/* * chardev.c: Creates a read-only char device that says how many times * you've read from the dev file */ #include <linux/kernel.h> #include <linux/module.h> #include <linux/fs.h> #include <asm/uaccess.h> /* for put_user */ /* * Prototypes - this would normally go in a .h file */ int init_module(void); void cleanup_module(void); static int device_open(struct inode *, struct file *); static int device_release(struct inode *, struct file *); static ssize_t device_read(struct file *, char *, size_t, loff_t *); static ssize_t device_write(struct file *, const char *, size_t, loff_t *); #define SUCCESS 0 #define DEVICE_NAME "chardev" /* Dev name as it appears in /proc/devices */ #define BUF_LEN 80 /* Max length of the message from the device */ /* * Global variables are declared as static, so are global within the file. */ static int Major; /* Major number assigned to our device driver */ static int Device_Open = 0; /* Is device open? * Used to prevent multiple access to device */ static char msg[BUF_LEN]; /* The msg the device will give when asked */ static char *msg_Ptr; static struct file_operations fops = { .read = device_read, .write = device_write, .open = device_open, .release = device_release }; /* * Functions */ int init_module(void) { Major = register_chrdev(0, DEVICE_NAME, &fops); if (Major < 0) { printk("Registering the character device failed with %d\n", Major); return Major; } printk("<1>I was assigned major number %d. To talk to\n", Major); printk("<1>the driver, create a dev file with\n"); printk("'mknod /dev/hello c %d 0'.\n", Major); printk("<1>Try various minor numbers. Try to cat and echo to\n"); printk("the device file.\n"); printk("<1>Remove the device file and module when done.\n"); return 0; } void cleanup_module(void) { /* * Unregister the device */ int ret = unregister_chrdev(Major, DEVICE_NAME); if (ret < 0) printk("Error in unregister_chrdev: %d\n", ret); } /* * Methods */ /* * Called when a process tries to open the device file, like * "cat /dev/mycharfile" */ static int device_open(struct inode *inode, struct file *file) { static int counter = 0; if (Device_Open) return -EBUSY; Device_Open++; sprintf(msg, "I already told you %d times Hello world!\n", counter++); msg_Ptr = msg; try_module_get(THIS_MODULE); return SUCCESS; } /* * Called when a process closes the device file. */ static int device_release(struct inode *inode, struct file *file) { Device_Open--; /* We're now ready for our next caller */ /* * Decrement the usage count, or else once you opened the file, you'll * never get get rid of the module. */ module_put(THIS_MODULE); return 0; } /* * Called when a process, which already opened the dev file, attempts to * read from it. */ static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */ char *buffer, /* buffer to fill with data */ size_t length, /* length of the buffer */ loff_t * offset) { /* * Number of bytes actually written to the buffer */ int bytes_read = 0; /* * If we're at the end of the message, * return 0 signifying end of file */ if (*msg_Ptr == 0) return 0; /* * Actually put the data into the buffer */ while (length && *msg_Ptr) { /* * The buffer is in the user data segment, not the kernel * segment so "*" assignment won't work. We have to use * put_user which copies data from the kernel data segment to * the user data segment. */ put_user(*(msg_Ptr++), buffer++); length--; bytes_read++; } /* * Most read functions return the number of bytes put into the buffer */ return bytes_read; } /* * Called when a process writes to dev file: echo "hi" > /dev/hello */ static ssize_t device_write(struct file *filp, const char *buff, size_t len, loff_t * off) { printk("<1>Sorry, this operation isn't supported.\n"); return -EINVAL; }