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MODULE(9) NetBSD Kernel Developer's Manual MODULE(9)
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module, module_load, module_autoload, module_unload, module_init_class,
module_hold, module_rele, module_find_section -- kernel module loader
MODULE(class, name, required);
module_load(const char *name, int flags, prop_dictionary_t props,
module_autoload(const char *name, modclass_t class);
module_unload(const char *name);
module_hold(const char *name);
module_rele(const char *);
module_find_section(const char *, void **, size_t *);
Modules are sections of code that can be independently linked and selec-
tively loaded into or unloaded from a running kernel. This provides a
mechanism to update the module without having to relink the kernel and
reboot. Modules can be loaded from within the kernel image, provided by
the boot loader, or loaded from the file system.
Two data types are relevant for module:
1. The module_t type provides storage to describe a module.
2. The modinfo_t type resides within the module itself, and con-
tains module header info.
The module subsystem is protected by the global kernconfig_mutex.
MODULE(class, name, required)
The MODULE() macro creates and initializes a modinfo_t structure.
In addition to the explicit arguments, the MODULE() macro creates a
reference to the module's modcmd() function. This function is
int modcmd(modcmd_t cmd, void *data)
The cmd argument requests one of the following operations:
MODULE_CMD_INIT Perform module-specific initialization
when the module is loaded.
MODULE_CMD_FINI Perform module-specific clean-up
before the module is unloaded.
MODULE_CMD_AUTOUNLOAD Notify the module that it is about to
MODULE_CMD_STAT Request the module to provide status
information (not currently imple-
All modules' modcmd() functions must implement the MODULE_CMD_INIT
and MODULE_CMD_FINI commands. The other commands are optional, and
should return ENOTTY if not implemented.
For the MODULE_CMD_INIT command, the data argument is used to pass
a pointer to the module's prop_dictionary(3). For the
MODULE_CMD_STAT command, the data argument points to a buffer where
the status information should be placed.
The __link_set mechanism is used to enable the module subsystem to
locate the modinfo_t structure.
module_load(name, flags, props, class)
Load a module, link it into the running kernel, and call the mod-
ule's modcmd() routine with a cmd argument of MODULE_CMD_INIT. If
the specified module requires other modules, they are loaded first;
if any required module cannot be loaded or if any of their modcmd()
control routines returns a non-zero status, loading of this module
and the specific required module will fail. The required modules
are marked for automatic unloading. Thus, if the loading of the
module failed, the required modules will be automatically unloaded
after a short delay.
The loader will look first for a built-in module with the specified
name that has not been disabled (see module_unload() below). If a
built-in module with that name is not found, the list of modules
prepared by the boot loader is searched. If the named module is
still not found, an attempt is made to locate the module within the
file system, provided it has been mounted by the initialization
The flags argument can include:
MODCTL_NO_PROP When loading a module from the file sys-
tem, do not attempt to locate a corre-
sponding prop_dictionary file.
MODCTL_LOAD_FORCE Force loading of disabled built-in modules
and modules built for a different version
of the operating system.
The props argument points to an externalized property list which is
passed to the module's modcmd() routine. If a module is being
loaded from the file system, and the MODCTL_NO_PROP flag is not
set, the system searches for a file with the same name as the mod-
ule file, but with the suffix ``.plist''. If this file is found,
the prop_dictionary it contains is loaded and merged with the
prop_dictionary from the props argument.
The class argument can be any of:
MODULE_CLASS_DRIVER Device driver
MODULE_CLASS_EXEC Executable image handler
MODULE_CLASS_MISC Miscellaneous module
MODULE_CLASS_SECMODEL Security model (see secmodel(9) for
MODULE_CLASS_VFS Virtual file system
If the class is not MODULE_CLASS_ANY, the class of the module being
loaded must match the requested class. Except when verifying a
module's class when it is being loaded, module classes other than
MODULE_CLASS_SECMODEL are transparent to the module subsystem.
They are provided only for the benefit of the subsystem's clients.
Modules with class MODULE_CLASS_SECMODEL are automatically regis-
tered with secmodel_register() after being successfully loaded, and
automatically deregistered with secmodel_deregister() when being
The module_load() routine is primarily intended as the implementa-
tion of the MODCTL_LOAD option of the modctl(2) system call.
Auto-load a module, making it available for automatic unloading.
The name and class arguments are the same as for the module_load()
The module subsystem uses a kernel thread to attempt to automati-
cally unload modules a short time (currently, 10 seconds) after
being loaded by module_autoload(). Before the module is unloaded,
its modcmd() is called with the cmd argument specified as
MODULE_CMD_AUTOUNLOAD. A module can prevent itself from being
unloaded by returning a non-zero value.
The module_autoload() function is intended for use by kernel compo-
nents to locate and load optional system components. The function
is also used to load modules that are required by other modules.
The directory from which the module is loaded will be searched for
a file with the same name as the module file, but with the suffix
``.plist''. If this file is found, the prop_dictionary it contains
will be loaded and passed to the module's modcmd() routine. If
this prop_dictionary contains a ``noautoload'' property which is
set to ``true'' then the system will refuse to load the module.
Unload a module. If the module's reference count is non-zero, the
function returns EBUSY. Otherwise, the module's modcmd() routine
is called with a cmd argument of MODULE_CMD_FINI. If the modcmd()
routine returns with an error, then the error is returned to the
caller otherwise the module is unloaded.
The reference counts of all modules that were required by this mod-
ule are decremented, but the required modules are not unloaded by
the call to module_unload(). Instead, the required modules may be
unloaded by subsequent calls to module_unload().
Unloading a built-in module causes the module to be marked as dis-
abled. This prevents the module from being re-loaded, except by
the module_load() function with the flags argument set to
The module_unload() function may be called by the modctl(2) system
call, by the module subsystem's internal auto-unload thread, or by
other kernel facilities. Generally, other kernel facilities should
not be calling this function.
Load and initialize all available modules of the specified class.
Any built-in modules that have not been disabled, and any modules
provided by the boot loader are loaded.
Increment the reference count of a module. A module cannot be
unloaded if its reference count is non-zero.
Decrement the reference count of a module.
module_find_section(name, addr, size)
Find the start address and size of linker section name within a
module. The miniroot module uses this routine to find the address
and size of the embedded file system image. This routine can only
examine the linker data for the module that is currently being ini-
tialized; it cannot examine data for any other module.
Initialize the module subsystem. Creates and initializes various
data structures, locates all built-in modules, and establishes the
sub-system's sysctl(8) tree. module_init() is called early in sys-
tem initialization to facilitate use of security model modules.
Create the thread that attempts to automatically unload modules
that were loaded via the module_autoload() routine. The function
is called only once, after the scheduler and timer functions are
Mark as "disabled" any built-in modules that have not been success-
fully initialized. Modules marked "disabled" can only be loaded if
the MODCTL_LOAD_FORCE is specified. module_builtin_require_force()
is called near the end of system initialization, after the init(8)
process is created.
The module subsystem is initialized early, long before any file
systems are available. After the root file system is mounted,
module_load_vfs_init() is used to enable loading modules from the
file system. Until this routine is called, modules can only be
loaded if they were built-in to the kernel image or provided by the
The module subsystem is designed to be called recursively, but only
within a single LWP. This permits one module's modcmd() routine to load
or unload other modules.
· A module is not permitted to load or unload itself. Attempts
to load or unload a module from within its own modcmd() routine
will fail with EEXIST or EBUSY, respectively.
· Although a module can be loaded by using either module_load()
or module_autoload(), it is not possible for the module's
modcmd() routine to distinguish between the two methods. Any
module which needs to ensure that it does not get auto-unloaded
must either handle the MODULE_CMD_AUTOUNLOAD command in its
modcmd() routine, or use module_hold() to increment its refer-
ence count. Note however that modules loaded manually with
modload(8) are never auto-unloaded.
The core of the kernel module implementation is in sys/kern/kern_module.c
The routines for linking the module are in sys/kern/subr_kobj.c.
The routines for reading a module from the file system are in
The header file <sys/sys/module.h> describes the public interface.
In addition, each architecture is expected to provide kobj_machdep(),
kobj_reloc(), and module_init_md(). kobj_machdep() is for any machine
dependent actions, such as flushing caches, that are needed when a module
is loaded or unloaded. kobj_reloc() deals with resolution of relocatable
symbols. module_init_md() is for finding modules passed in by the boot
The kernel module subsystem first appeared in NetBSD 5.0. It replaces
the ``LKM'' subsystem from earlier releases.
The module system was written by Andrew Doran <ad@NetBSD.org>. This man-
ual page was written by Paul Goyette <pgoyette@NetBSD.org>.
NetBSD 6.0 October 18, 2011 NetBSD 6.0