Check if a character device's output buffer is empty
#include <qvm/cdev.h>
static int cdev_output_empty(const struct cdev_state *const cds)
Calling this function makes sense only if you are using the internal buffers (i.e., you specified a non-zero buffer size when calling cdev_setup()).
If the buffer is not empty, 0 (zero); if it is empty, meaning there are no characters to output, 1 (one); otherwise, a negative errno value.