Linux poll机制

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

it waits for one of a set of file descriptors to become ready to perform I/O.

The set of file descriptors to be monitored is specified in the fds argument, which is an array of structures of the following form:

struct pollfd {

    int   fd;         /* file descriptor */

    short events;     /* requested events */

    short revents;    /* returned events */

};

关于timeout参数的说明：

　　timeout>0，设置超时时间为timeout

　　timeout=0，直接返回

　　timeout<0，无限长超时时间

返回值说明：

On success, a positive number is returned; this is the number of structures which have nonzero revents fields (in other words, those descriptors with events or errors reported).

A value of 0 indicates that the call timed out and no file descriptors were ready. On error, -1 is returned, and errno is set appropriately.

2.内核调用

asmlinkage long sys_poll(struct pollfd __user *ufds, unsigned int nfds,long timeout_msecs)

    -->ret = do_sys_poll(ufds, nfds, &timeout);

        -->struct poll_wqueues table;

        -->poll_initwait(&table);

            -->init_poll_funcptr(&pwq->pt, __pollwait);

        -->把ufds指向的用户空间数据拷贝到内核空间，由poll_list结构体保存

        -->fdcount = do_poll(nfds, head, &table, timeout);

        for (;;) {

            -->set_current_state(TASK_INTERRUPTIBLE);

            -->for (walk = list; walk != NULL; walk = walk->next) //遍历poll_list结构中的pollfd结构

                -->do_pollfd(pfd, pt)

                    -->if (file->f_op && file->f_op->poll) //驱动中的poll函数

                        --> void __pollwait(struct file *filp, wait_queue_head_t *wait_address,poll_table *p)

                            -->struct poll_table_entry *entry = poll_get_entry(p);

                            -->entry->filp = filp;

                            -->entry->wait_address = wait_address;

                            -->init_waitqueue_entry(&entry->wait, current);

                            -->add_wait_queue(wait_address, &entry->wait);//加入等待队列头

                    -->return mask;//返回设备是否可读写状态

            -->if (count || !*timeout || signal_pending(current))

                break;//设备可读写，超时时间到，有信号中断三种情况都返回

            -->__timeout = schedule_timeout(__timeout);//进程调度，休眠

        }

            //唤醒后返回

            -->__set_current_state(TASK_RUNNING);

            -->return count;

        -->把poll_list结构体保存的pollfd.revents返回给用户空间的ufds数组

        -->poll_freewait(&table);//释放poll_table_entry中的等待队列

3.相关结构体都在fs/select.c，include/linux/poll.h

poll_wqueues是最关键的一个结构体，

 struct poll_wqueues {

     poll_table pt;

     struct poll_table_page * table;

     int error;

     int inline_index;

     struct poll_table_entry inline_entries[N_INLINE_POLL_ENTRIES];

 };

poll_table结构体只包含一个函数指针，初始化时指向

void __pollwait(struct file *filp, wait_queue_head_t *wait_address,poll_table *p)

 typedef void (*poll_queue_proc)(struct file *, wait_queue_head_t *, struct poll_table_struct *);

 typedef struct poll_table_struct {

     poll_queue_proc qproc;

 } poll_table;

poll_table_page就是包含poll_table_entry结构的链表，作用与inline_entries[N]相同，检测文件数少的时候用不到这个结构

 struct poll_table_page {

     struct poll_table_page * next;

     struct poll_table_entry * entry;

     struct poll_table_entry entries[];

 };

poll_table_entry包含文件指针，等待队列及等待队列头

 struct poll_table_entry {

     struct file * filp;

     wait_queue_t wait;

     wait_queue_head_t * wait_address;

 };

4.操作

poll_wqueues初始化

 static inline void init_poll_funcptr(poll_table *pt, poll_queue_proc qproc)

 {

     pt->qproc = qproc;

 }

 void poll_initwait(struct poll_wqueues *pwq)

 {

     init_poll_funcptr(&pwq->pt, __pollwait);

     pwq->error = ;

     pwq->table = NULL;

     pwq->inline_index = ;

 }

那么poll_table结构中的函数什么时候使用及干什么呢？

我们自己在驱动中的poll函数中会调用poll_wait()函数

 static unsigned forth_drv_poll(struct file *file, poll_table *wait)

 {

     unsigned int mask = ;

     poll_wait(file, &button_waitq, wait); // 不会立即休眠

     if (ev_press)

         mask |= POLLIN | POLLRDNORM;

     return mask;

 }

而poll_wait()函数最终会调用__pollwait()函数

static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)

{

    if (p && wait_address)

        p->qproc(filp, wait_address, p);

}

 /* Add a new entry */

 static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,

                 poll_table *p)

 {

     struct poll_table_entry *entry = poll_get_entry(p);

     if (!entry)

         return;

     get_file(filp);

     entry->filp = filp;

     entry->wait_address = wait_address; //即forth_drv_poll中传入的button_waitq等待队列头

     init_waitqueue_entry(&entry->wait, current);

     add_wait_queue(wait_address, &entry->wait);

 }

巴特西

Linux poll机制

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