重新审视进程间的通信

Posted 2020-07-18 penguin_wwy

最近干活的时候又被Linux管道和消息队列搞的一脸懵逼。当初自己走马观花似的学习以为内容很简单，结果留下了大坑，借来Unix网络编程来补补，重新审视这两个部分，并且引以为戒！！！

首先看管道

#include<unistd.h>

int pipe(int fd[2]);

返回：成功为0，出错为1，两个文件描述符fd[0]用来读，fd[1]用来写

灵魂作图

单进程管道

刚fork后

父进程关闭管道读出端，子进程关闭管道写入端，在父子进程间提供一个单向数据流

管道只能用于父子进程或者兄弟进程间通信，也就是说管道只能用于具有亲缘关系的进程间通信

管道的缓冲区大小是受限制的。管道所传输的是无格式的字节流。这就需要管道输入方和输出方事先约定好数据格式

有名管道可用于没有亲缘关系的进程间通信（name pipe或者叫FIFO）

#include<sys/types.h>
#include<sys/stat.h>

int mkinfo(const char *pathname, mode_t mode);//<span style="font-family: 宋体, Arial; line-height: 26px;"><span style="font-size:12px;">pathname为创建有名管道的全路径名，mode为创建有名管道的模式</span></span>

返回：若成功则0，不成功则-1

实现分析

//管道缓冲区个数
#define PIPE_BUFFERS (16)
//管道缓存区对象结构
struct pipe_buffer {
    struct page *page; //管道缓冲区页框的描述符地址
    unsigned int offset, len; //页框内有效数据的当前位置，和有效数据的长度
    struct pipe_buf_operations *ops; //管道缓存区方法表的地址
};

//管道信息结构
struct pipe_inode_info {
    wait_queue_head_t wait; //管道等待队列
    unsigned int nrbufs, curbuf; //包含待读数据的缓冲区数和包含待读数据的第一个缓冲区的索引
    struct pipe_buffer bufs[PIPE_BUFFERS]; //管道缓冲区描述符数组
    struct page *tmp_page; //高速缓存区页框指针
    unsigned int start;  //当前管道缓存区读的位置
    unsigned int readers; //读进程的标志，或编号
    unsigned int writers; //写进程的标志，或编号
    unsigned int waiting_writers; //在等待队列中睡眠的写进程的个数
    unsigned int r_counter; //与readers类似，但当等待写入FIFO的进程是使用
    unsigned int w_counter; //与writers类似，但当等待写入FIFO的进程时使用
    struct fasync_struct *fasync_readers; //用于通过信号进行的异步I/O通知
    struct fasync_struct *fasync_writers; //用于通过信号的异步I/O通知
};

//管道读操作函数
static ssize_t
pipe_readv(struct file *filp, const struct iovec *_iov,
    unsigned long nr_segs, loff_t *ppos)
{
 struct inode *inode = filp->f_dentry->d_inode; //获取inode结点指针
 struct pipe_inode_info *info;
 int do_wakeup;
 ssize_t ret;
 struct iovec *iov = (struct iovec *)_iov; //获取读缓冲区的结构
 size_t total_len;
 total_len = iov_length(iov, nr_segs);
 /* Null read succeeds. */
 if (unlikely(total_len == 0))
  return 0;
 do_wakeup = 0;
 ret = 0;
 down(PIPE_SEM(*inode)); //获取inode中的i_sem信号量
 info = inode->i_pipe; //获取inode 结构的pipe_inode_info结构指针
 for (;;) {
  int bufs = info->nrbufs; //检查有几个管道缓冲区有被读取的数据
  if (bufs) { //说明有其中有缓冲区包含了读数据
   int curbuf = info->curbuf; //获取当前读数据的管道缓存区的索引
   struct pipe_buffer *buf = info->bufs + curbuf; //共有16个缓冲区，curbuf是当前的
   struct pipe_buf_operations *ops = buf->ops; //获取操作函数列表
   void *addr;
   size_t chars = buf->len; 
   int error;
   //若缓冲区长度大于要求读取的数据长度，chars设置成要求读的长度
   if (chars > total_len) 
    chars = total_len;
   //执行Map方法
   addr = ops->map(filp, info, buf);
   //从缓存区中复制数据
   error = pipe_iov_copy_to_user(iov, addr + buf->offset, chars);
   //执行umap方法
   ops->unmap(info, buf);
   if (unlikely(error)) { 
    if (!ret) ret = -EFAULT; //第一次读失败
    break;
   }
   //更新管道的offset和len字段
   ret += chars;
   buf->offset += chars;
   buf->len -= chars;
   
   //若现在的缓存区的数据长度为0
   if (!buf->len) {
    buf->ops = NULL;
    ops->release(info, buf);
    curbuf = (curbuf + 1) & (PIPE_BUFFERS-1);
    info->curbuf = curbuf;
    info->nrbufs = --bufs;
    do_wakeup = 1;
   }
   total_len -= chars;  //更新读的总长度
   if (!total_len)  //该读的已读完成
    break; /* common path: read succeeded */
  }
  if (bufs) /* More to do? */
   continue;
  //若bufs为0，说明所有管道为NULL，此时进行一下操作
  if (!PIPE_WRITERS(*inode)) //是否有写操作正在进行
   break;
  if (!PIPE_WAITING_WRITERS(*inode)) { //是否需要等待
   /* syscall merging: Usually we must not sleep
    * if O_NONBLOCK is set, or if we got some data.
    * But if a writer sleeps in kernel space, then
    * we can wait for that data without violating POSIX.
    */
   if (ret)
    break;
   if (filp->f_flags & O_NONBLOCK) { //要等待但又设置了NONBLOCK标记，矛盾了
    ret = -EAGAIN;
    break;
   }
  }
  if (signal_pending(current)) { //设置进程阻塞标志
   if (!ret) ret = -ERESTARTSYS;
   break;
  }
  if (do_wakeup) {
   wake_up_interruptible_sync(PIPE_WAIT(*inode));
    kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT);
  }
  pipe_wait(inode); 
 }
 up(PIPE_SEM(*inode));
 /* Signal writers asynchronously that there is more room.  */
 if (do_wakeup) {
  wake_up_interruptible(PIPE_WAIT(*inode));
  kill_fasync(PIPE_FASYNC_WRITERS(*inode), SIGIO, POLL_OUT);
 }
 if (ret > 0)
  file_accessed(filp);  //更新文件结构的atime对象
 return ret;
}
static ssize_t
pipe_read(struct file *filp, char __user *buf, size_t count, loff_t *ppos)
{
 struct iovec iov = { .iov_base = buf, .iov_len = count };
 return pipe_readv(filp, &iov, 1, ppos);
}

/* Drop the inode semaphore and wait for a pipe event, atomically */
void pipe_wait(struct inode * inode)
{
    DEFINE_WAIT(wait);
 //把current添加到管道的等待队列中
    prepare_to_wait(PIPE_WAIT(*inode), &wait, TASK_INTERRUPTIBLE);
 //释放i_sem
    up(PIPE_SEM(*inode));
    schedule();
 //被呼醒，把它从等待队列中删除
    finish_wait(PIPE_WAIT(*inode), &wait);
 //再次获取i_sem索引节点信号量
    down(PIPE_SEM(*inode));
}

static ssize_t
pipe_writev(struct file *filp, const struct iovec *_iov,
     unsigned long nr_segs, loff_t *ppos)
{
    struct inode *inode = filp->f_dentry->d_inode;
    struct pipe_inode_info *info;
    ssize_t ret;
    int do_wakeup;
    struct iovec *iov = (struct iovec *)_iov;
    size_t total_len;

    total_len = iov_length(iov, nr_segs);
    /* Null write succeeds. */
    if (unlikely(total_len == 0))
        return 0;

    do_wakeup = 0;
    ret = 0;
    down(PIPE_SEM(*inode));
    info = inode->i_pipe;

    //是否有读者进程存在，若没有写管道操作就没有任何意义

    //此时产生SIGPIPE信号

    if (!PIPE_READERS(*inode)) {
        send_sig(SIGPIPE, current, 0);
        ret = -EPIPE;
        goto out;
    }

    /* We try to merge small writes */
    //若有待读数据的缓冲区，而且写入的数据长度小于PAGE_SIZE

    if (info->nrbufs && total_len < PAGE_SIZE) {
        //第一个待读缓冲区+可读缓冲区数-1得到第一个可写缓冲区的地址

        int lastbuf = (info->curbuf + info->nrbufs - 1) & (PIPE_BUFFERS-1);
        struct pipe_buffer *buf = info->bufs + lastbuf;
        struct pipe_buf_operations *ops = buf->ops;
        int offset = buf->offset + buf->len;
        //若可写缓冲区的剩余的空间大于写入的数据总量total_len

        if (ops->can_merge && offset + total_len <= PAGE_SIZE) { 
            void *addr = ops->map(filp, info, buf);
            //把数据复制到管道缓冲区

            int error = pipe_iov_copy_from_user(offset + addr, iov, total_len);
            ops->unmap(info, buf);
            ret = error;
            do_wakeup = 1;
            if (error)
                goto out;
            //更新有效数据长度字段

            buf->len += total_len;
            ret = total_len;
            goto out;
        }
            
    }

    // 若全部可写（可读缓冲区数为0），

    // 或写入数据长度大于管道缓冲区的长度单位(PAGE_SIZE)

    for (;;) {
        int bufs;
        //是否有读者进程存在

        if (!PIPE_READERS(*inode)) {
            send_sig(SIGPIPE, current, 0);
            if (!ret) ret = -EPIPE;
            break;
        }
        //获取读缓冲区数

        bufs = info->nrbufs;
        if (bufs < PIPE_BUFFERS) {
            ssize_t chars;
            //用第一个可读缓冲区+可读缓冲区数得到可写(空)缓冲区的地址

            int newbuf = (info->curbuf + bufs) & (PIPE_BUFFERS-1);
            struct pipe_buffer *buf = info->bufs + newbuf;
            struct page *page = info->tmp_page;
            int error;

            //若page的值为空，从伙伴系统中获取一页

            if (!page) {
                page = alloc_page(GFP_HIGHUSER);
                if (unlikely(!page)) {
                    ret = ret ? : -ENOMEM;
                    break;
                }
                info->tmp_page = page;
            }
            /* Always wakeup, even if the copy fails. Otherwise
             * we lock up (O_NONBLOCK-)readers that sleep due to
             * syscall merging.
             * FIXME! Is this really true?
             */
            do_wakeup = 1;
            chars = PAGE_SIZE;
            if (chars > total_len)
                chars = total_len;

            //写chars字节到缓冲区中

            error = pipe_iov_copy_from_user(kmap(page), iov, chars);
            kunmap(page);
            if (unlikely(error)) {
                if (!ret) ret = -EFAULT;
                break;
            }
            ret += chars;

            /* Insert it into the buffer array */
            /更新nrbufs，和len字段。
            buf->page = page;
            buf->ops = &anon_pipe_buf_ops;
            buf->offset = 0;
            buf->len = chars;
            info->nrbufs = ++bufs;
            info->tmp_page = NULL;

            //若没有写完继续写入剩下的数据

            total_len -= chars;
            if (!total_len)
                break;
        }
        //还有可写缓冲区，继续写

        if (bufs < PIPE_BUFFERS)
            continue;
        //若设置非阻塞，

        //若没有写入任何的数据ret=0,此时返回错误

        //若已经写完了数据，结束写操作。

        if (filp->f_flags & O_NONBLOCK) {
            if (!ret) ret = -EAGAIN;
            break;
        }
        if (signal_pending(current)) {
            if (!ret) ret = -ERESTARTSYS;
            break;
        }
        if (do_wakeup) {
            wake_up_interruptible_sync(PIPE_WAIT(*inode));
            kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN);
            do_wakeup = 0;
        }
        PIPE_WAITING_WRITERS(*inode)++;
        pipe_wait(inode);
        PIPE_WAITING_WRITERS(*inode)--;
    }
out:
    up(PIPE_SEM(*inode));
    if (do_wakeup) {
        wake_up_interruptible(PIPE_WAIT(*inode));
        kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN);
    }
    if (ret > 0)
        inode_update_time(inode, 1);    /* mtime and ctime */
    return ret;
}

PS： 管道是作为一组VFS对象来实现的，因此没有对应的磁盘映像。所以管道的安装和实现都是VFS类似，此处不进行探讨