简单的读写-simple_read_from_buffer

之前在《简单的读和写》中提到过file_operations结构中的读/写方法中的buff参数是指向用户空间的缓冲区。而用户空间的指针，内核代码不能直接引用其中的内容。

所以用了copy_to_user和copy_from_user来完成最后的读写操作。

这里介绍另一对可以完成读写的函数：simple_read_from_buffer和simple_write_to_buffer。

这两个方法定义在：kernel-4.9/fs/libfs.c

/**
* simple_read_from_buffer - copy data from the buffer to user space
* @to: the user space buffer to read to
* @count: the maximum number of bytes to read
* @ppos: the current position in the buffer
* @from: the buffer to read from
* @available: the size of the buffer
*
* The simple_read_from_buffer() function reads up to @count bytes from the
* buffer @from at offset @ppos into the user space address starting at @to.
*
* On success, the number of bytes read is returned and the offset @ppos is
* advanced by this number, or negative value is returned on error.
**/
ssize_t simple_read_from_buffer(void __user *to, size_t count, loff_t *ppos,
const void *from, size_t available)
{
loff_t pos = *ppos;
size_t ret;

if (pos < 0)
return -EINVAL;
if (pos >= available || !count)
return 0;
if (count > available - pos)
count = available - pos;
ret = copy_to_user(to, from + pos, count);
if (ret == count)
return -EFAULT;
count -= ret;
*ppos = pos + count;
return count;
}
EXPORT_SYMBOL(simple_read_from_buffer);


/**
* simple_write_to_buffer - copy data from user space to the buffer
* @to: the buffer to write to
* @available: the size of the buffer
* @ppos: the current position in the buffer
* @from: the user space buffer to read from
* @count: the maximum number of bytes to read
*
* The simple_write_to_buffer() function reads up to @count bytes from the user
* space address starting at @from into the buffer @to at offset @ppos.
*
* On success, the number of bytes written is returned and the offset @ppos is
* advanced by this number, or negative value is returned on error.
**/
ssize_t simple_write_to_buffer(void *to, size_t available, loff_t *ppos,
const void __user *from, size_t count)
{
loff_t pos = *ppos;
size_t res;

if (pos < 0)
return -EINVAL;
if (pos >= available || !count)
return 0;
if (count > available - pos)
count = available - pos;
res = copy_from_user(to + pos, from, count);
if (res == count)
return -EFAULT;
count -= res;
*ppos = pos + count;
return count;
}
EXPORT_SYMBOL(simple_write_to_buffer);
举个读取GPIO的例子：

static ssize_t amy_proc_read(struct file *file, char __user *buffer, size_t count, loff_t *data) {
int ret;
char kbuf[32];
int value = 0;
if (*data)
return 0;

value = gpio_get_value(my_dev->gpio_pin);

sprintf(kbuf, "%d ", value);
if (clear_user(buffer, count)) {
printk(KERN_ERR "clear error ");
return -EIO;
}

ret = simple_read_from_buffer(buffer,count,data,kbuf,strlen(kbuf));

return ret;
}
 
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原文链接：https://blog.csdn.net/weixin_39821531/java/article/details/88556480