Many parts of the kernel are well served as a module
(dynamically-loadable parts of the kernel). Using the
module_exit() macros it is easy to write code
without #ifdefs which can operate both as a module or built into
module_init() macro defines which
function is to be called at module insertion time (if the file is
compiled as a module), or at boot time: if the file is not
compiled as a module the
becomes equivalent to
through linker magic ensures that the function is called on boot.
The function can return a negative error number to cause module loading to fail (unfortunately, this has no effect if the module is compiled into the kernel). This function is called in user context with interrupts enabled, so it can sleep.