FreeBSD模块开发最简单示例

FreeBSD运行时，能够将需要的功能编译到内核中去，这就是宏内核模式，但目前的FreeBSD内核支持动态内核的载入和卸载。能够在使用的时候，动态的将模块加载到内存中。
举个例子，可以将网卡驱动编译成单独的模块，而不是将网卡编导内核中去，这样内核能够减小体积，效率更高，在需要的时候可以动态的加载到内核中去。使用kldload命令，kldunload命令卸载模块。kldstat命令查看当前运行的模块。
下面给出一个最简单的模块实例：（/usr/src/share/example/kld）

/*-
* Copyright (c) 1999 Assar Westerlund
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* $FreeBSD: src/share/examples/kld/syscall/module/syscall.c,v 1.5 2007/07/22 06:48:34 kevlo Exp $
*/
#include sys/types.h>
#include sys/param.h>
#include sys/proc.h>
#include sys/module.h>
#include sys/sysproto.h>
#include sys/sysent.h>
#include sys/kernel.h>
#include sys/systm.h>
/*
* The function for implementing the syscall.
*/
static int
hello (struct thread *td, void *arg)
{
    printf ("hello kernel\n");
    return 0;
}
/*
* The `sysent' for the new syscall
*/
static struct sysent hello_sysent = {
    0,            /* sy_narg */
    hello            /* sy_call */
};
/*
* The offset in sysent where the syscall is allocated.
*/
static int offset = NO_SYSCALL;
/*
* The function called at load/unload.
*/
static int
load (struct module *module, int cmd, void *arg)

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