linux内核模块编程5

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Chapter 5. The /proc File System
关于 /proc 文件系统
在Linux中有另一种内核和内核模块向进程传递信息的方法，那就是通过 /proc文件系统。它原先设计的目的是为查看进程信息提供一个方便的途径，现在它被用来向用户提供各种内核中被感兴趣的内容。像文件 /proc/modules里是已加载模块的列表，文件/proc/meminfo 里是关于内存使用的信息。

使用 proc 文件系统的方法同使用设备文件很相似。你建立一个包含 /proc文件需要的所有信息的结构体，这其中包括处理各种事务的函数的指针（在我们的例子中，只用到从/proc文件读取信息的函数）。然后在init_module 时向内核注册这个结构体，在cleanup_module时注销这个结构体。

我们使用proc_register_dynamic[1]的原因是我们不用去设置inode，而留给内核去自动分配从而避免系统冲突错误。普通的文件系统是建立在磁盘上的，而 /proc 的文件仅仅是建立在内存中的。在前种情况中，inode的数值是一个指向存储在磁盘某个位置的文件的索引节点（inode就是index-node的缩写）。该索引节点储存着文件的信息，像文件的权限；同时还有在哪儿能找到文件中的数据。

因为我们无法得知该文件是被打开的或关闭的，我们也无法去使用宏 try_module_get和try_module_put在下面的模块中，我们无法避免该文件被打开而同时模块又被卸载。在下章中我将介绍一个较难实现，却更灵活，更安全的处理 /proc文件的方法。

Example 5-1. procfs.c

/*
*  procfs.c -  create a "file" in /proc
*/

#include <linux/module.h> /* Specifically, a module */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/proc_fs.h> /* Necessary because we use the proc fs */

struct proc_dir_entry *Our_Proc_File;

/* Put data into the proc fs file.
*
* Arguments
* =========
* 1. The buffer where the data is to be inserted, if
* you decide to use it.
* 2. A pointer to a pointer to characters. This is
* useful if you don't want to use the buffer
* allocated by the kernel.
* 3. The current position in the file
* 4. The size of the buffer in the first argument.
* 5. Write a "1" here to indicate EOF.
* 6. A pointer to data (useful in case one common
* read for multiple /proc/... entries)
*
* Usage and Return Value
* ======================
* A return value of zero means you have no further
* information at this time (end of file). A negative
* return value is an error condition.
*
* For More Information
* ====================
* The way I discovered what to do with this function
* wasn't by reading documentation, but by reading the
* code which used it. I just looked to see what uses
* the get_info field of proc_dir_entry struct (I used a
* combination of find and grep, if you're interested),
* and I saw that  it is used in <kernel source
* directory>/fs/proc/array.c.
*
* If something is unknown about the kernel, this is
* usually the way to go. In Linux we have the great
* advantage of having the kernel source code for
* free - use it.
*/
ssize_t
procfile_read(char *buffer,
      char **buffer_location,
      off_t offset, int buffer_length, int *eof, void *data)
{
printk(KERN_INFO "inside /proc/test : procfile_read\n" );

int len = 0; /* The number of bytes actually used */
static int count = 1;

/*
   * We give all of our information in one go, so if the
   * user asks us if we have more information the
   * answer should always be no.
   *
   * This is important because the standard read
   * function from the library would continue to issue
   * the read system call until the kernel replies
   * that it has no more information, or until its
   * buffer is filled.
   */
if (offset > 0) {
printk(KERN_INFO "offset %d : /proc/test : procfile_read, \
         wrote %d Bytes\n", (int)(offset), len);
*eof = 1;
return len;
}

/*
   * Fill the buffer and get its length
   */
len = sprintf(buffer,
      "For the %d%s time, go away!\n", count,
      (count % 100 > 10 && count % 100 < 14) ? "th" :
      (count % 10 == 1) ? "st" :
      (count % 10 == 2) ? "nd" :
      (count % 10 == 3) ? "rd" : "th" );
count++;

/*
   * Return the length
   */
printk(KERN_INFO
         "leaving /proc/test : procfile_read, wrote %d Bytes\n", len);
return len;
}

int init_module()
{
int rv = 0;
Our_Proc_File = create_proc_entry("test", 0644, NULL);
Our_Proc_File->read_proc = procfile_read;
Our_Proc_File->owner = THIS_MODULE;
Our_Proc_File->mode = S_IFREG | S_IRUGO;
Our_Proc_File->uid = 0;
Our_Proc_File->gid = 0;
Our_Proc_File->size = 37;

printk(KERN_INFO "Trying to create /proc/test:\n" );

if (Our_Proc_File == NULL) {
rv = -ENOMEM;
remove_proc_entry("test", &proc_root);
printk(KERN_INFO "Error: Could not initialize /proc/test\n" );
} else {
printk(KERN_INFO "Success!\n" );
}

return rv;
}

void cleanup_module()
{
remove_proc_entry("test", &proc_root);
printk(KERN_INFO "/proc/test removed\n" );
}
Notes
[1] 这是在2.0版本中的做法，在版本2.2中，当我们把inode设为0时，就已经这样自动处理了。