proc 文件系统

在Linux中，内核和内核模块有一个额外的机制来向进程发送信息——/proc文件系统。最初的设计是为了方便访问有关进程的信息(因此得名)，现在它被内核大量使用，例如/proc/modules有模块列表，/proc/meminfo有内存使用统计。

使用proc文件系统的方法与使用设备驱动程序的方法非常相似——您创建一个结构，其中包含/proc文件所需的所有信息，包括指向任何处理程序函数的指针(在我们的示例中，只有一个处理程序函数，当有人试图从/proc文件中读取时调用)。然后，init_module向内核注册这个结构，cleanup_module注销它。

我们使用proc_register_dynamic3.1的原因是我们不想提前确定文件使用的inode号，而是允许内核确定它以防止冲突。普通文件系统位于磁盘上，而不仅仅位于内存中(/proc位于内存中)，在这种情况下，inode号是指向文件索引节点(简称inode)所在的磁盘位置的指针。inode包含有关文件的信息，例如文件的权限，以及指向磁盘位置的指针，或者可以找到文件数据的位置。

因为当文件被打开或关闭时我们没有被调用，所以我们没有在这个模块中使用MOD_INC_USE_COUNT和MOD_DEC_USE_COUNT的，如果文件被打开，然后模块被删除，就可能导致不可预知的问题。在下一章中，我们将看到一种更难实现，但更灵活的处理/proc文件的方法，它也将允许我们防止这个问题。

procfs.c

/* procfs.c -  create a "file" in /proc
 * Copyright (C) 1998-1999 by Ori Pomerantz
 */


/* The necessary header files */

/* Standard in kernel modules */
#include <linux/kernel.h>   /* We're doing kernel work */
#include <linux/module.h>   /* Specifically, a module */

/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif

/* Necessary because we use the proc fs */
#include <linux/proc_fs.h>

/* In 2.2.3 /usr/include/linux/version.h includes a
 * macro for this, but 2.0.35 doesn't - so I add it
 * here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif

/* 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. Zero (for future use?).


   Usage and Return Value
   ======================
   If you use your own buffer, like I do, put its
   location in the second argument and return the
   number of bytes used in the buffer.

   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.
 */
int procfile_read(char *buffer,
                  char **buffer_location,
                  off_t offset,
                  int buffer_length,
                  int zero)
{
  int len;  /* The number of bytes actually used */

  /* This is static so it will still be in memory
   * when we leave this function */
  static char my_buffer[80];

  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)
    return 0;

  /* Fill the buffer and get its length */
  len = sprintf(my_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++;

  /* Tell the function which called us where the
   * buffer is */
  *buffer_location = my_buffer;

  /* Return the length */
  return len;
}


struct proc_dir_entry Our_Proc_File =
  {
    0, /* Inode number - ignore, it will be filled by
        * proc_register[_dynamic] */
    4, /* Length of the file name */
    "test", /* The file name */
    S_IFREG | S_IRUGO, /* File mode - this is a regular
                        * file which can be read by its
                        * owner, its group, and everybody
                        * else */
    1,  /* Number of links (directories where the
         * file is referenced) */
    0, 0,  /* The uid and gid for the file - we give it
            * to root */
    80, /* The size of the file reported by ls. */
    NULL, /* functions which can be done on the inode
           * (linking, removing, etc.) - we don't
           * support any. */
    procfile_read, /* The read function for this file,
                    * the function called when somebody
                    * tries to read something from it. */
    NULL /* We could have here a function to fill the
          * file's inode, to enable us to play with
          * permissions, ownership, etc. */
  };



/* Initialize the module - register the proc file */
int init_module()
{
  /* Success if proc_register[_dynamic] is a success,
   * failure otherwise. */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
  /* In version 2.2, proc_register assign a dynamic
   * inode number automatically if it is zero in the
   * structure , so there's no more need for
   * proc_register_dynamic
   */
  return proc_register(&proc_root, &Our_Proc_File);
#else
  return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif

  /* proc_root is the root directory for the proc
   * fs (/proc). This is where we want our file to be
   * located.
   */
}


/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
  proc_unregister(&proc_root, Our_Proc_File.low_ino);
}