Character device I/O across users - c

I'm learning Linux and I have the task that I can't handle. I have to create character device and make possible to read and write across users. For instance, I open first terminal and I use echo 'test' > /tmp/ringdev (where ringdev is my character device). I also open second terminal and I use cat /tmp/ringdev and I expect that output will be test. I have already read some example snippets, but all of them are just about the basics which I already have.
The question is how to read from / write to character device across users?
#define pr_fmt(fmt) "ringdev: " fmt
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/miscdevice.h>
#include <linux/sched.h>
#include <linux/uaccess.h>
#include <linux/wait.h>
#include <linux/sched.h>
DECLARE_WAIT_QUEUE_HEAD(head);
/*
* mutex used for access synchronization to buffer (ringdev_buf and ringdev_len)
*/
static struct mutex ringdev_lock;
/*
* buffer and number of written bytes in the buffer
*/
static char ringdev_buf[4096];
static size_t ringdev_len;
static int ringdev_open(struct inode *inode, struct file *filp)
{
printk("device_open called \n");
return 0;
}
static ssize_t ringdev_read(struct file *filp, char __user *buf, size_t count,
loff_t *off)
{
ssize_t ret = 0;
printk("device_read called \n");
/*
* access to ringdev_buf i ringdev_len is protected by ringdev_lock,
* take that lock
*/
//wait_event_interruptible(head,ringdev_len!=0);
mutex_lock(&ringdev_lock);
if (*off > ringdev_len)
count = 0;
else if (count >= ringdev_len - *off)
count = ringdev_len - *off;
/*
* for access to user memory special functions must be used,
* to copy to user memory copy_to_user must be used.
*/
ret = -EFAULT;
if (copy_to_user(buf, ringdev_buf + *off, count))
goto out_unlock;
ret = count;
*off += ret;
out_unlock:
mutex_unlock(&ringdev_lock);
ringdev_len=0;
return ret;
}
static ssize_t ringdev_write(struct file *filp, const char __user *buf,
size_t count, loff_t *off)
{
ssize_t ret=0;
ringdev_len=0;
printk("device_write called \n");
mutex_lock(&ringdev_lock);
ret=-EFAULT;
if(copy_from_user(ringdev_buf+ringdev_len ,buf, count)==0){
ringdev_len= ringdev_len + count;
ret=count;
wake_up_interruptible(&head);
goto out_unlock;
}
else{
ret=-ENOSPC;
}
/*
* not supported yet. Do not forget about copy_from_user().
*/
out_unlock:
mutex_unlock(&ringdev_lock);
return ret;
}
static int ringdev_release(struct inode *inode, struct file *filp)
{
mutex_unlock(&ringdev_lock);
printk("device_release called \n");
return 0;
}
static const struct file_operations ringdev_fops = {
.owner = THIS_MODULE,
.open = ringdev_open,
.read = ringdev_read,
.write = ringdev_write,
.release = ringdev_release,
};
static struct miscdevice ringdev_miscdevice = {
.minor = MISC_DYNAMIC_MINOR,
.name = "ringdev",
.fops = &ringdev_fops
};
static int __init ringdev_init(void)
{
int ret;
mutex_init(&ringdev_lock);
ret = misc_register(&ringdev_miscdevice);
if (ret < 0) {
pr_err("can't register miscdevice.\n");
return ret;
}
pr_info("minor %d\n", ringdev_miscdevice.minor);
return 0;
}
static void __exit ringdev_exit(void)
{
misc_deregister(&ringdev_miscdevice);
mutex_destroy(&ringdev_lock);
}
module_init(ringdev_init);
module_exit(ringdev_exit);
MODULE_DESCRIPTION("Device");
MODULE_AUTHOR("xxx");
MODULE_LICENSE("GPL");

Related

How to send signal from Linux kernel space to user space in order to notify about an input hardware event

My kernel module code needs to send signal to a user land program, to transfer its execution to registered signal handler.
In fact, I have developed a C program for my embedded board which make LED turns on and off when I push BUTTON ( the input event ). On the other hand, I have just developed a simple Linux module with its basic functions ( OPEN, CLOSE, READ, WRITE ).
I just don't have any idea how to modify my principal program and my kernel module in order to arrive to my objective.
I share with you my user space program :
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <sys/select.h>
#include <sys/time.h>
#include <errno.h>
#include <linux/input.h>
#define BTN_FILE_PATH "/dev/input/event0"
#define LED_PATH "/sys/class/leds"
#define green "green"
void change_led_state(char *led_path, int led_value)
{
char lpath[64];
FILE *led_fd;
strncpy(lpath, led_path, sizeof(lpath) - 1);
lpath[sizeof(lpath) - 1] = '\0';
led_fd = fopen(lpath, "w");
if (led_fd == NULL) {
fprintf(stderr, "simplekey: unable to access led\n");
return;
}
fprintf(led_fd, "%d\n", led_value);
fclose(led_fd);
}
void reset_leds(void)
{
change_led_state(LED_PATH "/" green "/brightness", 0);
}
int configure_leds(void)
{
FILE *r_fd;
char *none_str = "none";
/* Configure leds for hand control */
r_fd = fopen(LED_PATH "/" green "/trigger", "w");
fprintf(r_fd, "%s\n", none_str);
fclose(r_fd);
/* Switch off leds */
reset_leds();
return 0;
}
void eval_keycode(int code)
{
static int green_state = 0;
switch (code) {
case 260:
printf("BTN left pressed\n");
/* figure out green state */
green_state = green_state ? 0 : 1;
change_led_state(LED_PATH "/" green "/brightness", green_state);
break;
}
}
int main(void)
{
int file;
/* how many bytes were read */
size_t rb;
int ret;
int yalv;
/* the events (up to 64 at once) */
struct input_event ev[64];
char *str = BTN_FILE_PATH;
printf("Starting simplekey app\n");
ret = configure_leds();
if (ret < 0)
exit(1);
printf("File Path: %s\n", str);
if((file = open(str, O_RDONLY)) < 0) {
perror("simplekey: File can not open");
exit(1);
}
for (;;) {
/* Blocking read */
rb= read(file, &ev, sizeof(ev));
if (rb < (int) sizeof(struct input_event)) {
perror("simplekey: short read");
exit(1);
}
for (yalv = 0;
yalv < (int) (rb / sizeof(struct input_event));
yalv++) {
if (ev[yalv].type == EV_KEY) {
printf("%ld.%06ld ",
ev[yalv].time.tv_sec,
ev[yalv].time.tv_usec);
printf("type %d code %d value %d\n",
ev[yalv].type,
ev[yalv].code, ev[yalv].value);
/* Change state on button pressed */
if (ev[yalv].value == 0)
eval_keycode(ev[yalv].code);
}
}
}
close(file);
And this is the basic kernel module :
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/kernel.h>
#include <linux/uaccess.h>
#include <linux/input.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Gaston");
MODULE_DESCRIPTION("A simple Linux char driver");
MODULE_VERSION("0.1");
ssize_t exer_open(struct inode *pinode, struct file *pfile) {
printk(KERN_INFO "Device has been opened\n");
return 0;
}
ssize_t exer_read(struct file *pfile, char __user *buffer, size_t length, loff_t *offset) {
return 0;
}
ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset) {
return 0;
}
ssize_t exer_close(struct inode *pinode, struct file *pfile) {
printk(KERN_INFO "Device successfully closed\n");
return 0;
}
struct file_operations exer_file_operations = {
.owner = THIS_MODULE,
.open = exer_open,
.read = exer_read,
.write = exer_write,
.release = exer_close,
};
int exer_simple_module_init(void) {
printk(KERN_INFO "Initializing the LKM\n");
register_chrdev(240, "Simple Char Drv", &exer_file_operations);
return 0;
}
void exer_simple_module_exit(void) {
unregister_chrdev(240, "Simple Char Drv");
}
module_init(exer_simple_module_init);
module_exit(exer_simple_module_exit);
I hope you will help me. Thank you!
I will concentrate on sending a signal, since that is what you asked for, although sending signals to a process is quite brutal. It would be better to implement poll and read file operations so the user code can wait for events from the device and read them.
Anyway, for sending a signal to the processes that opened the device, the things you need are:
You need a struct fasync_struct * in the private data of your device:
struct fasync_struct *pasync_queue;
It needs to be initialized to NULL by some means during initialization of your device private data. How you do that is up to you.
You need a fasync file operation handler pointed to by the fasync member of your struct file_operations. The implementation of the fasync handler is very simple as it just needs to call fasync_helper() using supplied parameters and a pointer to your device's private struct fasync_struct *:
static int exer_fasync(int fd, struct file *pfile, int mode)
{
// N.B. Change this code to use the pasync_queue member from your device private data.
struct fasync_struct **fapp = &pasync_queue;
return fasync_helper(fd, pfile, mode, fapp);
}
struct file_operations exer_file_operations = {
.owner = THIS_MODULE,
.open = exer_open,
.read = exer_read,
.write = exer_write,
.release = exer_close,
.fasync = exer_fasync,
};
Your device driver can send a SIGIO signal by calling kill_fasync() as follows:
// N.B. Change this code to use the pasync_queue member from your device private data.
struct fasync_struct **fapp = &pasync_queue;
kill_fasync(fapp, SIGIO, POLL_IN);
N.B. The last parameter (value POLL_IN in this case) affects the value of the si_band member of the siginfo_t that your application sees in its signal handler.
Your application needs to set a signal handler for the SIGIO signal. I recommend usingsigaction() to set this up.
Your application needs to set the O_ASYNC flag when it opens the device file, or set it by calling fcntl(fd, F_SETFL, O_ASYNC); after opening the device file.

Linux kernel module : Is it possible to use an open function inside another open function for my module?

Maybe this question makes no sense, but I was wondering if there was a "recommended practice" on how to open a file descriptor for a device inside an open function of the created module.
In fact, I developped a simple Linux kernel module with its basic functions :
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/kernel.h>
#include <linux/uaccess.h>
#include <linux/input.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Gaston");
MODULE_DESCRIPTION("A simple Linux char driver");
MODULE_VERSION("0.1");
ssize_t exer_open(struct inode *pinode, struct file *pfile) {
printk(KERN_INFO "Device has been opened\n");
return 0;
}
ssize_t exer_read(struct file *pfile, char __user *buffer, size_t length, loff_t *offset) {
return 0;
}
ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset) {
return 0;
}
ssize_t exer_close(struct inode *pinode, struct file *pfile) {
printk(KERN_INFO "Device successfully closed\n");
return 0;
}
struct file_operations exer_file_operations = {
.owner = THIS_MODULE,
.open = exer_open,
.read = exer_read,
.write = exer_write,
.release = exer_close,
};
int exer_simple_module_init(void) {
printk(KERN_INFO "Initializing the LKM\n");
register_chrdev(240, "Simple Char Drv", &exer_file_operations);
return 0;
}
void exer_simple_module_exit(void) {
unregister_chrdev(240, "Simple Char Drv");
}
module_init(exer_simple_module_init);
module_exit(exer_simple_module_exit);
I compile it and no errors occured.
Now I want to open the file descriptor of my device ( BUTTON ) in order to manipulate it later from user space program, so I made some modifications by adding the BUTTON device path and another open function like this :
#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/kernel.h>
#include <linux/uaccess.h>
#include <linux/input.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Gaston");
MODULE_DESCRIPTION("A simple Linux char driver");
MODULE_VERSION("0.1");
#define BTN_FILE_PATH "/dev/input/event0"
int file;
char *str = BTN_FILE_PATH;
ssize_t exer_open(struct inode *pinode, struct file *pfile) {
printk(KERN_INFO "Device has been opened\n");
if((file = open(str, O_RDONLY)) < 0) {
printk("simplekey: File can not open");
return(-1);
}
return 0;
}
ssize_t exer_read(struct file *pfile, char __user *buffer, size_t length, loff_t *offset) {
return 0;
}
ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset) {
return 0;
}
ssize_t exer_close(struct inode *pinode, struct file *pfile) {
printk(KERN_INFO "Device successfully closed\n");
return 0;
}
struct file_operations exer_file_operations = {
.owner = THIS_MODULE,
.open = exer_open,
.read = exer_read,
.write = exer_write,
.release = exer_close,
};
int exer_simple_module_init(void) {
printk(KERN_INFO "Initializing the LKM\n");
register_chrdev(240, "Simple Char Drv", &exer_file_operations);
return 0;
}
void exer_simple_module_exit(void) {
unregister_chrdev(240, "Simple Char Drv");
}
module_init(exer_simple_module_init);
module_exit(exer_simple_module_exit);
But the problem, when I try to compile the module now errors are printed :
/home/gaston/ledshared/exer_simple_char_drv.c: In function
‘exer_open’: /home/gaston/ledshared/exer_simple_char_drv.c:32:13:
error: implicit declaration of function ‘open’
[-Werror=implicit-function-declaration]
if((file = open(str,O_RDONLY)) < 0) {
How can I fix the problem please ?
open() is a user-space function. The equivalent kernel-space function is filp_open(), but it returns a struct file * instead of a int file descriptor. The returned struct file * could be an error code instead of a valid pointer. Use the IS_ERR(ptr) macro to check for that, and the PTR_ERR(ptr) macro to extract the error code (which will be a negated errno value).
Use of the filp_open function is discouraged, but here are some modifications to your code to use this function:
int exer_open(struct inode *pinode, struct file *pfile) {
struct file *f;
f = filp_open(str, O_RDONLY);
if (IS_ERR(f)) {
printk("simplekey: File can not open");
return(PTR_ERR(f));
}
pfile->private_data = f;
printk(KERN_INFO "Device has been opened\n");
return 0;
}
The close function should look something like this:
int exer_close(struct inode *pinode, struct file *pfile) {
struct file *f = pfile->private_data;
int rc;
rc = filp_close(f, NULL);
if (rc == 0) {
printk(KERN_INFO "Device successfully closed\n");
}
return rc;
}
There is no legitimate way for a module to read from a struct file * directly into a user-space buffer or write from a user-space buffer to a struct file *, so an intermediate buffer in kernel memory is needed, so that kernel_read() or kernel_write() can be used to read or write the file:
ssize_t exer_read(struct file *pfile, char __user *buffer, size_t length, loff_t *offset) {
struct file *f = pfile->private_data;
enum { MAX_BUF_SIZE = 4096 };
size_t buf_size = 0;
char *buf = NULL;
ssize_t total = 0;
ssize_t rc = 0;
/* Allocate temporary buffer. */
if (length) {
buf_size = min_t(size_t, MAX_BUF_SIZE, length);
buf = kmalloc(buf_size, GFP_KERNEL);
if (buf == NULL) {
return -ENOMEM;
}
}
/* Read file to buffer in chunks. */
do {
size_t amount = min_t(size_t, length, buf_size);
rc = kernel_read(f, buf, amount, offset);
if (rc > 0) {
/* Have read some data from file. */
if (copy_to_user(buffer, buf, rc) != 0) {
/* Bad user memory! */
rc = -EFAULT;
} else {
/* Update totals. */
total += rc;
buffer += rc;
*offset += rc;
length -= rc;
if (rc < amount) {
/* Didn't read the full amount, so terminate early. */
rc = 0;
}
}
}
} while (rc > 0 && length > 0);
/* Free temporary buffer. */
kfree(buf);
if (total > 0) {
return total;
}
return rc;
}
ssize_t exer_write(struct file *pfile, const char __user *buffer, size_t length, loff_t *offset) {
struct file *f = pfile->private_data;
enum { MAX_BUF_SIZE = 4096 };
size_t buf_size = 0;
char *buf = NULL;
ssize_t total = 0;
ssize_t rc = 0;
/* Allocate temporary buffer. */
if (length) {
buf_size = min_t(size_t, MAX_BUF_SIZE, length);
buf = kmalloc(buf_size, GFP_KERNEL);
if (buf == NULL) {
return -ENOMEM;
}
}
/* Write file from buffer in chunks. */
do {
size_t amount = min_t(size_t, length, buf_size);
if (copy_from_user(buf, buffer, amount) != 0) {
/* Bad user memory! */
rc = -EFAULT;
} else {
rc = kernel_write(f, buf, amount, offset);
if (rc > 0) {
/* Have written some data to file. */
/* Update totals. */
total += rc;
buffer += rc;
*offset += rc;
length -= rc;
if (rc < amount) {
/* Didn't write the full amount, so terminate early. */
rc = 0;
}
}
}
} while (rc > 0 && length > 0);
/* Free temporary buffer. */
kfree(buf);
if (total > 0) {
return total;
}
return rc;
}

Correct way to initialize semaphore in linux driver

I wrote this linux char driver just to control open call,
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#include <linux/semaphore.h>
#include <linux/device.h>
#include <linux/cdev.h>
MODULE_LICENSE("GPL");
#define CLASS_NAME "myclass"
#define MINOR_NUM 0
#define MINOR_CNT 1
static struct class *myclass=NULL;
static struct device *mydevice=NULL;
static dev_t mycdevt;
static struct cdev *mycdev;
static struct semaphore *sem;
static int myopen(struct inode *inod, struct file *fp)
{
down(sem);
printk(KERN_INFO "critical section\n");
return 0;
}
static int myclose(struct inode *inod, struct file *fp )
{
up(sem);
printk(KERN_INFO "critical section freed\n");
return 0;
}
static ssize_t myread(struct file *fp, char *buf, size_t len, loff_t *off)
{
return 0;
}
static ssize_t mywrite(struct file *fp, char *buf, size_t len, loff_t *off)
{
return 0;
}
static struct file_operations fops =
{
.open = myopen,
.release = myclose,
.read = myread,
.write = mywrite,
};
static int __init myinit(void)
{
int ret;
ret = alloc_chrdev_region ( &mycdevt, MINOR_NUM, MINOR_CNT, "mycdevt");
if(ret<0)
{
printk(KERN_INFO "chardev can't be allocated\n");
// goto label;//todo
}
mycdev = cdev_alloc();//instead of cdev_alloc, we can use cdev_init(&mycdev, &fops);
if(mycdev == NULL)
{
printk(KERN_INFO"cdev_alloc failed\n");
// goto label;//todo
}
mycdev->ops = &fops;
ret = cdev_add(mycdev, mycdevt, 1);
if(ret < 0)
{
printk(KERN_INFO"cdev_add failed\n");
// goto label;//todo
}
myclass = class_create(THIS_MODULE, CLASS_NAME);
if(myclass == NULL)
{
printk(KERN_INFO"class create failed\n");
// goto label;//todo
}
mydevice = device_create(myclass, NULL, mycdevt, NULL, "mydevice");
if(mydevice == NULL)
{
printk(KERN_INFO"device create failed\n");
// goto label;//todo
}
sema_init(sem, 1);//here is the problem
printk(KERN_INFO"myinit done\n");
return 0;
}
static void __exit myexit(void)
{
device_destroy(myclass, mycdevt);
class_unregister(myclass);
class_destroy(myclass);
cdev_del(mycdev);
unregister_chrdev(MAJOR(mycdevt), "mycdevt");
printk(KERN_INFO "exited\n");
}
module_init(myinit);
module_exit(myexit);
I am following ldd3 book, and tried to use simple APIs to use semahore for an app.
What happened is my kernel crashed when it called sema_init function. I read that semaphore is used in mutex mode
http://www.makelinux.net/ldd3/chp-5-sect-3
I also read that semaphore is not a mutex as there is no ownership. I have yet to explore that 'ownership' thing, but for now I am not able to make a simple semaphore.
What wrong I am doing here?
There's no actual semaphore.
This line:
static struct semaphore *sem;
creates a pointer to a semaphore, not a semaphore itself. It's almost certain initialized to NULL.
You probably want something like
static struct semaphore sem;
and therefore
sema_init( &sem, 1);
along with
up( &sem );
and
down( &sem );
Note that these calls are taking the address of an actual semaphore.

IOCTL call not working with driver

I wrote a IOCTL driver and a corresponding ioctl app with a header file containing commands.
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kdev_t.h>
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <asm/uaccess.h>
#include "myioctl.h"
#include <linux/ioctl.h>
#define NAME MyCharDevice
//Function Prototypes
int NAME_open(struct inode *inode, struct file *filp);
int NAME_release(struct inode *indoe, struct file *filp);
ssize_t NAME_write(struct file *filp, char __user *Ubuff, size_t count, loff_t *offp);
ssize_t NAME_read(struct file *filp, char __user *Ubuff, size_t count, loff_t *offp);
int NAME_flush (struct file *filp);
int NAME_IOCTL (struct inode *inode, struct file *filp, unsigned long cmd, unsigned long val);
//Structure that defines the operations that the driver provides
struct file_operations fops =
{
.owner = THIS_MODULE,
.open = NAME_open,
.read = NAME_read,
.write = NAME_write,
.unlocked_ioctl = NAME_IOCTL,
.release = NAME_release,
.flush = NAME_flush,
};
//Structure for a character driver
struct cdev *my_cdev;
//Init Module
static int __init CharDevice_init(void)
{
int result;
int MAJOR,MINOR;
dev_t Mydev;
Mydev = MKDEV(255,0);//Create a device number
MAJOR=MAJOR(Mydev);
MINOR=MINOR(Mydev);
printk("\nThe Major Number is %d...THe Minor Number is %d\n",MAJOR,MINOR);
result=register_chrdev_region(Mydev,1,"MyCharDevice");//register device region.....
if(result<0)
{
printk(KERN_ALERT "\nThe Region requested for is not obtainable\n");
return(-1);
}
my_cdev = cdev_alloc();//allocate memory to Char Device structure
my_cdev->ops = &fops;//link our file operations to the char device
result=cdev_add(my_cdev,Mydev,1);//Notify the kernel abt the new device
if(result<0)
{
printk(KERN_ALERT "\nThe Char Devide has not been created......\n");
return (-1);
}
return 0;
}
//Cleanup Module
void __exit CharDevice_exit(void)
{
dev_t Mydev;
int MAJOR,MINOR;
Mydev=MKDEV(255,0);
MAJOR=MAJOR(Mydev);
MINOR=MINOR(Mydev);
printk("\nThe Major Number is %d...THe Minor Number is %d\n",MAJOR,MINOR);
unregister_chrdev_region(Mydev,1);//unregister the device numbers and the device created
cdev_del(my_cdev);
printk(KERN_ALERT "\nI have unregistered the stuff that was allocated.....Goodbye for ever.....\n");
return;
}
int NAME_IOCTL (struct inode *inode, struct file *filp, unsigned long cmd, unsigned long val)
{
int BAUD=0, STOP;
char PARITY, CONFIG;
printk ("In IOCTL\n");
printk("command = %d %d val = %d\n", cmd, SET_BAUD, val);
switch (cmd) {
case SET_BAUD:
get_user (BAUD, (int *)val);
printk ("The baud is %d", BAUD);
case SET_PARITY:
case SET_STOP:
case READ_CONFIG:
default:
return -1;
}
return 0;
}
//Open System Call
int NAME_open(struct inode *inode, struct file *filp)
{
printk(KERN_ALERT "\nThis is the Kernel....Open Call.....I have nothing to do.....but YOU ALL HAVE....HAHAHAHA...\n");
return 0;
}
//Close System Call
int NAME_release(struct inode *indoe, struct file *filp)
{
printk(KERN_ALERT "\nThis is the release method of my Character Driver......Bye Dudes......\n");
return 0;
}
//Write Functionality
ssize_t NAME_write(struct file *filp, char __user *Ubuff, size_t count, loff_t *offp)
{
char Kbuff[80];
unsigned long result;
ssize_t retval;
//strcpy(Kbuff,Ubuff);
result=copy_from_user((char *)Kbuff,(char *)Ubuff,count); //get user data
if(result==0)
{
printk(KERN_ALERT "\nMessage from the user......\n>>>> %s <<<<\n",Kbuff);
printk(KERN_ALERT "\n Data Successfully Written.....\n");
retval=count;
return retval;
}
else
{
printk(KERN_ALERT "\n Error Writing Data\n");
retval=-EFAULT;
return retval;
}
}
//read Functionality
ssize_t NAME_read(struct file *filp, char __user *Ubuff, size_t count, loff_t *offp)
{
char Kbuff[]="THis is some date from the kernel to the user....User,ENJOY......";
unsigned long result;
ssize_t retval;
//strcpy(Kbuff,Ubuff);
result=copy_to_user((char *)Ubuff,(char *)Kbuff,sizeof(Kbuff)); //copy to user
if(result==0)
{
//printk("\nMessage from the user......\n>>>> %s <<<<\n");
printk(KERN_ALERT "\n Data Successfully read.....\n");
retval=count;
return retval;
}
else
{
printk(KERN_ALERT"\n Error Writing Data to User\n");
retval=-EFAULT;
return retval;
}
}
int NAME_flush (struct file *filp)
{
printk("\n This is the close function of the file....");
return 0;
}
//Module over ride functions
module_init(CharDevice_init);
module_exit(CharDevice_exit);
header file
#define MAGIC 'x'
#define SET_BAUD _IOW(MAGIC,0, int)
#define SET_PARITY _IOW(MAGIC, 1, char)
#define SET_STOP _IOW(MAGIC, 2, int)
#define READ_CONFIG _IOR(MAGIC, 3, int)
c file
#include <stdio.h>
#include <sys/types.h>
#include <fcntl.h>
#include <linux/ioctl.h>
#include "myioctl.h"
int main()
{
int FileDesc, Baud=9600;
// char Ubuff[]="THis is the User Buffer......Sending Data to the Kernel....";
// char Kbuff[100];
FileDesc=open("/dev/MyCharDevice",O_RDWR);
if(FileDesc <0)
{
printf("\nError Opening Device\n");
exit(1);
}
ioctl (FileDesc, SET_BAUD, &Baud);
printf("%d %d \n", SET_BAUD, &Baud);
// write(FileDesc,Ubuff,sizeof(Ubuff));
// read(FileDesc,Kbuff,sizeof(Ubuff));
// printf("\n The Data read from the Kernel is\n>>>> %s <<<<\n",Kbuff);
close(FileDesc);
}
I am printing in driver what are the command and value of the argument that print like this
command = 1622004312 1074034688 val = 1622004312
So sent command is equal to the argument I sent. Why this is happening?
I used older IOCTL prototype in my driver.
it should be of this type
#if (LINUX_VERSION_CODE < KERNEL_VERSION(2,6,35))
static int my_ioctl(struct inode *i, struct file *f, unsigned int cmd, unsigned long arg)
#else
static long my_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
#endif
In my case against my kernel
static long my_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
is the correct type.

endlessly looping when reading from character device

For a homework assignment, I have written a character device driver. It seems to work OK. I can read and write it. The problem is that when I read the device, it endlessly loops, printing out the contents of the message buffer over and over.
This seems like it should be fairly straight forward. Just use copy_to_user(), but it's proven to be very problematic.
Anyway, here is the code. I think the problem is in the gdev_read() function.
The printk's are there to serve as debugging as well as talking points, since I have to present the project in class.
/*
* Implement a generic character pseudo-device driver
*/
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/types.h>
#include <linux/vmalloc.h>
#include <asm/uaccess.h>
/* you need these, or the kernel will be tainted */
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("A simple sample character device driver");
/*
* function prototypes
*/
int init_module(void);
void cleanup_module(void);
static ssize_t gdev_read(struct file *, char *, size_t, loff_t *);
static ssize_t gdev_write(struct file *, const char *, size_t, loff_t *);
static int gdev_open(struct inode *, struct file *);
static int gdev_release(struct inode *, struct file *);
/* macros */
#define TRUE 1
#define FALSE 0
#define MAX_MSG_LEN 64
/*
* global variables
*/
static dev_t dev_num; /* device number, for new device */
static char *mesg; /* buffer for message */
/* file operations structure, so my device knows how to act */
static struct file_operations fops = {
.owner = THIS_MODULE,
.read = gdev_read,
.write = gdev_write,
.open = gdev_open,
.release = gdev_release,
};
/* character device struct. Declaired here, but initialized elsewhere */
struct cdev *gdev;
int init_module(void)
{
int err;
printk(KERN_ALERT "in init_module\n");
if(alloc_chrdev_region(&dev_num, 0, 1, "/dev/gdev")){
printk(KERN_INFO "Could not allocate device numbers\n");
printk(KERN_INFO "Module gdev not loaded\n");
return -1;
}
/* now I need to make the device and register it */
gdev = cdev_alloc();
gdev->owner = THIS_MODULE;
gdev->ops = &fops;
err = cdev_add(gdev, dev_num, 1);
if(err){
printk(KERN_NOTICE "Error %d adding gdev", err);
return err;
}
mesg = (char *)vmalloc(MAX_MSG_LEN);
printk(KERN_INFO "Module gdev successfully loaded.\n");
printk(KERN_INFO "gdev Major Number: %d\n", MAJOR(dev_num));
return 0;
}
void cleanup_module(void)
{
printk(KERN_ALERT "in cleanup_module\n");
unregister_chrdev_region(dev_num, 3);
vfree(mesg);
cdev_del( gdev );
printk(KERN_INFO "Module gdev unregistered\n");
}
static ssize_t gdev_read(struct file *filp, char *page,
size_t len, loff_t *offset)
{
ssize_t bytes = len < MAX_MSG_LEN ? len : MAX_MSG_LEN;
printk(KERN_ALERT "in gdev_read\n");
if(copy_to_user(page, mesg, bytes)){
return -EFAULT;
}
return bytes;
}
static ssize_t gdev_write(struct file *filp, const char *page,
size_t len, loff_t *offset)
{
ssize_t bytes = len < MAX_MSG_LEN ? len : MAX_MSG_LEN;
printk(KERN_ALERT "in gdev_write\n");
if(copy_from_user(mesg, page, bytes)){
return -EFAULT;
}
return bytes;
}
static int gdev_open(struct inode *inode, struct file *filp)
{
printk(KERN_ALERT "in gdev_open\n");
return 0;
}
static int gdev_release(struct inode *inode, struct file *filp)
{
printk(KERN_ALERT "in gdev_release\n");
/* doesn't do anything because it doesn't need too */
return 0;
}
If zero is not returned from read() (in your case gdev_read()), the read function will be called again.
To stop this you use the loff_t *offset parameter. Increment it by how many bytes you have read using (*offset) += bytes; after copy_to_user(). Next time read() is called, offset will be what you have incremented it to. Now just check how many bytes you have previously sent, and only send what you still have remaining. Your function should look like this:
static ssize_t gdev_read(struct file *filp, char *page,
size_t len, loff_t *offset)
{
ssize_t bytes = len < (MAX_MSG_LEN-(*offset)) ? len : (MAX_MSG_LEN-(*offset));
printk(KERN_ALERT "in gdev_read\n");
if(copy_to_user(page, mesg, bytes)){
return -EFAULT;
}
(*offset) += bytes;
return bytes;
}
You could use 'simple_read_from_buffer' function from 'linux/fs.h':
static ssize_t gdev_read(struct file *filep, char __user *buff, size_t count, loff_t *offp)
{
return simple_read_from_buffer(buff, count, offp, my_buffer, buffer_len);
}
'my_buffer' and 'buffer_len' are defined in your module.

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