9.2. Dynamic Kernel Linker Facility - KLD
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## 9.2. Dynamic Kernel Linker Facility - KLD

The kld interface allows system administrators to dynamically add and remove functionality from a running system. This allows device driver writers to load their new changes into a running kernel without constantly rebooting to test changes.

The kld interface is used through:

- `kldload` - loads a new kernel module
- `kldunload` - unloads a kernel module
- `kldstat` - lists loaded modules

Skeleton Layout of a kernel module

/* * KLD Skeleton * Inspired by Andrew Reiter's Daemonnews article */ #include <sys/types.h> #include <sys/module.h> #include <sys/systm.h> /* uprintf */ #include <sys/errno.h> #include <sys/param.h> /* defines used in kernel.h */ #include <sys/kernel.h> /* types used in module initialization */ /* * Load handler that deals with the loading and unloading of a KLD. */ static int skel_loader(struct module *m, int what, void *arg) { int err = 0; switch (what) { case MOD_LOAD: /* kldload */ uprintf("Skeleton KLD loaded.\n"); break; case MOD_UNLOAD: uprintf("Skeleton KLD unloaded.\n"); break; default: err = EOPNOTSUPP; break; } return(err); } /* Declare this module to the rest of the kernel */ static moduledata_t skel_mod = { "skel", skel_loader, NULL }; DECLARE_MODULE(skeleton, skel_mod, SI_SUB_KLD, SI_ORDER_ANY);

### 9.2.1. Makefile

FreeBSD provides a system makefile to simplify compiling a kernel module.

SRCS=skeleton.c KMOD=skeleton .include <bsd.kmod.mk>

Running `make` with this makefile will create a file `skeleton.ko` that can be loaded into the kernel by typing:

`#` `kldload -v ./skeleton.ko`

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