(flex.info)Makefiles and Flex

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A.1 Makefiles and Flex
======================

In this appendix, we provide tips for writing Makefiles to build your
scanners.

   In a traditional build environment, we say that the '.c' files are
the sources, and the '.o' files are the intermediate files.  When using
'flex', however, the '.l' files are the sources, and the generated '.c'
files (along with the '.o' files) are the intermediate files.  This
requires you to carefully plan your Makefile.

   Modern 'make' programs understand that 'foo.l' is intended to
generate 'lex.yy.c' or 'foo.c', and will behave accordingly(1)(2).  The
following Makefile does not explicitly instruct 'make' how to build
'foo.c' from 'foo.l'.  Instead, it relies on the implicit rules of the
'make' program to build the intermediate file, 'scan.c':

         # Basic Makefile -- relies on implicit rules
         # Creates "myprogram" from "scan.l" and "myprogram.c"
         #
         LEX=flex
         myprogram: scan.o myprogram.o
         scan.o: scan.l
     

   For simple cases, the above may be sufficient.  For other cases, you
may have to explicitly instruct 'make' how to build your scanner.  The
following is an example of a Makefile containing explicit rules:

         # Basic Makefile -- provides explicit rules
         # Creates "myprogram" from "scan.l" and "myprogram.c"
         #
         LEX=flex
         myprogram: scan.o myprogram.o
                 $(CC) -o $@  $(LDFLAGS) $^
     
         myprogram.o: myprogram.c
                 $(CC) $(CPPFLAGS) $(CFLAGS) -o $@ -c $^
     
         scan.o: scan.c
                 $(CC) $(CPPFLAGS) $(CFLAGS) -o $@ -c $^
     
         scan.c: scan.l
                 $(LEX) $(LFLAGS) -o $@ $^
     
         clean:
                 $(RM) *.o scan.c
     

   Notice in the above example that 'scan.c' is in the 'clean' target.
This is because we consider the file 'scan.c' to be an intermediate
file.

   Finally, we provide a realistic example of a 'flex' scanner used with
a 'bison' parser(3).  There is a tricky problem we have to deal with.
Since a 'flex' scanner will typically include a header file (e.g.,
'y.tab.h') generated by the parser, we need to be sure that the header
file is generated BEFORE the scanner is compiled.  We handle this case
in the following example:

         # Makefile example -- scanner and parser.
         # Creates "myprogram" from "scan.l", "parse.y", and "myprogram.c"
         #
         LEX     = flex
         YACC    = bison -y
         YFLAGS  = -d
         objects = scan.o parse.o myprogram.o
     
         myprogram: $(objects)
         scan.o: scan.l parse.c
         parse.o: parse.y
         myprogram.o: myprogram.c
     

   In the above example, notice the line,

         scan.o: scan.l parse.c

   , which lists the file 'parse.c' (the generated parser) as a
dependency of 'scan.o'.  We want to ensure that the parser is created
before the scanner is compiled, and the above line seems to do the
trick.  Feel free to experiment with your specific implementation of
'make'.

   For more details on writing Makefiles, see Note: (make)Top.

   ---------- Footnotes ----------

   (1) GNU 'make' and GNU 'automake' are two such programs that provide
implicit rules for flex-generated scanners.

   (2) GNU 'automake' may generate code to execute flex in
lex-compatible mode, or to stdout.  If this is not what you want, then
you should provide an explicit rule in your Makefile.am

   (3) This example also applies to yacc parsers.