bytestream-example.c

/*
 * Sample kfifo byte stream implementation.
 */

#include <linux/init.h>
#include <linux/kfifo.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/proc_fs.h>
#include <linux/version.h>

#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0)
#define HAVE_PROC_OPS
#endif

/*
 * This module shows how to create a byte stream fifo.
 */

/* fifo size in elements (bytes) */
#define FIFO_SIZE 32

/* name of the proc entry */
#define PROC_FIFO "bytestream-fifo"

/* lock for procfs read access */
static DEFINE_MUTEX(read_lock);

/* lock for procfs write access */
static DEFINE_MUTEX(write_lock);

static struct kfifo test;

static const unsigned char expected_result[FIFO_SIZE] = {
    3,  4,  5,  6,  7,  8,  9,  0,  1,  20, 21, 22, 23, 24, 25, 26,
    27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42,
};

static int __init testfunc(void)
{
    unsigned char buf[6];
    unsigned char i, j;
    unsigned int ret;

    printk(KERN_INFO "byte stream fifo test start\n");

    /* put string into the fifo */
    kfifo_in(&test, "hello", 5);

    /* put values into the fifo */
    for (i = 0; i != 10; i++)
        kfifo_put(&test, i);

    /* show the number of used elements */
    printk(KERN_INFO "fifo len: %u\n", kfifo_len(&test));

    /* get max of 5 bytes from the fifo */
    i = kfifo_out(&test, buf, 5);
    printk(KERN_INFO "buf: %.*s\n", i, buf);

    /* get max of 2 elements from the fifo */
    ret = kfifo_out(&test, buf, 2);
    printk(KERN_INFO "ret: %d\n", ret);
    /* and put it back to the end of the fifo */
    ret = kfifo_in(&test, buf, ret);
    printk(KERN_INFO "ret: %d\n", ret);

    /* skip first element of the fifo */
    printk(KERN_INFO "skip 1st element\n");
    kfifo_skip(&test);

    /* put values into the fifo until is full */
    for (i = 20; kfifo_put(&test, i); i++)
        ;

    printk(KERN_INFO "queue len: %u\n", kfifo_len(&test));

    /* show the first value without removing from the fifo */
    if (kfifo_peek(&test, &i))
        printk(KERN_INFO "%d\n", i);

    /* check the correctness of all values in the fifo */
    j = 0;
    while (kfifo_get(&test, &i)) {
        printk(KERN_INFO "item = %d\n", i);
        if (i != expected_result[j++]) {
            printk(KERN_WARNING "value mismatch: test failed\n");
            return -EIO;
        }
    }
    if (j != ARRAY_SIZE(expected_result)) {
        printk(KERN_WARNING "size mismatch: test failed\n");
        return -EIO;
    }
    printk(KERN_INFO "test passed\n");

    return 0;
}

static ssize_t fifo_write(struct file *file,
                          const char __user *buf,
                          size_t count,
                          loff_t *ppos)
{
    int ret;
    unsigned int copied;

    if (mutex_lock_interruptible(&write_lock))
        return -ERESTARTSYS;

    ret = kfifo_from_user(&test, buf, count, &copied);

    mutex_unlock(&write_lock);
    if (ret)
        return ret;

    return copied;
}

static ssize_t fifo_read(struct file *file,
                         char __user *buf,
                         size_t count,
                         loff_t *ppos)
{
    int ret;
    unsigned int copied;

    if (mutex_lock_interruptible(&read_lock))
        return -ERESTARTSYS;

    ret = kfifo_to_user(&test, buf, count, &copied);

    mutex_unlock(&read_lock);
    if (ret)
        return ret;

    return copied;
}

#ifdef HAVE_PROC_OPS
static const struct proc_ops fifo_proc_ops = {
    .proc_read = fifo_read,
    .proc_write = fifo_write,
    .proc_lseek = noop_llseek,
};
#else
static const struct file_operations fifo_proc_ops = {
    .read = fifo_read,
    .write = fifo_write,
    .llseek = noop_llseek,
};
#endif

static int __init example_init(void)
{
    int ret;

    ret = kfifo_alloc(&test, FIFO_SIZE, GFP_KERNEL);
    if (ret) {
        printk(KERN_ERR "error kfifo_alloc\n");
        return ret;
    }
    if (testfunc() < 0) {
        kfifo_free(&test);
        return -EIO;
    }

    if (proc_create(PROC_FIFO, 0, NULL, &fifo_proc_ops) == NULL) {
        kfifo_free(&test);
        return -ENOMEM;
    }
    return 0;
}

static void __exit example_exit(void)
{
    remove_proc_entry(PROC_FIFO, NULL);
    kfifo_free(&test);
}

module_init(example_init);
module_exit(example_exit);
MODULE_LICENSE("GPL");