... int major . ... Here is an example of foo_write() : static int foo_write(struct inode * inode, struct file * file, char * buf, int count) { unsigned int minor = MINOR(inode->i_rdev); char ret; while (count > 0) { ret = foo_write_byte(minor); if (ret < 0) { foo_handle_error(WRITE, ret, minor); continue; } buf++ = ret; count-- } return count; } foo_write_byte() and foo_handle_error() are either functions defined elsewhere in foo.c or pseudocode. ... 2. return value of foo_write(...) by My name here . ...