Check how many characters are in the output buffer
#include <qvm/cdev.h>
static unsigned cdev_output_num_pending(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 successful, the number of characters in the output buffer; otherwise, a negative errno value.