I'm trying to query what the size of the buffer is without actually reading it. I currently write to /dev/ttyS0 then query and then read the data. I'm just writing ABC and then receiving the same thing when I read, but the value I get when I query is some random number.

This is my call:

int bytes_avaiable;
ioctl(num, FIONREAD, &bytes_available);

I've found that it may be a timing issue. As I change the time I sleep after my write, but before my query it will return more bytes the longer I wait. When I use usleep(8000) after I have written about 4 bytes to /dev/ttyS0 it will query the right amount of data. I can't use that number however because it won't work for every size written ...

FINALLY

Ok, I figured it out, all the echo had to be turned off:

stty -F /dev/ttyS0 -echo -echoe -echoke .....

Echo isn't so nice for a loopback test

Glad you figured it out... but for next time:

Please post your thread in only one forum. Posting a single thread in the most relevant forum will make it easier for members to help you and will keep the discussion in one place. This thread is being closed because it is a duplicate.