_MOD_UMAKernel Helper Framework


#include < sys/khelp.h>
#include < sys/module_khelp.h>

int khelp_init_osd( uint32_t classes, struct osd *hosd);

int khelp_destroy_osd( struct osd *hosd);

int32_t khelp_get_id( char *hname);

void * khelp_get_osd( struct osd *hosd, int32_t id);

int khelp_add_hhook( struct hookinfo *hki, uint32_t flags);

int khelp_remove_hhook( struct hookinfo *hki);

KHELP_DECLARE_MOD( hname, hdata, hhooks, version);

KHELP_DECLARE_MOD_UMA( hname, hdata, hhooks, version, ctor, dtor);


khelp provides a framework for managing khelp modules, which indirectly use the hhook(9) KPI to register their hook functions with hook points of interest within the kernel. Khelp modules aim to provide a structured way to dynamically extend the kernel at runtime in an ABI preserving manner. Depending on the subsystem providing hook points, a khelp module may be able to associate per-object data for maintaining relevant state between hook calls. The hhook(9) and khelp frameworks are tightly integrated and anyone interested in khelp should also read the hhook(9) manual page thoroughly.

Information for Khelp Module Implementors

khelp modules are represented within the khelp framework by a struct helper which has the following members:

struct helper { 
 int (*mod_init) (void); 
 int (*mod_destroy) (void); 
 char   h_name[HELPER_NAME_MAXLEN]; 
 uma_zone_t  h_zone; 
 struct hookinfo  *h_hooks; 
 uint32_t  h_nhooks; 
 uint32_t  h_classes; 
 int32_t   h_id; 
 volatile uint32_t h_refcount; 
 uint16_t  h_flags; 
 TAILQ_ENTRY(helper) h_next; 

Modules must instantiate a struct helper, but are only required to set the h_classes field, and may optionally set the h_flags, mod_init and mod_destroy fields where required. The framework takes care of all other fields and modules should refrain from manipulating them. Using the C99 designated initialiser feature to set fields is encouraged.

If specified, the mod_init function will be run by the khelp framework prior to completing the registration process. Returning a non-zero value from the mod_init function will abort the registration process and fail to load the module. If specified, the mod_destroy function will be run by the khelp framework during the deregistration process, after the module has been deregistered by the khelp framework. The return value is currently ignored. Valid khelp classes are defined in < sys/khelp.h>. Valid flags are defined in < sys/module_khelp.h>. The HELPER_NEEDS_OSD flag should be set in the h_flags field if the khelp module requires persistent per-object data storage. There is no programmatic way (yet) to check if a khelp class provides the ability for khelp modules to associate persistent per-object data, so a manual check is required.

The KHELP_DECLARE_MOD() and KHELP_DECLARE_MOD_UMA() macros provide convenient wrappers around the DECLARE_MODULE(9) macro, and are used to register a khelp module with the khelp framework. KHELP_DECLARE_MOD_UMA() should only be used by modules which require the use of persistent per-object storage i.e. modules which set the HELPER_NEEDS_OSD flag in their struct helper's h_flags field.

The first four arguments common to both macros are as follows. The hname argument specifies the unique ascii(7) name for the khelp module. It should be no longer than HELPER_NAME_MAXLEN-1 characters in length. The hdata argument is a pointer to the module's struct helper. The hhooks argument points to a static array of struct hookinfo structures. The array should contain a struct hookinfo for each hhook(9) point the module wishes to hook, even when using the same hook function multiple times for different hhook(9) points. The version argument specifies a version number for the module which will be passed to MODULE_VERSION(9). The KHELP_DECLARE_MOD_UMA() macro takes the additional ctor and dtor arguments, which specify optional uma(9) constructor and destructor functions. NULL should be passed where the functionality is not required.

The khelp_get_id() function returns the numeric identifier for the khelp module with name hname.

The khelp_get_osd() function is used to obtain the per-object data pointer for a specified khelp module. The hosd argument is a pointer to the underlying subsystem object's struct osd. This is provided by the hhook(9) framework when calling into a khelp module's hook function. The id argument specifies the numeric identifier for the khelp module to extract the data pointer from hosd for. The id is obtained using the khelp_get_id() function.

The khelp_add_hhook() and khelp_remove_hhook() functions allow a khelp module to dynamically hook/unhook hhook(9) points at run time. The hki argument specifies a pointer to a struct hookinfo which encapsulates the required information about the hhook(9) point and hook function being manipulated. The HHOOK_WAITOK flag may be passed in via the flags argument of khelp_add_hhook() if malloc(9) is allowed to sleep waiting for memory to become available.

Integrating Khelp Into a Kernel Subsystem

Most of the work required to allow khelp modules to do useful things relates to defining and instantiating suitable hhook(9) points for khelp modules to hook into. The only additional decision a subsystem needs to make is whether it wants to allow khelp modules to associate persistent per-object data. Providing support for persistent data storage can allow khelp modules to perform more complex functionality which may be desirable. Subsystems which want to allow Khelp modules to associate persistent per-object data with one of the subsystem's data structures need to make the following two key changes:
  • Embed a struct osd pointer in the structure definition for the object.
  • Add calls to khelp_init_osd() and khelp_destroy_osd() to the subsystem code paths which are responsible for respectively initialising and destroying the object.

The khelp_init_osd() function initialises the per-object data storage for all currently loaded khelp modules of appropriate classes which have set the HELPER_NEEDS_OSD flag in their h_flags field. The classes argument specifies a bitmask of khelp classes which this subsystem associates with. If a khelp module matches any of the classes in the bitmask, that module will be associated with the object. The hosd argument specifies the pointer to the object's struct osd which will be used to provide the persistent storage for use by khelp modules.

The khelp_destroy_osd() function frees all memory that was associated with an object's struct osd by a previous call to khelp_init_osd(). The hosd argument specifies the pointer to the object's struct osd which will be purged in preparation for destruction.


khelp modules are protected from being prematurely unloaded by a reference count. The count is incremented each time a subsystem calls khelp_init_osd() causing persistent storage to be allocated for the module, and decremented for each corresponding call to khelp_destroy_osd(). Only when a module's reference count has dropped to zero can the module be unloaded.


The khelp_init_osd() function returns zero if no errors occurred. It returns ENOMEM if a khelp module which requires per-object storage fails to allocate the necessary memory.

The khelp_destroy_osd() function only returns zero to indicate that no errors occurred.

The khelp_get_id() function returns the unique numeric identifier for the registered khelp module with name hname. It return -1 if no module with the specified name is currently registered.

The khelp_get_osd() function returns the pointer to the khelp module's persistent object storage memory. If the module identified by id does not have persistent object storage registered with the object's hosd struct osd, NULL is returned.

The khelp_add_hhook() function returns zero if no errors occurred. It returns ENOENT if it could not find the requested hhook(9) point. It returns ENOMEM if malloc(9) failed to allocate memory. It returns EEXIST if attempting to register the same hook function more than once for the same hhook(9) point.

The khelp_remove_hhook() function returns zero if no errors occurred. It returns ENOENT if it could not find the requested hhook(9) point.


A well commented example Khelp module can be found at: /usr/share/examples/kld/khelp/h_example.c

The Enhanced Round Trip Time (ERTT) h_ertt(4) khelp module provides a more complex example of what is possible.


Development and testing of this software were made possible in part by grants from the FreeBSD Foundation and Cisco University Research Program Fund at Community Foundation Silicon Valley.


The khelp kernel helper framework first appeared in FreeBSD 9.0.

The khelp framework was first released in 2010 by Lawrence Stewart whilst studying at Swinburne University of Technology's Centre for Advanced Internet Architectures, Melbourne, Australia. More details are available at:



The khelp framework was written by Lawrence Stewart <lstewart@FreeBSD.org>.

This manual page was written by David Hayes <david.hayes@ieee.org> and Lawrence Stewart <lstewart@FreeBSD.org>.

February 15, 2011 FreeBSD