After some investigation I have found that the operative system drops data when syswrite is called with more than 512 bytes or when consecutive enough syswrites summing up more than 512 bytes are performed.
syswrite returns the number of bytes written, and the caller is responsible for re-syswriting the bytes that didn't get written. Is it possible that the return value is not being checked? Or is the OS simply reporting that the syswrite call succeeded even though data is being dropped?

In reply to Re: Expect and PTY dropping data by pc88mxer
in thread Expect and PTY dropping data by salva

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