Driver programming : cat command not showing output - linux

Question

I am new to driver programming and I have written a simple char device driver code. When I wrote it without using pointers, it crashed.
When writing to a driver using echo, it works. But when reading from it, there is no output. Someone please help. File operations part of the code is shown below. 'p' and 'q' are normal character pointers. 'max' value was set as 10. 'ptr' is of static int type initialized as '0'.
int my_open(struct inode *inode,struct file *filp)
{
printk("In open.\n");
if((filp->f_flags & O_ACCMODE) == O_WRONLY){
p = (char *)buffer;
ptr = 0;
}
else if((filp->f_flags & O_ACCMODE) == O_RDONLY)
q = (char *)buffer;
return 0;
}
int my_close(struct inode *inode,struct file *filp)
{
printk("In close.\n");
return 0;
}
ssize_t my_read(struct file *filp,char *buff,size_t count,loff_t *pos)
{
long ret;
printk("In read.\n");
ret = copy_to_user(buff,q,max);
q += max;
*pos += max;
if(ptr -= max)
return max;
else
return 0;
}
ssize_t my_write(struct file *filp,const char *buff,size_t count,loff_t *pos)
{
long ret;
printk("In write.\n");
ret = copy_from_user(p,buff,max);
p += max;
*pos += max;
ptr += max;
return max;
}
module_init(my_init);
module_exit(my_exit);

Answer 1

In both read and write you are not taking into account the "count" parameter, as your code seems to assume that "count>=max", that is not guaranteed. This by itself may lead to any sort of troubles in the process executing read. Also, you copy_to/from_user before checking if the current read or write position is over the buffer limit. Moreover, the assignment/test if (ptr -= max) only works if ptr is exactly equal to max, also not guaranteed it you execute the read more than once.
NOTE: since definitions of p, q, buffer, ptr and max are missing, I'll assume that they look like:
static char *p;
static char *q;
statint int ptr = 0;
static char buffer[10];
static int max=10;