simple makefile for lex yacc and C

Question

I am trying to compile my program which has a lex file and a yacc file and a couple of C files.I came across this example in the Flex manual.

I have a couple of questions regarding this makefile.It doesn't specify a compiler such as gcc how does the makefile know how to create the targets such as scan.o and parse.o and myprogram.o?

     # 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

Answer 1

That's a Makefile feature: some implicit rules exist and are documented in https://www.gnu.org/software/make/manual/html_node/Catalogue-of-Rules.html#Catalogue-of-Rules

In you case, the relevant part is

Compiling C programs

n.o is made automatically from n.c with a recipe of the form ‘$(CC) $(CPPFLAGS) $(CFLAGS) -c’.

[...]

Yacc for C programs

n.c is made automatically from n.y by running Yacc with the recipe ‘$(YACC) $(YFLAGS)’.

Lex for C programs

n.c is made automatically from n.l by running Lex. The actual recipe is ‘$(LEX) $(LFLAGS)’.

According to https://ftp.gnu.org/old-gnu/Manuals/make-3.79.1/html_chapter/make_12.html some of this rules are not portable...