Can't it be because you are using size = 1byte in copy_from_user? Make it 'count'. -- Thanks, Nilesh --------------------------------------------------------- From: kernelnewbies-bounce@xxxxxxxxxxxx [mailto:kernelnewbies-bounce@xxxxxxxxxxxx] On Behalf Of Bond Sent: Wednesday, September 15, 2010 5:27 PM To: kernelnewbies Subject: my driver is dropping characters I wrote my hello world type of character device driver.(First driver that I wrote) I created a device as follows mknod /dev/bond c 60 0 and then tried to write something to that device as follows echo -n abcde > /dev/bond cat /dev/bond will show me the last e of entered abcde above but will drop abcd. I can see the last character which was passed on to as argument to echo but not the previous characters. Any idea what improvement should I make. Here is the code. #include <linux/init.h> #include <linux/module.h> #include <linux/kernel.h> /* printk() */ #include <linux/slab.h> /* kmalloc() */ #include <linux/fs.h> /* everything... */ #include <linux/errno.h> /* error codes */ #include <linux/types.h> /* size_t */ #include <linux/proc_fs.h> #include <linux/fcntl.h> /* O_ACCMODE */ #include <asm/system.h> /* cli(), *_flags */ #include <asm/uaccess.h> /* copy_from/to_user */ /* Declaration of memory.c functions */ int bond_open(struct inode *inode, struct file *filp) { return 0;}; int bond_release(struct inode *inode, struct file *filp){ return 0;}; ssize_t bond_read(struct file *filp, char *buf, size_t count, loff_t *f_pos) { /* Transfering data to user space */ copy_to_user(buf,bond_buffer,1); /* Changing reading position as best suits */ if (*f_pos == 0) { *f_pos+=1; return 1; } else { return 0; } }; ssize_t bond_write(struct file *filp, char *buf, size_t count, loff_t *f_pos) { char *tmp; tmp=buf+count-1; copy_from_user(bond_buffer,tmp,1); return 1; } ; void bond_exit(void); int bond_init(void); /* Structure that declares the usual file */ /* access functions */ struct file_operations bond_fops = { read: bond_read, write: bond_write, open: bond_open, release: bond_release }; /* Global variables of the driver */ /* Major number */ int bond_major = 60; /* Buffer to store data */ char *bond_buffer; static int bond_init(void) { printk("<1> Hello bond new driver!\n"); int result; /* Registering device */ result = register_chrdev(bond_major, "bond", &bond_fops); if (result < 0) { printk( "<1>memory: cannot obtain major number %d\n", bond_major); return result; } /* Allocating memory for the buffer */ bond_buffer = kmalloc(1, GFP_KERNEL); if (!bond_buffer) { result = -ENOMEM; goto fail; } memset(bond_buffer, 0, 1); printk("<1>Inserting bond module\n"); return 0; fail: bond_exit(); return result; return 0; } static void bond_exit(void) { printk("<1> Bye, bond world\n"); /* Freeing the major number */ unregister_chrdev(bond_major, "bond"); /* Freeing buffer memory */ if (bond_buffer) { kfree(bond_buffer); } printk("<1>Removing bond module\n"); } module_init(bond_init); module_exit(bond_exit); -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
