我是 Linux 内核模块的新手。我正在学习基于网络课程的字符驱动模块。我有一个非常简单的模块来创建一个/dev/chardevexample，我有一个问题需要我理解：
当我这样做时，我会按预期echo "hello4" > /dev/chardevexample看到执行一次。write但是，当我这样做时cat /dev/chardevexample，我看到读取执行了两次。
我在我的代码和课程材料中都看到了这一点。所有数据都在 first 中返回read()，为什么cat还要调用它呢？
到目前为止，我所做的所有事情如下：
insmod chardev.ko加载我的模块echo "hello4" > /dev/chardevexample. 这是写的，我看到它在 dmesg 中只发生过一次cat /dev/chardevexample. 这是读取，并dmesg显示它发生了两次。我做到了
strace cat /dev/chardevexample，我确实看到函数调用被调用两次以进行读取。中间也有写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)dmesg读后（cat 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附加了读取、写入和文件操作的代码片段。任何指导都会有所帮助。我广泛搜索并无法理解。于是发帖。
/*! * @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,};