Why the operator >> doesn't work with my char device?
c, linux, linux-device-driver
Solution
You need to handle `O_APPEND` in your write routine. The `>>` operator opens the file with the `O_APPEND` flag, which requests your driver to seek to the end before each write operation. In your case your `mad_write` routine should check the file flags, and seek to the end before writing if `O_APPEND` is set.
See the manual definition here. I had a look around the Linux kernel source for examples, but very few character drivers actually handle `O_APPEND`. The best example I could find was in the generic file code.
Problem
I'm currently learning linux device drivers. I have begun with an example driver, which is just a memory buffer. My code is available on my github. I test my driver by doing this: ``` # echo "Hello World" > /dev/mad # cat /dev/mad Hello World ``` This is going well but when I use the redirection operator to append something (>>), the behaviour is not the one that I expected. ``` # echo foo > /dev/mad # echo bar >> /dev/mad # cat /dev/mad bar ``` I expected rather to have: ``` foo bar ``` I have implemented the `llseek` callback and take care of the `offp` in the `read` and `write` callbacks, but it still doesn't work.