How to Use Flex with Multiple Source Files
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1) Write a Flex specification file, and use an extension of ".l" (dot
ell). I will assume the file is named "WordCount.l".

2) Use Flex to generate a lexer. Flex will generate C code, but it is
also valid C++ code so we will use an extension of ".cc" for the
generated code.

flex --nounput -o WordCountLexer.cc WordCount.l

3) Compile the generated lexer.

g++ -Wall -c WordCountLexer.cc

This MAY produce warnings about comparisons between signed and
unsigned numbers. The warnings will not result in problems and can be
ignored.

3) Write a driver file that invokes the generated lexer/scanner
function "yylex", defined in "WordCountLexer.cc". I will assume the
driver file is "WordCountDriver.cc".

4) Compile the driver file.

g++ -Wall -c WordCountDriver.cc

5) Link the two generated object files together to produce an executable.

g++ WordCountDriver.o WordCountLexer.o -o WordCountDriver -lfl

Don't forget to link with the Flex library with "-lfl". 

6) You will likely have errors :-) If you modify the Flex file, you
need to re-run flex and recompile the generated lexer. You do NOT have
to recompile the driver unless you modified it as well. Repeat the
link step to generate an updated executable.

7) The above steps can be used with more than two source files. Just
compile all source files and be sure to include all of them in the
link command in step 5.