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");
}