linux环形缓冲区kfifo实践3：IO多路复用poll和select

千册2023-08-10 22:24

基础知识

poll和select方法在Linux用户空间的API接口函数定义如下。

cpp 复制代码 
int poll(struct pollfd *fds, nfds_t nfds, int timeout);

poll()函数的第一个参数fds是要监听的文件描述符集合，类型为指向struct pollfd的指针。struct pollfd数据结构定义如下。

cpp 复制代码 
struct pollfd {
    int　 fd;
    short events;
    short revents;
};

fd表示要监听的文件描述符，events表示监听的事件，revents表示返回的事件。常用的监听的事件有如下类型（掩码）。

  • POLLIN：数据可以立即被读取。
  • POLLRDNORM：等同于POLLIN，表示数据可以立即被读取。
  • POLLERR：设备发生了错误。
  • POLLOUT：设备可以立即写入数据。

poll()函数的第二个参数nfds是要监听的文件描述符的个数；第三个参数timeout是单位为ms的超时，负数表示一直监听，直到被监听的文件描述符集合中有设备发生了事件。
Linux内核的file_operations方法集提供了poll方法的实现。

cpp 复制代码 
<include/linux/fs.h>
struct file_operations {
    ...
    unsigned int (*poll) (struct file *, struct poll_table_struct *);
    ...
};

当用户程序打开设备文件后执行poll或者select系统调用时，驱动程序的poll方法就会被调用。设备驱动程序的poll方法会执行如下步骤。

1）在一个或者多个等待队列中调用poll_wait()函数。poll_wait()函数会把当前进程添加到指定的等待列表（poll_table）中，当请求数据准备好之后，会唤醒这些睡眠的进程。

2）返回监听事件，也就是POLLIN或者POLLOUT等掩码。因此，poll方法的作用就是让应用程序同时等待多个数据流。

驱动代码：

cpp 复制代码 
#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/types.h>
#include <linux/miscdevice.h>
#include <linux/kfifo.h>
#include <linux/wait.h>
#include <linux/poll.h>

#define DEBUG_INFO(format, ...) printk("%s:%d -- "format"\n",\
__func__,__LINE__,##__VA_ARGS__)

struct ch5_kfifo_struct{
    struct miscdevice misc;
    struct file_operations fops;
    struct kfifo fifo;
    char buf[64];
    char name[64];
    wait_queue_head_t read_queue;
    wait_queue_head_t write_queue;
};

static int ch5_open (struct inode *inode, struct file *file){
    struct ch5_kfifo_struct *p = (struct ch5_kfifo_struct *)container_of(file->f_op,struct ch5_kfifo_struct,fops);
    file->private_data = p;
    DEBUG_INFO("major = %d, minor = %d\n",MAJOR(inode->i_rdev),MINOR(inode->i_rdev));
    DEBUG_INFO("name = %s",p->misc.name);
    return 0;
}

static int ch5_release (struct inode *inode, struct file *file){
    DEBUG_INFO("close");
    return 0;
}

static unsigned int ch5_poll(struct file *file, poll_table *wait){
    int mask = 0;
    struct ch5_kfifo_struct *p __attribute__((unused)) = (struct ch5_kfifo_struct *)file->private_data;
    DEBUG_INFO("begin wait:%s",p->name);
    poll_wait(file, &p->read_queue, wait);
    poll_wait(file, &p->write_queue, wait);
    DEBUG_INFO("poll:%s",p->name);
    if (!kfifo_is_empty(&p->fifo)){
        mask |= POLLIN | POLLRDNORM;
        DEBUG_INFO("POLLIN:%s",p->name);
    }
        
    if (!kfifo_is_full(&p->fifo)){
        mask |= POLLOUT | POLLWRNORM;
        DEBUG_INFO("POLLOUT:%s",p->name);
    }
        
    return mask;
}

static ssize_t ch5_read (struct file *file, char __user *buf, size_t size, loff_t *pos){
    struct ch5_kfifo_struct *p __attribute__((unused)) = (struct ch5_kfifo_struct *)file->private_data;
    int ret;
    int actual_readed = 0;

    if(kfifo_is_empty(&p->fifo)){
        if(file->f_flags & O_NONBLOCK){
            DEBUG_INFO("kfifo is null");
            return -EAGAIN;
        }
        ret = wait_event_interruptible(p->read_queue,kfifo_is_empty(&p->fifo) == 0);
        if(ret){
            DEBUG_INFO("wait_event_interruptible error");
            return ret;
        }
        DEBUG_INFO("");
    }
    
    ret = kfifo_to_user(&p->fifo, buf, size, &actual_readed);
    if (ret){
        DEBUG_INFO("kfifo_to_user error");
		return -EIO;
    }
    
    DEBUG_INFO("size = %d,actual_readed = %d\n",size,actual_readed);

    if (!kfifo_is_full(&p->fifo)){
        wake_up_interruptible(&p->write_queue);
    }

    memset(p->buf,0,sizeof(p->buf));
    ret = copy_from_user(p->buf, buf, actual_readed);
    if(ret != 0){
        DEBUG_INFO("copy_from_user error ret = %d\n",ret);
    }else{
        DEBUG_INFO("read p->buf = %s\n",p->buf);
    }
    *pos = *pos + actual_readed;
    return actual_readed;
}
static ssize_t ch5_write (struct file *file, const char __user *buf, size_t size, loff_t* pos){
    struct ch5_kfifo_struct *p = (struct ch5_kfifo_struct *)file->private_data;
    int actual_writed = 0;
    int ret;
    if(kfifo_is_full(&p->fifo)){
        if(file->f_flags & O_NONBLOCK){
            DEBUG_INFO("kfifo is full");
            return -EAGAIN;
        }
        ret = wait_event_interruptible(p->write_queue, kfifo_is_full(&p->fifo) == 0);
        if(ret){
            DEBUG_INFO("wait_event_interruptible error");
            return ret;
        }
        DEBUG_INFO("");
    }
    ret = kfifo_from_user(&p->fifo, buf, size, &actual_writed);
    if (ret){
        DEBUG_INFO("kfifo_from_user error");
		return -EIO;
    }
    
    DEBUG_INFO("actual_writed = %d\n",actual_writed);

    if (!kfifo_is_empty(&p->fifo)){
        wake_up_interruptible(&p->read_queue);
    }
    memset(p->buf,0,sizeof(p->buf));
    ret = copy_from_user(p->buf, buf, actual_writed);
    if(ret != 0){
        DEBUG_INFO("copy_from_user error ret = %d\n",ret);
    }else{
        DEBUG_INFO("write:p->buf = %s\n",p->buf);
    }
    *pos = *pos + actual_writed;
    return actual_writed;
}

struct ch5_kfifo_struct ch5_kfifo[8];
//  = {
//     .misc = { 
//         .name = "ch5-04-block",
//         .minor = MISC_DYNAMIC_MINOR,
//     },
//     .fops = {
//         .owner = THIS_MODULE,
//         .read = ch5_read,
//         .write = ch5_write,
//         .open = ch5_open,
//         .release = ch5_release,
//     },
// };

static int __init ch5_init(void){
    int ret = 0;
    int i = 0;
    struct ch5_kfifo_struct *p;

    DEBUG_INFO("start init\n");
    for(i = 0;i < sizeof(ch5_kfifo)/sizeof(ch5_kfifo[0]);i++){
        p = &ch5_kfifo[i];
        snprintf(p->name,sizeof(p->name),"ch5-05-poll-%d",i);
        p->misc.name = p->name;
        p->misc.minor = MISC_DYNAMIC_MINOR;

        p->fops.owner = THIS_MODULE;
        p->fops.read = ch5_read;
        p->fops.write = ch5_write;
        p->fops.open = ch5_open;
        p->fops.release = ch5_release;
        p->fops.poll = ch5_poll;

        p->misc.fops = &p->fops;
        ret = kfifo_alloc(&p->fifo,
                    8,
                    GFP_KERNEL);
        if (ret) {
            DEBUG_INFO("kfifo_alloc error: %d\n", ret);
            ret = -ENOMEM;
            return ret;
        }
        DEBUG_INFO("kfifo_alloc size = %d",kfifo_avail(&p->fifo));

        init_waitqueue_head(&p->read_queue);
        init_waitqueue_head(&p->write_queue);

        ret = misc_register(&p->misc);
        if(ret < 0){
            DEBUG_INFO("misc_register error: %d\n", ret);
            return ret;
        }
    }
    
    DEBUG_INFO("misc_register ok");
    return 0;
}

static void __exit ch5_exit(void){
    int i = 0;
    struct ch5_kfifo_struct *p;
    for(i = 0;i < sizeof(ch5_kfifo)/sizeof(ch5_kfifo[0]);i++){
        p = &ch5_kfifo[i];
        misc_deregister(&p->misc);
        kfifo_free(&p->fifo);
    }

    DEBUG_INFO("exit\n");
}

module_init(ch5_init);
module_exit(ch5_exit);

MODULE_LICENSE("GPL");

应用测试代码：

cpp 复制代码 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <errno.h>
#include <poll.h>
#include <linux/input.h>
#include <unistd.h>

#define DEBUG_INFO(format, ...) printf("%s:%d -- "format"\n",\
__func__,__LINE__,##__VA_ARGS__)

int main(int argc, char**argv){
    char bufs[8][1024];
    char name[8][1024];
    int fd[8];
    int i = 0;
    int ret;
    int len;
    struct pollfd fds[8];

    for(i = 0;i < 8;i++){
        memset(name[i], 0, sizeof(name[i]));
        snprintf(name[i], sizeof(name[i]),"/dev/ch5-05-poll-%d",i);
        fd[i] = open(name[i],O_RDONLY | O_NONBLOCK);
        if(fd[i] < 0){
            perror("open");
            DEBUG_INFO("open %s failed",name[i]);
            return -1;
        }
        fds[i].fd = fd[i];
        fds[i].events = POLLIN;
        fds[i].revents = 0;
    }
    DEBUG_INFO("start poll\n");
    while(1){
        ret = poll(fds,8,-1);
        if(ret < 0){
            perror("poll");
            return 0;
        }
        DEBUG_INFO("ret %d\n",ret);
        for(i = 0;i < 8;i++){
            if(fds[i].revents & POLLIN){
                while(1){
                    memset(bufs[i],0,sizeof(bufs[i]));
                    len = read(fd[i],(void*)&bufs[i][0],sizeof(bufs[i]));
                    if(len <= 0){
                        DEBUG_INFO("read %s finish",name[i]);
                        break;
                    }
                    DEBUG_INFO("read %s :buf = %s",name[i],bufs[i]);
                }
            }
        }
    }
    return 0;
}

测试：

加载模块生成8个设备

后台运行应用：

bash 复制代码 
/mnt # ./app &
/mnt # [  519.185812] ch5_open:30 -- major = 10, minor = 58
[  519.185812] 
[  519.186175] ch5_open:31 -- name = ch5-05-poll-0
[  519.186849] ch5_open:30 -- major = 10, minor = 57
[  519.186849] 
[  519.187772] ch5_open:31 -- name = ch5-05-poll-1
[  519.190276] ch5_open:30 -- major = 10, minor = 56
[  519.190276] 
[  519.190553] ch5_open:31 -- name = ch5-05-poll-2
[  519.191129] ch5_open:30 -- major = 10, minor = 55
[  519.191129] 
[  519.191604] ch5_open:31 -- name = ch5-05-poll-3
[  519.192090] ch5_open:30 -- major = 10, minor = 54
[  519.192090] 
[  519.192283] ch5_open:31 -- name = ch5-05-poll-4
[  519.192759] ch5_open:30 -- major = 10, minor = 53
[  519.192759] 
[  519.193020] ch5_open:31 -- name = ch5-05-poll-5
[  519.193486] ch5_open:30 -- major = 10, minor = 52
[  519.193486] 
[  519.193677] ch5_open:31 -- name = ch5-05-poll-6
[  519.194148] ch5_open:30 -- major = 10, minor = 51
[  519.194148] 
[  519.194340] ch5_open:31 -- name = ch5-05-poll-7
main:38 -- start poll

[  519.199679] ch5_poll:43 -- begin wait:ch5-05-poll-0
[  519.200029] ch5_poll:46 -- poll:ch5-05-poll-0
[  519.200236] ch5_poll:54 -- POLLOUT:ch5-05-poll-0
[  519.200358] ch5_poll:43 -- begin wait:ch5-05-poll-1
[  519.200598] ch5_poll:46 -- poll:ch5-05-poll-1
[  519.200770] ch5_poll:54 -- POLLOUT:ch5-05-poll-1
[  519.201213] ch5_poll:43 -- begin wait:ch5-05-poll-2
[  519.201531] ch5_poll:46 -- poll:ch5-05-poll-2
[  519.201646] ch5_poll:54 -- POLLOUT:ch5-05-poll-2
[  519.201727] ch5_poll:43 -- begin wait:ch5-05-poll-3
[  519.201847] ch5_poll:46 -- poll:ch5-05-poll-3
[  519.201998] ch5_poll:54 -- POLLOUT:ch5-05-poll-3
[  519.202154] ch5_poll:43 -- begin wait:ch5-05-poll-4
[  519.202347] ch5_poll:46 -- poll:ch5-05-poll-4
[  519.202504] ch5_poll:54 -- POLLOUT:ch5-05-poll-4
[  519.202661] ch5_poll:43 -- begin wait:ch5-05-poll-5
[  519.202850] ch5_poll:46 -- poll:ch5-05-poll-5
[  519.203122] ch5_poll:54 -- POLLOUT:ch5-05-poll-5
[  519.203276] ch5_poll:43 -- begin wait:ch5-05-poll-6
[  519.203490] ch5_poll:46 -- poll:ch5-05-poll-6
[  519.203643] ch5_poll:54 -- POLLOUT:ch5-05-poll-6
[  519.203798] ch5_poll:43 -- begin wait:ch5-05-poll-7
[  519.203964] ch5_poll:46 -- poll:ch5-05-poll-7
[  519.204109] ch5_poll:54 -- POLLOUT:ch5-05-poll-7

写数据

bash 复制代码 
/mnt # echo "hello world" > /dev/ch5-05-poll-0 
[  576.205375] ch5_open:30 -- major = 10, minor = 58
[  576.205375] 
[  576.206863] ch5_open:31 -- name = ch5-05-poll-0
[  576.208285] ch5_write:122 -- actual_writed = 8
[  576.208285] 
[  576.208757] ch5_write:132 -- write:p->buf = hello wo
[  576.208757] 
[  576.209448] ch5_poll:43 -- begin wait:ch5-05-poll-0
[  576.209675] ch5_poll:46 -- poll:ch5-05-poll-0
[  576.209874] ch5_poll:49 -- POLLIN:ch5-05-poll-0
[  576.210058] ch5_poll:43 -- begin wait:ch5-05-poll-1
[  576.210291] ch5_poll:46 -- poll:ch5-05-poll-1
[  576.210451] ch5_poll:54 -- POLLOUT:ch5-05-poll-1
[  576.210645] ch5_poll:43 -- begin wait:ch5-05-poll-2
[  576.211496] ch5_poll:46 -- poll:ch5-05-poll-2
[  576.212140] ch5_poll:54 -- POLLOUT:ch5-05-poll-2
[  576.212315] ch5_poll:43 -- begin wait:ch5-05-poll-3
[  576.212517] ch5_poll:46 -- poll:ch5-05-poll-3
[  576.212707] ch5_poll:54 -- POLLOUT:ch5-05-poll-3
[  576.212928] ch5_poll:43 -- begin wait:ch5-05-poll-4
[  576.213169] ch5_poll:46 -- poll:ch5-05-poll-4
[  576.213333] ch5_poll:54 -- POLLOUT:ch5-05-poll-4
[  576.213515] ch5_poll:43 -- begin wait:ch5-05-poll-5
[  576.213752] ch5_poll:46 -- poll:ch5-05-poll-5
[  576.213869] ch5_poll:54 -- POLLOUT:ch5-05-poll-5
[  576.214067] ch5_poll:43 -- begin wait:ch5-05-poll-6
[  576.214238] ch5_poll:46 -- poll:ch5-05-poll-6
[  576.214392] ch5_poll:54 -- POLLOUT:ch5-05-poll-6
[  576.214545] ch5_poll:43 -- begin wait:ch5-05-poll-7
[  576.214963] ch5_poll:46 -- poll:ch5-05-poll-7
[  576.215300] ch5_poll:54 -- POLLOUT:ch5-05-poll-7
main:45 -- ret 1
[  576.216522] ch5_read:84 -- size = 1024,actual_readed = 8
[  576.216522] 
[  576.216920] ch5_read:95 -- read p->buf = hello wo
[  576.216920] 
[  576.217219] ch5_write:114 -- 
main:55 -- read /dev/ch5-05-poll-0 :buf = hello wo
[  576.217652] ch5_write:122 -- actual_writed = 4
[  576.217652] 
[  576.217989] ch5_write:132 -- write:p->buf = rld
[  576.217989] 
[  576.217989] 
main:55 -- read /dev/ch5-05-poll-0 :buf = rld[  576.218140] ch5_read:84 -- size = 1024,actual_readed = 4
[  576.218140] 

[  576.218294] ch5_read:95 -- read p->buf = rld
[  576.218294] 
[  576.218294] 

[  576.219434] ch5_read:67 -- kfifo is null
main:52 -- read /dev/ch5-05-poll-0 finish[  576.220169] ch5_release:36 -- close

[  576.220874] ch5_poll:43 -- begin wait:ch5-05-poll-0
[  576.221959] ch5_poll:46 -- poll:ch5-05-poll-0
[  576.222639] ch5_poll:54 -- POLLOUT:ch5-05-poll-0
[  576.224333] ch5_poll:43 -- begin wait:ch5-05-poll-1
[  576.225068] ch5_poll:46 -- poll:ch5-05-poll-1
[  576.225443] ch5_poll:54 -- POLLOUT:ch5-05-poll-1
[  576.225890] ch5_poll:43 -- begin wait:ch5-05-poll-2
[  576.226298] ch5_poll:46 -- poll:ch5-05-poll-2
[  576.226829] ch5_poll:54 -- POLLOUT:ch5-05-poll-2
[  576.227435] ch5_poll:43 -- begin wait:ch5-05-poll-3
/mnt # [  576.227639] ch5_poll:46 -- poll:ch5-05-poll-3
[  576.227892] ch5_poll:54 -- POLLOUT:ch5-05-poll-3
[  576.228124] ch5_poll:43 -- begin wait:ch5-05-poll-4
[  576.228344] ch5_poll:46 -- poll:ch5-05-poll-4
[  576.228573] ch5_poll:54 -- POLLOUT:ch5-05-poll-4
[  576.228902] ch5_poll:43 -- begin wait:ch5-05-poll-5
[  576.229155] ch5_poll:46 -- poll:ch5-05-poll-5
[  576.229349] ch5_poll:54 -- POLLOUT:ch5-05-poll-5
[  576.229573] ch5_poll:43 -- begin wait:ch5-05-poll-6
[  576.229761] ch5_poll:46 -- poll:ch5-05-poll-6
[  576.230005] ch5_poll:54 -- POLLOUT:ch5-05-poll-6
[  576.230233] ch5_poll:43 -- begin wait:ch5-05-poll-7
[  576.230421] ch5_poll:46 -- poll:ch5-05-poll-7
[  576.231095] ch5_poll:54 -- POLLOUT:ch5-05-poll-7

小结

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