(flex.info)How can I have multiple input sources feed into the same scanner at the same time?

---

Next: Can I build nested parsers that work with the same input file? Prev: My actions are executing out of order or sometimes not at all. Up: FAQ

---

Enter node , (file) or (file)node

How can I have multiple input sources feed into the same scanner at the same time?
==================================================================================

If ...
   * your scanner is free of backtracking (verified using 'flex''s '-b'
     flag),
   * AND you run your scanner interactively ('-I' option; default unless
     using special table compression options),
   * AND you feed it one character at a time by redefining '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.