Implementing open, read, write, release
Character device drivers in Linux interact with user programs through a set of standard file operations. These operations define how the kernel should respond when a user program opens, reads from, writes to, or closes a device file. The most fundamental file operations you will implement are open, read, write, and release. Each operation corresponds to a user action, such as opening a device file with open(), reading data with read(), writing data with write(), or closing the file with close(). Implementing these functions is the foundation for any character device driver, as they form the interface between user space and your device logic.
simple_chrdev_open_release.c
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950#include <linux/module.h> #include <linux/fs.h> #include <linux/cdev.h> #include <linux/uaccess.h> #define DEVICE_NAME "simple_chrdev" static int major; static struct cdev simple_cdev; static int simple_open(struct inode *inode, struct file *file) { pr_info("simple_chrdev: device opened\n"); return 0; } static int simple_release(struct inode *inode, struct file *file) { pr_info("simple_chrdev: device closed\n"); return 0; } static struct file_operations simple_fops = { .owner = THIS_MODULE, .open = simple_open, .release = simple_release, }; static int __init simple_chrdev_init(void) { major = register_chrdev(0, DEVICE_NAME, &simple_fops); if (major < 0) { pr_err("simple_chrdev: failed to register device\n"); return major; } pr_info("simple_chrdev: registered with major number %d\n", major); return 0; } static void __exit simple_chrdev_exit(void) { unregister_chrdev(major, DEVICE_NAME); pr_info("simple_chrdev: unregistered device\n"); } module_init(simple_chrdev_init); module_exit(simple_chrdev_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Device Driver Course"); MODULE_DESCRIPTION("Simple character device with open and release");
The open and release operations in the example above are called when a user process opens or closes the device file, respectively. These are often used to allocate or free resources, or to perform bookkeeping, but in this simple example they just print a message to the kernel log. Alongside these, the read and write operations are key to transferring data between user space and the device. When a user program reads from the device file, the kernel calls your driver's read function; when it writes, the write function is invoked. These operations must handle copying data between kernel and user space safely and efficiently. Typically, you will implement logic within these functions to manage device buffers, validate user input, and handle any device-specific data transfer needs. The open and release functions shown earlier provide the structure into which you will integrate your read and write logic.
simple_chrdev_read.c
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556#include <linux/module.h> #include <linux/fs.h> #include <linux/uaccess.h> #define DEVICE_NAME "simple_chrdev" #define BUF_LEN 100 static int major; static char device_buffer[BUF_LEN] = "Hello from kernel space!\n"; static ssize_t simple_read(struct file *file, char __user *buf, size_t len, loff_t *offset) { ssize_t bytes_read = 0; if (*offset >= strlen(device_buffer)) return 0; if (len > strlen(device_buffer) - *offset) len = strlen(device_buffer) - *offset; if (copy_to_user(buf, device_buffer + *offset, len)) return -EFAULT; *offset += len; bytes_read = len; return bytes_read; } static struct file_operations simple_fops = { .owner = THIS_MODULE, .read = simple_read, }; static int __init simple_chrdev_init(void) { major = register_chrdev(0, DEVICE_NAME, &simple_fops); if (major < 0) { pr_err("simple_chrdev: failed to register device\n"); return major; } pr_info("simple_chrdev: registered with major number %d\n", major); return 0; } static void __exit simple_chrdev_exit(void) { unregister_chrdev(major, DEVICE_NAME); pr_info("simple_chrdev: unregistered device\n"); } module_init(simple_chrdev_init); module_exit(simple_chrdev_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Device Driver Course"); MODULE_DESCRIPTION("Simple character device with read operation");
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Completion rate improved to 3.85Implementing open, read, write, release
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Character device drivers in Linux interact with user programs through a set of standard file operations. These operations define how the kernel should respond when a user program opens, reads from, writes to, or closes a device file. The most fundamental file operations you will implement are open, read, write, and release. Each operation corresponds to a user action, such as opening a device file with open(), reading data with read(), writing data with write(), or closing the file with close(). Implementing these functions is the foundation for any character device driver, as they form the interface between user space and your device logic.
simple_chrdev_open_release.c
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950#include <linux/module.h> #include <linux/fs.h> #include <linux/cdev.h> #include <linux/uaccess.h> #define DEVICE_NAME "simple_chrdev" static int major; static struct cdev simple_cdev; static int simple_open(struct inode *inode, struct file *file) { pr_info("simple_chrdev: device opened\n"); return 0; } static int simple_release(struct inode *inode, struct file *file) { pr_info("simple_chrdev: device closed\n"); return 0; } static struct file_operations simple_fops = { .owner = THIS_MODULE, .open = simple_open, .release = simple_release, }; static int __init simple_chrdev_init(void) { major = register_chrdev(0, DEVICE_NAME, &simple_fops); if (major < 0) { pr_err("simple_chrdev: failed to register device\n"); return major; } pr_info("simple_chrdev: registered with major number %d\n", major); return 0; } static void __exit simple_chrdev_exit(void) { unregister_chrdev(major, DEVICE_NAME); pr_info("simple_chrdev: unregistered device\n"); } module_init(simple_chrdev_init); module_exit(simple_chrdev_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Device Driver Course"); MODULE_DESCRIPTION("Simple character device with open and release");
The open and release operations in the example above are called when a user process opens or closes the device file, respectively. These are often used to allocate or free resources, or to perform bookkeeping, but in this simple example they just print a message to the kernel log. Alongside these, the read and write operations are key to transferring data between user space and the device. When a user program reads from the device file, the kernel calls your driver's read function; when it writes, the write function is invoked. These operations must handle copying data between kernel and user space safely and efficiently. Typically, you will implement logic within these functions to manage device buffers, validate user input, and handle any device-specific data transfer needs. The open and release functions shown earlier provide the structure into which you will integrate your read and write logic.
simple_chrdev_read.c
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556#include <linux/module.h> #include <linux/fs.h> #include <linux/uaccess.h> #define DEVICE_NAME "simple_chrdev" #define BUF_LEN 100 static int major; static char device_buffer[BUF_LEN] = "Hello from kernel space!\n"; static ssize_t simple_read(struct file *file, char __user *buf, size_t len, loff_t *offset) { ssize_t bytes_read = 0; if (*offset >= strlen(device_buffer)) return 0; if (len > strlen(device_buffer) - *offset) len = strlen(device_buffer) - *offset; if (copy_to_user(buf, device_buffer + *offset, len)) return -EFAULT; *offset += len; bytes_read = len; return bytes_read; } static struct file_operations simple_fops = { .owner = THIS_MODULE, .read = simple_read, }; static int __init simple_chrdev_init(void) { major = register_chrdev(0, DEVICE_NAME, &simple_fops); if (major < 0) { pr_err("simple_chrdev: failed to register device\n"); return major; } pr_info("simple_chrdev: registered with major number %d\n", major); return 0; } static void __exit simple_chrdev_exit(void) { unregister_chrdev(major, DEVICE_NAME); pr_info("simple_chrdev: unregistered device\n"); } module_init(simple_chrdev_init); module_exit(simple_chrdev_exit); MODULE_LICENSE("GPL"); MODULE_AUTHOR("Device Driver Course"); MODULE_DESCRIPTION("Simple character device with read operation");
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