/* * chardev2.c - Create an input/output character device */ #include #include #include #include #include #include #include /* We are doing kernel work */ #include /* Specifically, a module */ #include #include "chardev.h" #define SUCCESS 0 #define DEVICE_NAME "char_dev" #define BUF_LEN 80 /* Is the device open right now? Used to prevent concurrent access into * the same device */ static atomic_t already_open = ATOMIC_INIT(0); /* The message the device will give when asked */ static char message[BUF_LEN]; /* How far did the process reading the message get? Useful if the message * is larger than the size of the buffer we get to fill in device_read. */ static char *message_ptr; static struct class *cls; /* This is called whenever a process attempts to open the device file */ static int device_open(struct inode *inode, struct file *file) { pr_info("device_open(%p)\n", file); /* We don't want to talk to two processes at the same time. */ if (atomic_cmpxchg(&already_open, 0, 1)) return -EBUSY; /* Initialize the message */ message_ptr = message; try_module_get(THIS_MODULE); return SUCCESS; } static int device_release(struct inode *inode, struct file *file) { pr_info("device_release(%p,%p)\n", inode, file); /* We're now ready for our next caller */ atomic_set(&already_open, 0); module_put(THIS_MODULE); return SUCCESS; } /* This function is called whenever a process which has already opened the * device file attempts to read from it. */ static ssize_t device_read(struct file *file, /* see include/linux/fs.h */ char __user *buffer, /* buffer to be filled */ size_t length, /* length of the buffer */ loff_t *offset) { /* Number of bytes actually written to the buffer */ int bytes_read = 0; pr_info("device_read(%p,%p,%ld)\n", file, buffer, length); /* If at the end of message, return 0 (which signifies end of file). */ if (*message_ptr == 0) return 0; /* Actually put the data into the buffer */ while (length && *message_ptr) { /* Because the buffer is in the user data segment, not the kernel * data segment, assignment would not work. Instead, we have to * use put_user which copies data from the kernel data segment to * the user data segment. */ put_user(*(message_ptr++), buffer++); length--; bytes_read++; } pr_info("Read %d bytes, %ld left\n", bytes_read, length); /* Read functions are supposed to return the number of bytes actually * inserted into the buffer. */ return bytes_read; } /* called when somebody tries to write into our device file. */ static ssize_t device_write(struct file *file, const char __user *buffer, size_t length, loff_t *offset) { int i; pr_info("device_write(%p,%s,%ld)", file, buffer, length); for (i = 0; i < length && i < BUF_LEN; i++) get_user(message[i], buffer + i); message_ptr = message; /* Again, return the number of input characters used. */ return i; } /* This function is called whenever a process tries to do an ioctl on our * device file. We get two extra parameters (additional to the inode and file * structures, which all device functions get): the number of the ioctl called * and the parameter given to the ioctl function. * * If the ioctl is write or read/write (meaning output is returned to the * calling process), the ioctl call returns the output of this function. */ static long device_ioctl(struct file *file, /* ditto */ unsigned int ioctl_num, /* number and param for ioctl */ unsigned long ioctl_param) { int i; char *temp; char ch; /* Switch according to the ioctl called */ switch (ioctl_num) { case IOCTL_SET_MSG: /* Receive a pointer to a message (in user space) and set that to * be the device's message. Get the parameter given to ioctl by * the process. */ temp = (char *)ioctl_param; /* Find the length of the message */ get_user(ch, (char __user *)temp); for (i = 0; ch && i < BUF_LEN; i++, temp++) get_user(ch, (char __user *)temp); device_write(file, (char __user *)ioctl_param, i, NULL); break; case IOCTL_GET_MSG: /* Give the current message to the calling process - the parameter * we got is a pointer, fill it. */ i = device_read(file, (char __user *)ioctl_param, 99, NULL); /* Put a zero at the end of the buffer, so it will be properly * terminated. */ put_user('\0', (char __user *)ioctl_param + i); break; case IOCTL_GET_NTH_BYTE: /* This ioctl is both input (ioctl_param) and output (the return * value of this function). */ return message[ioctl_param]; break; } return SUCCESS; } /* Module Declarations */ /* This structure will hold the functions to be called when a process does * something to the device we created. Since a pointer to this structure * is kept in the devices table, it can't be local to init_module. NULL is * for unimplemented functions. */ static struct file_operations fops = { .read = device_read, .write = device_write, .unlocked_ioctl = device_ioctl, .open = device_open, .release = device_release, /* a.k.a. close */ }; /* Initialize the module - Register the character device */ static int __init chardev2_init(void) { /* Register the character device (atleast try) */ int ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &fops); /* Negative values signify an error */ if (ret_val < 0) { pr_alert("%s failed with %d\n", "Sorry, registering the character device ", ret_val); return ret_val; } cls = class_create(THIS_MODULE, DEVICE_FILE_NAME); device_create(cls, NULL, MKDEV(MAJOR_NUM, 0), NULL, DEVICE_FILE_NAME); pr_info("Device created on /dev/%s\n", DEVICE_FILE_NAME); return 0; } /* Cleanup - unregister the appropriate file from /proc */ static void __exit chardev2_exit(void) { device_destroy(cls, MKDEV(MAJOR_NUM, 0)); class_destroy(cls); /* Unregister the device */ unregister_chrdev(MAJOR_NUM, DEVICE_NAME); } module_init(chardev2_init); module_exit(chardev2_exit); MODULE_LICENSE("GPL");