linux 驱动编写之虚拟字符设备的编写实例详解
<p><strong> linux 驱动编写</strong></p>
<p>
<strong>前言：</strong></p>
<p>
昨天我们说了一些简单模块编写方法，但是终归没有涉及到设备的编写内容，今天我们就可以了解一下相关方面的内容，并且用一个实例来说明在Linux上面设备是如何编写的。虽然我不是专门做linux驱动的，却也经常收到一些朋友们的来信。在信件中，很多做驱动的朋友对自己的工作不是很满意，认为自己的工作就是把代码拷贝来拷贝去，或者说是改来改去，没有什么技术含量。有这种想法的朋友不在少数，我想这主要还是因为他们对自己的工作缺少了解导致。如果有可能，我们可以问问自己这样几个问题：</p>
<p>
（1）我真的搞懂设备的开发驱动流程了吗？我是否可以从0开始，编写一个独立的驱动代码呢？<br>
（2）我真的了解设备的初始化、关闭、运行的流程吗？<br>
（3）当前的设备驱动流程是否合理，有没有可以改进的地方？<br>
（4）对于内核开发中涉及的api调用，我自己是否真正了解、是否明白它们在使用上有什么区别？<br>
（5）如果我要驱动的设备只是在一个前后台系统中运行，在没有框架帮助的情况下，我是否有信心把它启动和运行起来？</p>
<p>
当然，上面的内容只是我个人的想法，也不一定都正确。但是，知其然，更要知其所以然，熟悉了当前开发流程的优缺点才能真正掌握和了解驱动开发的本质。这听上去有些玄乎，其实也很简单，就是要有一种刨根问底、不断改进的精神，这样才能做好自己的工作。因为我们是在pc linux上学习驱动的，因此暂时没有真实的外接设备可以使用，但是这丝毫不影响我们学习的热情。通过定时器、进程，我们可以仿真出真实设备的各种需求，所以对于系统来说，它是无所谓真设备、假设备的，基本的处理流程对它来说都是一样的。只要大家一步一步做下去，肯定可以了解linux驱动设备的开发工程的。</p>
<p>
下面，为了说明问题，我们可以编写一段简单的char设备驱动代码，文件名为char.c，</p>
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<code class="cpp preprocessor">#include <linux/module.h> </code>
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<code class="cpp preprocessor">#include <linux/kernel.h> </code>
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<code class="cpp preprocessor">#include <linux/fs.h> </code>
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<code class="cpp preprocessor">#include <linux/cdev.h> </code>
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<code class="cpp spaces"> </code>
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<code class="cpp keyword bold">static</code> <code class="cpp keyword bold">struct</code> <code class="cpp plain">cdev chr_dev; </code>
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<code class="cpp keyword bold">static</code> <code class="cpp plain">dev_t ndev; </code>
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<code class="cpp spaces"> </code>
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<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">chr_open(</code><code class="cpp keyword bold">struct</code> <code class="cpp plain">inode* nd, </code><code class="cpp keyword bold">struct</code> <code class="cpp plain">file* filp) </code>
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<code class="cpp plain">{ </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">major ; </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">minor; </code>
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<code class="cpp spaces"> </code>
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<code class="cpp spaces"> </code><code class="cpp plain">major = MAJOR(nd->i_rdev); </code>
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<code class="cpp spaces"> </code><code class="cpp plain">minor = MINOR(nd->i_rdev); </code>
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<code class="cpp spaces"> </code>
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<code class="cpp spaces"> </code><code class="cpp plain">printk(</code><code class="cpp string">"chr_open, major = %d, minor = %d\n"</code><code class="cpp plain">, major, minor); </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0; </code>
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<code class="cpp plain">} </code>
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<code class="cpp spaces"> </code>
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<code class="cpp keyword bold">static</code> <code class="cpp plain">ssize_t chr_read(</code><code class="cpp keyword bold">struct</code> <code class="cpp plain">file* filp, </code><code class="cpp color1 bold">char</code> <code class="cpp plain">__user* u, </code><code class="cpp color1 bold">size_t</code> <code class="cpp plain">sz, loff_t* off) </code>
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<code class="cpp plain">{ </code>
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<code class="cpp spaces"> </code><code class="cpp plain">printk(</code><code class="cpp string">"chr_read process!\n"</code><code class="cpp plain">); </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0; </code>
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<code class="cpp plain">} </code>
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<code class="cpp spaces"> </code>
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<code class="cpp keyword bold">struct</code> <code class="cpp plain">file_operations chr_ops = { </code>
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<code class="cpp spaces"> </code><code class="cpp plain">.owner = THIS_MODULE, </code>
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<code class="cpp spaces"> </code><code class="cpp plain">.open = chr_open, </code>
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<code class="cpp spaces"> </code><code class="cpp plain">.read = chr_read </code>
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<code class="cpp plain">}; </code>
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<code class="cpp spaces"> </code>
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<code class="cpp keyword bold">static</code> <code class="cpp color1 bold">int</code> <code class="cpp plain">demo_init(</code><code class="cpp keyword bold">void</code><code class="cpp plain">) </code>
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<code class="cpp plain">{ </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">ret; </code>
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<code class="cpp spaces"> </code>
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<code class="cpp spaces"> </code><code class="cpp plain">cdev_init(&chr_dev, &chr_ops); </code>
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<code class="cpp spaces"> </code><code class="cpp plain">ret = alloc_chrdev_region(&ndev, 0, 1, </code><code class="cpp string">"chr_dev"</code><code class="cpp plain">); </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(ret < 0 ) </code>
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<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">ret; </code>
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<code class="cpp spaces"> </code><code class="cpp plain">} </code>
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<code class="cpp spaces"> </code>
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<code class="cpp spaces"> </code><code class="cpp plain">printk(</code><code class="cpp string">"demo_init(): major = %d, minor = %d\n"</code><code class="cpp plain">, MAJOR(ndev), MINOR(ndev)); </code>
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<code class="cpp spaces"> </code><code class="cpp plain">ret = cdev_add(&chr_dev, ndev, 1); </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(ret < 0) </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">ret; </code>
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<code class="cpp spaces"> </code><code class="cpp plain">} </code>
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<code class="cpp spaces"> </code>
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<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0; </code>
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<code class="cpp plain">} </code>
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<code class="cpp spaces"> </code>
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<code class="cpp keyword bold">static</code> <code class="cpp keyword bold">void</code> <code class="cpp plain">demo_exit(</code><code class="cpp keyword bold">void</code><code class="cpp plain">) </code>
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<code class="cpp plain">{ </code>
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<code class="cpp spaces"> </code><code class="cpp plain">printk(</code><code class="cpp string">"demo_exit process!\n"</code><code class="cpp plain">); </code>
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<code class="cpp spaces"> </code><code class="cpp plain">cdev_del(&chr_dev); </code>
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<code class="cpp spaces"> </code><code class="cpp plain">unregister_chrdev_region(ndev, 1); </code>
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<code class="cpp plain">} </code>
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<code class="cpp spaces"> </code>
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<code class="cpp plain">module_init(demo_init); </code>
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<code class="cpp plain">module_exit(demo_exit); </code>
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<code class="cpp spaces"> </code>
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<code class="cpp plain">MODULE_LICENSE(</code><code class="cpp string">"GPL"</code><code class="cpp plain">); </code>
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<code class="cpp plain">MODULE_AUTHOR(</code><code class="cpp string">"feixiaoxing@163.com"</code><code class="cpp plain">); </code>
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<code class="cpp plain">MODULE_DESCRIPTION(</code><code class="cpp string">"A simple device example!"</code><code class="cpp plain">); </code>
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<p>
在module_init中的函数是模块加载时处理的函数，而模块卸载的函数则是在module_exit中。每一个设备都要对应一个基本的设备数据，当然为了使得这个设备注册在整个系统当中，我们还需要分配一个设备节点，alloc_chrdev_region就完成这样一个功能。等到cdev_add的时候，整个设备注册的过程就全部完成了，就是这么简单。当然为了编写这个文件，我们还需要编写一个Makefile文件，</p>
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<code class="cpp plain">ifneq ($(KERNELRELEASE),) </code>
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<code class="cpp plain">obj-m := </code><code class="cpp color1 bold">char</code><code class="cpp plain">.o </code>
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<code class="cpp keyword bold">else</code>
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<code class="cpp plain">PWD := $(shell pwd) </code>
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<code class="cpp plain">KVER := $(shell uname -r) </code>
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<code class="cpp plain">KDIR := /lib/modules/$(KVER)/build </code>
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<code class="cpp plain">all: </code>
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<code class="cpp spaces"> </code><code class="cpp plain">$(MAKE) -C $(KDIR) M=$(PWD) modules </code>
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<code class="cpp plain">clean: </code>
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<code class="cpp spaces"> </code><code class="cpp plain">rm -rf .*.cmd *.o *.mod.c *.ko .tmp_versions modules.* Module.* </code>
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<code class="cpp plain">endif </code>
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<p>
这个Makefile文件和我们之前编写的makefile基本上没有区别，唯一的区别就是文件名称改成了char.o，仅此而已。为了编写模块，我们直接输入make即可。这时候，char.ko文件就可以生成了。然后，模块需要被注册在系统当中，insmod char.ko是少不了的。如果此时，我们还不确信是否模块已经加入到系统当中，完全可以通过输入lsmod | grep char进行查找和验证。为了创建设备节点，我们需要知道设备为我们创建的major、minor数值是多少，所以dmesg | tail 查找一下数值。在我hp的机器上，这两个数值分别是249和0，所以下面可以利用它们直接创建设备节点了，输入mknod /dev/chr_dev c 249 0即可，此时可以输入ls /dev/chr_dev验证一下。那么，按照这种方法，真的可以访问这个虚拟设备了吗，我们可以编写一段简单的代码验证一下，</p>
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<code class="cpp preprocessor">#include <stdio.h> </code>
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<code class="cpp preprocessor">#include <fcntl.h> </code>
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<code class="cpp preprocessor">#include <unistd.h> </code>
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<code class="cpp spaces"> </code>
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<code class="cpp preprocessor">#define CHAR_DEV_NAME "/dev/chr_dev" </code>
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<code class="cpp color1 bold">int</code> <code class="cpp plain">main() </code>
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<code class="cpp plain">{ </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">ret; </code>
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<code class="cpp spaces"> </code><code class="cpp color1 bold">int</code> <code class="cpp plain">fd; </code>
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<div class="line number11 index10 alt2">
<code class="cpp spaces"> </code><code class="cpp color1 bold">char</code> <code class="cpp plain">buf; </code>
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<div class="line number12 index11 alt1">
<code class="cpp spaces"> </code>
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<div class="line number13 index12 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">fd = open(CHAR_DEV_NAME, O_RDONLY | O_NDELAY); </code>
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<div class="line number14 index13 alt1">
<code class="cpp spaces"> </code><code class="cpp keyword bold">if</code><code class="cpp plain">(fd < 0) </code>
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<div class="line number15 index14 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">{ </code>
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<div class="line number16 index15 alt1">
<code class="cpp spaces"> </code><code class="cpp functions bold">printf</code><code class="cpp plain">(</code><code class="cpp string">"open failed!\n"</code><code class="cpp plain">); </code>
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<div class="line number17 index16 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">-1; </code>
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<div class="line number18 index17 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">} </code>
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<div class="line number19 index18 alt2">
<code class="cpp spaces"> </code>
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<div class="line number20 index19 alt1">
<code class="cpp spaces"> </code><code class="cpp plain">read(fd, buf, 32); </code>
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<div class="line number21 index20 alt2">
<code class="cpp spaces"> </code><code class="cpp plain">close(fd); </code>
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<div class="line number22 index21 alt1">
<code class="cpp spaces"> </code>
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<div class="line number23 index22 alt2">
<code class="cpp spaces"> </code><code class="cpp keyword bold">return</code> <code class="cpp plain">0; </code>
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<div class="line number24 index23 alt1">
<code class="cpp plain">} </code>
</div>
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<div class="codetool" id="codetool">
<div class="code_n">
<textarea></textarea>
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<p>
代码的内容非常简单，就是利用CHAR_DEV_NAME直接打开设备，读写设备。当然。首先还是需要对这个文件进行编译，文件名为test.c，输入gcc test.c -o test，其次就是运行这个文件，直接输入./test即可。如果没有问题的话，那么说明我们的代码是ok的，但是我们还是没有看到任何内容。没关系，我们还是通过dmesg这个命令查看内核中是否存在相关的打印内容，直接输入dmesg | tail即可。此时如果没有意外的话，我们就可以看到之前在chr_open和chr_read中留下的printk打印，这说明我们的代码完全是ok的。</p>
<p>
上面的代码只是一段小例子，真实的内容要比这复杂一下。不过既然我们都已经入门了，那么后面的内容其实也没有什么好怕的了。最后有两个事情补充一下：（1）如果大家在创建节点后想删除设备节点，直接rm -rf /dev/chr_dev即可；（2）上面这段代码的原型来自于《深入linux设备驱动程序内核机制》这本书，稍作修改，如果大家对内核机制的内容感兴趣，可以参考这本书的内容。</p>
<p>
感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！</p>
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