为什么cat一次足够调用两次read（）？

Question

我是Linux内核模块的新手。我正在学习基于网络课程的char驱动程序模块。我有一个非常简单的模块，它创建一个/dev/chardevexample，我有一个问题需要我理解：

当我执行echo "hello4" > /dev/chardevexample时，我看到write仅按预期执行一次。但是，当我执行cat /dev/chardevexample时，我看到读取执行了[[two次。

我在代码和课程资料中都看到了这一点。所有数据都在第一个read()中返回，那么为什么cat再次调用它？

到目前为止，我所做的所有事情如下：

[insmod chardev.ko加载我的模块
1. echo "hello4" > /dev/chardevexample。这是写操作，我看到它在dmesg中恰好发生一次
2. cat /dev/chardevexample。这是读数，dmesg显示它发生了两次。
4. 我做了strace cat /dev/chardevexample，确实看到函数调用被两次调用以进行读取。之间也有一个写操作

   read(3, "hello4\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 131072) = 4096
   write(1, "hello4\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,    4096hello4) = 4096
   read(3, "", 131072)

5. [dmesg读取后（猫命令）

   [909836.517402] DEBUG-device_read: To User  hello4 and bytes_to_do 4096 ppos 0 # Read #1 
   [909836.517428] DEBUG-device_read: Data send to app hello4, nbytes=4096        # Read #1   
   [909836.519086] DEBUG-device_read: To User   and bytes_to_do 0 ppos 4096       # Read #2  
   [909836.519093] DEBUG-device_read: Data send to app hello4, nbytes=0           # Read #2

6. 随附用于读取，写入和file_operations的代码段。任何指导会有所帮助。我进行了广泛搜索，无法理解。因此，该职位。

   /*!
    * @brief Write to device from userspace to kernel space
    * @returns     Number of bytes written
    */
   
   static ssize_t device_write(struct file *file,  //!< File pointer
                                   const char *buf,//!< from for copy_from_user. Takes 'buf' from user space and writes to 
                                                   //!< kernel space in 'buffer'. Happens on fwrite or write 
                                   size_t lbuf,    //!< length of buffer
                                   loff_t *ppos)   //!< position to write to
   {
           int nbytes = lbuf - copy_from_user(
                                              buffer + *ppos,      /* to */
                                              buf,                 /* from */
                                              lbuf);               /* how many bytes */
           *ppos += nbytes;
           buffer[strcspn(buffer, "\n")] = 0; // Remove End of line character
           pr_info("Recieved data \"%s\" from apps, nbytes=%d\n", buffer, nbytes);
           return nbytes;
   }
   
   /*!
    * @brief Read from device - from kernel space to user space
    * @returns     Number of bytes read
    */
   static ssize_t device_read(struct file *file,//!< File pointer
                              char *buf,   //!< for copy_to_user. buf is 'to' from buffer
                          size_t lbuf, //!< Length of buffer
                          loff_t *ppos)//!< Position {
       int nbytes;
       int maxbytes;
       int bytes_to_do;
       maxbytes = PAGE_SIZE -  *ppos;
       if(maxbytes >lbuf)
               bytes_to_do = lbuf;
       else
               bytes_to_do = maxbytes;
   
       buffer[strcspn(buffer, "\n")] = 0; // Remove End of line character
       printk("DEBUG-device_read: To User  %s and bytes_to_do %d ppos %lld\n", buffer + *ppos, bytes_to_do, *ppos);
       nbytes = bytes_to_do - copy_to_user(
                                       buf, /* to */
                                       buffer + *ppos, /* from */
                                       bytes_to_do); /* how many bytes*/
       *ppos += nbytes;
       pr_info("DEBUG-device_read: Data send to app %s, nbytes=%d\n", buffer, nbytes);
       return nbytes;} /* Every Device is like a file - this is device file operation */ static struct file_operations device_fops = {
           .owner = THIS_MODULE,
           .write = device_write,
           .open  = device_open,
           .read  = device_read,};

Answer 1

用于指示文件结束的Unix约定是使read返回0字节。

在这种情况下，cat要求131072字节，仅接收4096。这是正常现象，不应解释为已到达文件末尾。例如，当您从键盘上读取但用户仅输入少量数据时，就会发生这种情况。

由于cat尚未看到EOF（即read并未返回0），因此它将继续发出read调用，直到发出EOF。这意味着如果有任何数据，您将始终至少看到两次读取调用：一次（或多次）读取数据，最后一次返回0。