Question

我正在为加速度计实现SPI驱动程序。 SPI部分已完成，但我无法从用户空间读取值。

static char charDevMessage[CD_BUFFER_SIZE] = { 0 };
...
static ssize_t char_dev_read(struct file *filep, char *buffer, size_t len, loff_t *position)
{

    int error_count = 0;
    struct xyz_values xyz;
    size_t size_requested;

    xyz = adxl345_get_xyz();
    memset(charDevMessage, 0, CD_BUFFER_SIZE);
    sprintf(charDevMessage, "%d,%d,%d", xyz.x, xyz.y, xyz.z);
    printk(KERN_INFO "MOB: %s, requested size: %d\n", charDevMessage, len);

    if (len >= CD_BUFFER_SIZE)
    {
        size_requested = CD_BUFFER_SIZE;
    }
    else
    {
        size_requested = len;
    }

    error_count = copy_to_user(buffer, charDevMessage, size_requested);      

    if (error_count == 0)          
    {
        printk(KERN_INFO "MOB: Sent %d characters to the user\n", size_requested);
        return (size_requested = 0);      

    }
    else
    {
        printk(KERN_INFO "MOB: Failed to send %d characters to the user\n", error_count);
        return -EFAULT;              

    }
}

安装驱动程序时已创建节点。但是，当我尝试cat或尝试通过python读取时，它返回一个空字符串。

dmesg表示已成功发送到用户空间：

[ 3094.495972] MOB: SPI Character device has been opened 1 time(s)
[ 3094.506075] MOB: -349,-512,511 , requested size:49
[ 3094.514487] MOB: Sent 256 characters to the user
[ 3094.522646] MOB: Character device successfully closed
[ 3120.658568] MOB: SPI Character device has been opened 2 time(s)
[ 3120.668609] MOB: 0,0,0 , requested size:48
[ 3120.676392] MOB: Sent 256 characters to the user
[ 3120.684740] MOB: Character device successfully closed

我做错了什么？

Answer 1

你误解了public class TestRing extends Activity { boolean isLastObj = false; Button add; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.testring); add = (Button) findViewById(R.id.btnAdd); add.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { if(!isLastObj) { for (int i = 1; i < addObject.size(); i++ ) { Object newboject = addObject.get(i); newboject.setTitle("stuff"); newboject.isLast = false; newboject.add(object); if(newboject.getIsLast) { isLastObj = true; // this will prevent furtheradding of objects } } } } }); }函数的概念：

读者（用户空间）只看到if (!File.Exists(path)) { System.IO.File.WriteAllText(path, rxHeader + textBox8.Text); } else { System.IO.File.AppendAllText(path, textBox8.Text); MessageBox.Show("Export of " + comboBox5.Text + " table is complete!"); textBox8.Clear(); }返回的值，并将其解释为已读取的字节数。

对于 .read（在您的情况下为.read）的最后一个参数，其解释完全取决于驱动程序的作者。当文件打开时，由.read指向的值由内核核心初始化为0。在那之后，内核本身永远不会修改它。

如果您想从头开始阅读，可以忽略position参数：

position

或者，在语义上更好，您可以增加值position，因此它将反映总读取的字节数。但否则忽略它：

return size_requested;

无法从linux中读取字符设备

1 个答案: