简单休眠

beyes

模块代码来自 LDD3。该实例实现了一个简单的休眠。休眠的原因是没有资源可读，当有数据写向设备时睡眠的读进程又会被唤醒。


代码如下：

[C++] 纯文本查看 复制代码

#include <linux/module.h>
#include <linux/init.h>


#include <linux/sched.h>  /* current and everything */
#include <linux/kernel.h> /* printk() */
#include <linux/fs.h>     /* everything... */
#include <linux/types.h>  /* size_t */
#include <linux/wait.h>


MODULE_LICENSE("Dual BSD/GPL");


static int sleepy_major = 0;


static DECLARE_WAIT_QUEUE_HEAD(wq);
static int flag = 0;


ssize_t sleepy_read (struct file *filp, char __user *buf, size_t count, loff_t *pos)
{
    printk(KERN_DEBUG "process %i (%s) going to sleep\n",
            current->pid, current->comm);
    wait_event_interruptible(wq, flag != 0);
    flag = 0;
    printk(KERN_DEBUG "awoken %i (%s)\n", current->pid, current->comm);
    return 0; /* EOF */
}


ssize_t sleepy_write (struct file *filp, const char __user *buf, size_t count,
        loff_t *pos)
{
    printk(KERN_DEBUG "process %i (%s) awakening the readers...\n",
            current->pid, current->comm);
    flag = 1;
    wake_up_interruptible(&wq);
    return count; /* succeed, to avoid retrial */
}




struct file_operations sleepy_fops = {
    .owner = THIS_MODULE,
    .read =  sleepy_read,
    .write = sleepy_write,
};




int sleepy_init(void)
{
    int result;


    /*
 *      * Register your major, and accept a dynamic number
 *           */
    result = register_chrdev(sleepy_major, "sleepy", &sleepy_fops);
    if (result < 0)
        return result;
    if (sleepy_major == 0)
        sleepy_major = result; /* dynamic */
    return 0;
}


void sleepy_cleanup(void)
{
    unregister_chrdev(sleepy_major, "sleepy");
}

module_init(sleepy_init);
module_exit(sleepy_cleanup);

编译好模块，插进去后，可以通过 /proc/devices 文件查看获取的主设备号为多少，然后在 mknod 建立设备文件。

用户态读测试程序：

[C++] 纯文本查看 复制代码

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>


int main(void)
{
        int fd;
        char buf[100];
        if((fd = open ("/dev/sleepy", O_RDONLY)) < 0) {
                perror ("open");
                exit (EXIT_FAILURE);
        }


        printf ("ready to read sleepy device and waiting data coming...\n");


        if(read(fd, buf, 10) == -1) {
                perror ("read");
                exit (EXIT_FAILURE);
        }




        printf ("OK, i got it, bye\n");


        return (0);
}

当读进程读取设备时，因为 flag 标志为 0，所以被 wait_event_interruptible() 置入睡眠态，它在等待设备被写入时将 flag 标志改变。


写用户态测试程序：

[C++] 纯文本查看 复制代码

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>


int main(void)
{
        int fd;
        char buf[10] = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'};
        if((fd = open ("/dev/sleepy", O_RDWR)) < 0) {
                perror ("open");
                exit (EXIT_FAILURE);
        }


        printf ("write 10 characters to buffer.\n");


        if(write(fd, buf, 10) == -1) {
                perror ("write");
                exit (EXIT_FAILURE);
        }


        printf ("write over.\n");


        return (0);
}

当写操作发生时，wake_up_interruptible() 函数会唤醒因为等待 flag 状态改变而睡眠的进程。


运行读程序时，会在 read() 这里阻塞，当写操作发生时，它才会往下运行然后退出。


最后通过 dmesg 可以看到驱动模块里的打印信息：

process 3217 (read) going to sleep
awoken 3217 (read)
process 3252 (read) going to sleep
process 3254 (write) awakening the readers...