问题
What should a .read operation return in a kernel module character device?
I know that copy_to_user(...) returns the number of bytes not copied and on success returns 0. I saw examples which make the read() function return -EFAULT if copy_to_user(...) returns anything other than 0.
But then upon success this says to return 0 and this says to return the number of bytes read. What should be returning?
Also should I check the params of
static ssize_t dev_read(struct file *filep, char *buffer, size_t len, loff_t *offset)
like checking if buffer==NULL or len == something? If so, what should I return on a bad condition?
回答1:
.read operation should return either:
- number of bytes copied into user-provided buffer, or
- negative error code
Additionally, operation should update *offset value, so futher reading from the file will return next portion of data.
If driver implementation follows this rules, standard commmands like cat, dd will interpret result of read system call correctly, and, by possibly repeating it, output to the user full "content" of the device.
Normally, correctness of buffer parameter is checked in copy_to_user call. If this call fails(returns non-zero), -EFAULT is usually returned by .read.
len argument is rarely checked for errors. Assuming data_len to be length of remaing data:
if
len<data_len, copy firstlenbytes into buffer and returnlenif
len>data_len, copy all remaining bytes into buffer and returndata_lenif
data_lenorlenis 0, return 0
Sometimes device's data can be read only by portions of predefined size. In that situation you can check len and return -EINVAL in case it has inappropriate value. buffer can also being checked for alignment.
As for example, where .read returns 0 on success, it is correct until device is used by own-written user-space program, which doesn't check actual length of returning data.
来源：https://stackoverflow.com/questions/33423312/error-checking-in-a-read-function-in-kernel-module