#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h> /* printk() */
#include <linux/errno.h> /* error codes */
#include <linux/types.h> /* size_t */
#include <linux/proc_fs.h>
#include <asm/uaccess.h> /* copy_from/to_user */

MODULE_LICENSE("GPL");

//Declarations
int memory_open(struct inode *inode, struct file *filp);
int memory_release(struct inode *inode, struct file *filp);
ssize_t memory_read(struct file *filp, char *buf, size_t count, loff_t *f_pos);
ssize_t memory_write(struct file *filp, char *buf, size_t count, loff_t *f_pos);
void memory_exit(void);
int memory_init(void);

/* Structure that declares the usual file access functions */
struct file_operations memory_fops = {
    read: memory_read,
    write: memory_write,
    open: memory_open,
    release: memory_release
};

//Default functions
module_init(memory_init);
module_exit(memory_exit);

/* Global variables of the driver */
/* Major number */
int memory_major = 60;
/* Buffer to store data */
char* tx_buffer;
char* rx_buffer;

int BUFFER_SIZE=64;
int actual_rx_size=0;

int memory_init(void) {
    int result;

    /* Registering device */
    result = register_chrdev(memory_major, "move_data", &memory_fops);
    if (result < 0) {
        printk(
        "<1>move_data: cannot obtain major number %d\n", memory_major);
        return result;
    }

    /* Allocating memory for the buffers */
    //Allocate buffers
    tx_buffer = kmalloc(BUFFER_SIZE,  GFP_KERNEL);
    rx_buffer = kmalloc(BUFFER_SIZE,  GFP_KERNEL);

    //Check allocation was ok
    if (!tx_buffer || !rx_buffer) {
        result = -ENOMEM;
        goto fail;
    }

    //Reset the buffers
    memset(tx_buffer,0, BUFFER_SIZE);
    memset(rx_buffer,0, BUFFER_SIZE);

    printk("<1>Inserting memory module\n"); 
    return 0;

    fail:
        memory_exit(); 
        return result;
}

void memory_exit(void) {
    /* Freeing the major number */
    unregister_chrdev(memory_major, "memory");

    /* Freeing buffers */
    if (tx_buffer) {
        kfree(tx_buffer); //Note kfree
    }

    if (rx_buffer) {
        kfree(rx_buffer); //Note kfree
    }
    printk("<1>Removing memory module\n");
}


//Read function
ssize_t memory_read(struct file *filp, char *buf, size_t count, loff_t *f_pos) { 

    printk("user requesting data, our buffer has (%d) \n", actual_rx_size);

    /* Transfering data to user space */ 
    int retval = copy_to_user(buf,rx_buffer,actual_rx_size);

    printk("copy_to_user returned (%d)", retval);

    return retval;
}

ssize_t memory_write( struct file *filp, char *buf,
                  size_t count, loff_t *f_pos) {

    //zero the input buffer
    memset(tx_buffer,0,BUFFER_SIZE);
    memset(rx_buffer,0,BUFFER_SIZE);

    printk("New message from userspace - count:%d\n",count);

    int retval = copy_from_user(tx_buffer,buf,count);

    printk("copy_from_user returned (%d) we read [%s]\n",retval , tx_buffer);
    printk("initialize rx buffer..\n");

    memcpy(rx_buffer,tx_buffer, count);
    printk("content of rx buffer [%s]\n", rx_buffer);

    actual_rx_size = count;

    return count; //inform that we read all (fixme?)
}

//Always successfull
int memory_open(struct inode *inode, struct file *filp) { return 0; }
int memory_release(struct inode *inode, struct file *filp) { return 0; }

#include <unistd.h>     //open, close | always first, defines compliance
#include <fcntl.h>      //O_RDONLY
#include <stdio.h>
#include <stdlib.h>     //printf
#include <string.h>

int main(int args, char *argv[])
{
int BUFFER_SIZE = 20;

char internal_buf[BUFFER_SIZE];
int to_read = 0;

memset(internal_buf,0,BUFFER_SIZE);

if (args < 3) {
    printf("2 Input arguments needed\nTo read 10 bytes: \"%s read 10\" \
    \nTo write string \"hello\": \"%s write hello\"\nExiting..\n", argv[0], argv[0]);
    return 1;
}


//Check the operation
if (strcmp(argv[1],"write") == 0) {

    printf("input lenght:%d", strlen(argv[2]));
    //Make sure our write fits to the internal buffer
    if(strlen(argv[2]) >= BUFFER_SIZE) {
        printf("too long input string, max buffer[%d]\nExiting..", BUFFER_SIZE);
        return 2;
    }

    printf("write op\n");
    memcpy(internal_buf,argv[2], strlen(argv[2]));

    printf("Writing [%s]\n", internal_buf);

    FILE * filepointer;
    filepointer = fopen("/dev/move_data", "w");
    fwrite(internal_buf, sizeof(char) , strlen(argv[2]), filepointer);
    fclose(filepointer);

} else if (strcmp(argv[1],"read") == 0) {
    printf("read op\n");

    to_read = atoi(argv[2]);

    FILE * filepointer;
    filepointer = fopen("/dev/move_data", "r");
    int retval = fread(internal_buf, sizeof(char) , to_read, filepointer);
    fclose(filepointer);

    printf("Read %d bytes from driver string[%s]\n", retval, internal_buf);
} else {
    printf("first argument has to be 'read' or 'write'\nExiting..\n");
    return 1;
}


return 0;
}

./rw write "testing testing"

kernel side:
[ 2696.607586] New message from userspace - count:15
[ 2696.607591] copy_from_user returned (0) we read [testing testing]
[ 2696.607593] initialize rx buffer..
[ 2696.607594] content of rx buffer [testing testing]

./rw read 15
read op
Read 0 bytes from driver string[]

kernel side: 
[  617.096521] user requesting data, our buffer has (15) 
[  575.797668] copy_to_user returned (0)
[  617.096528] copy_to_user returned (0)

ssize_t memory_read {
...
    //return retval;
    return 5;
}

And in userspace I read:

./rw read 20
read op
Read 20 bytes from driver string[testitestitestitesti�]

     return actual_rx_size;