On 2012-07-24 09:08, bforce1729 wrote:
Hi,I am new to kernel programming so can anyone point me to the right forum/reply for the question below. I would like to implement a character device and using a node of type character read and write data. However I am not sure of the length of data from kernel which a user app will receive, so I would like to code safely. If the data exceeds the application gets killed. Any idea or API to perform the task of knowing the data length when passing data to and fro from kernel to user level would be appreciated. Kernel code: copy_to_user(buff, kernel_msg, strlen(kernel_msg)); User App: fd=open("./my_device",O_RDONLY); ret=read(fd,buff,strlen(buf)); //How can I know the size of buff, which would be sent/filled from kernel???
I doubt you really want strlen(buf) there. You probably want sizeof(buf) or something similar. You might need to read up on what strlen() actually does.
Anyway, the driver's "read" file operation knows the maximum amount the user is asking for as it is specified in the third parameter. The function should return the (non-negative) amount it is actually passing back to the user (which will be returned by the read() function in userspace), or a negative error value (which will cause read() to return -1 and set errno appropriately).
Also, copy_to_user() might not succeed if the user passed a bad pointer or bad length. It returns 0 on success and the length of the uncopied part on failure. So if copy_to_user() returns a non-zero value you should either return -EFAULT or work out the partial amount successfully read (as long as it is greater than zero) and return that.
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