Hello!
I am trying to compile a character device kernel module, as
given in the example at
http://www.ibiblio.org/mdw/LDP/lkmpg/node14.html (attached
with this mail for reference).
When I compile the code on RHL7.3, I literally get hundreds
of errors. They begin with parse problem in module.h file. I
compile the code with -DMODULE -D__KERNEL__ -DLINUX -O2 flags.
Also, I checked my "linux/version.h" file and I found that the
kernel version there is not 2.4.18 but it was 2.4.9! How is
that? I couldn't get it. Ultimately, I changed the string
manually. But this does not help in any way.
I am desperate to be able to write kernel modules on linux, so
any help in this regard (compilation/documentation/information)
is highly appreciated!
Thanks!
Atul.
/* chardev.c
* Copyright (C) 1998-1999 by Ori Pomerantz
*
* Create a character device (read only)
*/
/* 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
/* For character devices */
#include <linux/fs.h> /* The character device
* definitions are here */
#include <linux/wrapper.h> /* A wrapper which does
* next to nothing at
* at present, but may
* help for compatibility
* with future versions
* of Linux */
/* 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
/* Conditional compilation. LINUX_VERSION_CODE is
* the code (as per KERNEL_VERSION) of this version. */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h> /* for put_user */
#endif
#define SUCCESS 0
/* Device Declarations **************************** */
/* The name for our device, as it will appear
* in /proc/devices */
#define DEVICE_NAME "char_dev"
/* The maximum length of the message from the device */
#define BUF_LEN 80
/* Is the device open right now? Used to prevent
* concurent access into the same device */
static int Device_Open = 0;
/* The message the device will give when asked */
static char Message[BUF_LEN];
/* How far did the process reading the message
* get? Useful if the message is larger than the size
* of the buffer we get to fill in device_read. */
static char *Message_Ptr;
/* This function is called whenever a process
* attempts to open the device file */
static int device_open(struct inode *inode,
struct file *file)
{
static int counter = 0;
#ifdef DEBUG
printk ("device_open(%p,%p)\n", inode, file);
#endif
/* This is how you get the minor device number in
* case you have more than one physical device using
* the driver. */
printk("Device: %d.%d\n",
inode->i_rdev >> 8, inode->i_rdev & 0xFF);
/* We don't want to talk to two processes at the
* same time */
if (Device_Open)
return -EBUSY;
/* If this was a process, we would have had to be
* more careful here.
*
* In the case of processes, the danger would be
* that one process might have check Device_Open
* and then be replaced by the schedualer by another
* process which runs this function. Then, when the
* first process was back on the CPU, it would assume
* the device is still not open.
*
* However, Linux guarantees that a process won't be
* replaced while it is running in kernel context.
*
* In the case of SMP, one CPU might increment
* Device_Open while another CPU is here, right after
* the check. However, in version 2.0 of the
* kernel this is not a problem because there's a lock
* to guarantee only one CPU will be kernel module at
* the same time. This is bad in terms of
* performance, so version 2.2 changed it.
* Unfortunately, I don't have access to an SMP box
* to check how it works with SMP.
*/
Device_Open++;
/* Initialize the message. */
sprintf(Message,
"If I told you once, I told you %d times - %s",
counter++,
"Hello, world\n");
/* The only reason we're allowed to do this sprintf
* is because the maximum length of the message
* (assuming 32 bit integers - up to 10 digits
* with the minus sign) is less than BUF_LEN, which
* is 80. BE CAREFUL NOT TO OVERFLOW BUFFERS,
* ESPECIALLY IN THE KERNEL!!!
*/
Message_Ptr = Message;
/* Make sure that the module isn't removed while
* the file is open by incrementing the usage count
* (the number of opened references to the module, if
* it's not zero rmmod will fail)
*/
MOD_INC_USE_COUNT;
return SUCCESS;
}
/* This function is called when a process closes the
* device file. It doesn't have a return value in
* version 2.0.x because it can't fail (you must ALWAYS
* be able to close a device). In version 2.2.x it is
* allowed to fail - but we won't let it.
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static int device_release(struct inode *inode,
struct file *file)
#else
static void device_release(struct inode *inode,
struct file *file)
#endif
{
#ifdef DEBUG
printk ("device_release(%p,%p)\n", inode, file);
#endif
/* We're now ready for our next caller */
Device_Open --;
/* Decrement the usage count, otherwise once you
* opened the file you'll never get rid of the module.
*/
MOD_DEC_USE_COUNT;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return 0;
#endif
}
/* This function is called whenever a process which
* have already opened the device file attempts to
* read from it. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t device_read(struct file *file,
char *buffer, /* The buffer to fill with data */
size_t length, /* The length of the buffer */
loff_t *offset) /* Our offset in the file */
#else
static int device_read(struct inode *inode,
struct file *file,
char *buffer, /* The buffer to fill with
* the data */
int length) /* The length of the buffer
* (mustn't write beyond that!) */
#endif
{
/* Number of bytes actually written to the buffer */
int bytes_read = 0;
/* If we're at the end of the message, return 0
* (which signifies end of file) */
if (*Message_Ptr == 0)
return 0;
/* Actually put the data into the buffer */
while (length && *Message_Ptr) {
/* Because the buffer is in the user data segment,
* not the kernel data segment, assignment wouldn't
* work. Instead, we have to use put_user which
* copies data from the kernel data segment to the
* user data segment. */
put_user(*(Message_Ptr++), buffer++);
length --;
bytes_read ++;
}
#ifdef DEBUG
printk ("Read %d bytes, %d left\n",
bytes_read, length);
#endif
/* Read functions are supposed to return the number
* of bytes actually inserted into the buffer */
return bytes_read;
}
/* This function is called when somebody tries to write
* into our device file - unsupported in this example. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t device_write(struct file *file,
const char *buffer, /* The buffer */
size_t length, /* The length of the buffer */
loff_t *offset) /* Our offset in the file */
#else
static int device_write(struct inode *inode,
struct file *file,
const char *buffer,
int length)
#endif
{
return -EINVAL;
}
/* Module Declarations ***************************** */
/* The major device number for the device. This is
* global (well, static, which in this context is global
* within this file) because it has to be accessible
* both for registration and for release. */
static int Major;
/* This structure will hold the functions to be
* called when a process does something to the device
* we created. Since a pointer to this structure is
* kept in the devices table, it can't be local to
* init_module. NULL is for unimplemented functions. */
struct file_operations Fops = {
NULL, /* seek */
device_read,
device_write,
NULL, /* readdir */
NULL, /* select */
NULL, /* ioctl */
NULL, /* mmap */
device_open,
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
NULL, /* flush */
#endif
device_release /* a.k.a. close */
};
/* Initialize the module - Register the character device */
int init_module()
{
/* Register the character device (atleast try) */
Major = module_register_chrdev(0,
DEVICE_NAME,
&Fops);
/* Negative values signify an error */
if (Major < 0) {
printk ("%s device failed with %d\n",
"Sorry, registering the character",
Major);
return Major;
}
printk ("%s The major device number is %d.\n",
"Registeration is a success.",
Major);
printk ("If you want to talk to the device driver,\n");
printk ("you'll have to create a device file. \n");
printk ("We suggest you use:\n");
printk ("mknod <name> c %d <minor>\n", Major);
printk ("You can try different minor numbers %s",
"and see what happens.\n");
return 0;
}
/* Cleanup - unregister the appropriate file from /proc */
void cleanup_module()
{
int ret;
/* Unregister the device */
ret = module_unregister_chrdev(Major, DEVICE_NAME);
/* If there's an error, report it */
if (ret < 0)
printk("Error in unregister_chrdev: %d\n", ret);
}
