KLD

It is a Dynamic Kernel Linker facility unique to FreeBSD >= 4.0 (similar to Linux's and NetBSD/OpenBSD's lkm). It 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 the following commands:

• kldload - loads a new kernel module


• kldunload - (guess what..) unloads a kernel module


• kldstat - lists the currently loaded modules

Here is the 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 = EINVAL;
        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);

In order to compile a module from source, a Makefile is needed:

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

Simply running make with this Makefile will create a file skeleton.ko that can be loaded into your system by typing:

# kldload -v ./skeleton.ko