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If …
flex’s ‘-b’ flag),
YY_INPUT to do so,
then every time it matches a token, it will have exhausted its input
buffer (because the scanner is free of backtracking). This means you
can safely use select() at the point and only call yylex() for another
token if select() indicates there’s data available.
That is, move the select() out from the input function to a point where
it determines whether yylex() gets called for the next token.
With this approach, you will still have problems if your input can arrive
piecemeal; select() could inform you that the beginning of a token is
available, you call yylex() to get it, but it winds up blocking waiting
for the later characters in the token.
Here’s another way: Move your input multiplexing inside of YY_INPUT. That
is, whenever YY_INPUT is called, it select()’s to see where input is
available. If input is available for the scanner, it reads and returns the
next byte. If input is available from another source, it calls whatever
function is responsible for reading from that source. (If no input is
available, it blocks until some input is available.) I’ve used this technique in an
interpreter I wrote that both reads keyboard input using a flex scanner and
IPC traffic from sockets, and it works fine.