/* procfsreadxtime.c 
 *   Variation of Ch 5.1 proc file module from
 *   Linux Kernel Module Programmer's Guide for 2.4 Kernel
 * 
 *   Create /proc/xtime and supply xtime.tv_sec and xtime.tv_uxec upon read
 */

/* kernel module programming */
#ifndef MODULE
#define MODULE
#endif

#ifndef LINUX
#define LINUX
#endif

#ifndef __KERNEL__
#define __KERNEL__
#endif

#define MAX_BUF 80

#include <linux/module.h>    /* needed by all modules */
#include <linux/kernel.h>    /* needed for KERN_ALERT */
#include <linux/proc_fs.h>   /* working with the proc fs */


/* prototypes and externs*/

/* For the proper signaure of this function see the
 * /usr/src/linux/include/procfs.h with the definition
 * of the proc_dir_entry structure
 * and the read_proc function field in that structure
 */
int my_read_proc(char *sys_buffer,   /* system supplied buffer */
                 char **user_buffer, /* user provided buffer */
		 off_t file_pos,     /* position in proc file read */
		 int buflen,         /* for system buf_len ?? */
		 int *eof,           /* I could guess */
		 void *data);        /* no clue */
int init_module(void);
void cleanup_module(void);

/* I'm not sure where this really is. sched.h has this extern reference */
extern struct timeval xtime;

/* My function to supply the proc file read information */
int my_read_proc(char *sys_buffer,   /* system supplied buffer */
                 char **user_buffer, /* user provided buffer */
		 off_t file_pos,     /* position in proc file read */
		 int buflen,         /* buffer size (system buffer?) */
		 int *eof,           /* I could guess */
		 void *data)         /* no clue */
{ int len;  /* number of bytes actually used */

  static char my_buffer[MAX_BUF];
  static int count = 1;            /* access count */

  printk(KERN_ALERT "my_read_proc: entering.\n");

  /* Make sure we only respond once to a read call from the user. If we
   * are called directly from a user program that knows what this proc
   * file is intended to produce, then we won't get called twice. But
   * if we are called from a standard file read (such as doing a cat
   * on a proc file) then the standard file reading system code will
   * continue to issue a read until an EOF is returned. An EOF is
   * signaled by a return of a 0 (zero) length for the read. We can
   * use any indicator to determine if we have already been called
   * once. This example from the LKMPG just checks to see if the
   * file_pos has been moved at all. Simpler here might have been
   * just to see if our count was greater than zero.
   */
   
  if (file_pos > 0)
  { printk(KERN_ALERT "my_read_proc: leaving via EOF simulator.\n");
    return 0;
  }

  len = sprintf(my_buffer, "count: %d  xtime.tv_sec: %ld  xtime.tv_usec: %ld\n",
                            count, xtime.tv_sec, xtime.tv_usec);
  count++;

  *user_buffer = my_buffer;
  printk(KERN_ALERT "my_read_proc: leaving.\n");
  return len;    /* return the length of the string returned */
}

/* Directory structure for proc files (a psuedo inode?) 
 * This is created dynamically in kernel 2.4, see init_module below
 */
struct proc_dir_entry *my_proc_file;


/* create_proc_read_entry registers the read function. See proc_fs.h */
int init_module(void)
{ printk(KERN_ALERT "/proc/xtime module loaded\n");
  my_proc_file = create_proc_read_entry( "xtime",  /* proc file name */
                                          0444,   /* uid gid mode */
                                          NULL,   /* dir entry base */
					  my_read_proc, /* read function */
					  NULL);  /* data for function ?? */

  /* A non-zero return means init_module failed; module can't be loaded */
  if (my_proc_file == NULL)
    return -ENOMEM;
  return 0;
}

void cleanup_module(void)
{ printk(KERN_ALERT "/proc/xtime module unloaded\n");
  remove_proc_entry("xtime", NULL); /* file name and parent */
}

MODULE_LICENSE("GPL");