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From: ac1@chive.cs.reading.ac.uk (Andrew Cunningham)
Newsgroups: comp.unix.wizards
Subject: Re: yacc & lex - cupla questions
Keywords: yacc lex compiler parse
Message-ID: <2481@onion.reading.ac.uk>
Date: 28 Jul 90 21:44:53 GMT
References: <1990Jul26.175831.1216@uicbert.eecs.uic.edu>
Sender: news@reading.ac.uk
Reply-To: ac1@rosemary.cs.reading.ac.uk (Andrew Cunningham)
Distribution: comp
Organization: Comp. Sci. Dept., Reading Univ., UK.
Lines: 83

In article <1990Jul26.175831.1216@uicbert.eecs.uic.edu> woodward@uicbert.eecs.uic.edu writes:
>
>i have been trying to parse a straightforward stream of bytes using the
>c-preprocessors lex & yacc.  being a new user of these utilities, i have
>a couple of problems for which i'd like to solicit your suggestions:
> 
>---------------------------------------------------------------------
>1.)  how does one redefine the i/o in a yacc/lex piece of code?  i.e.
>the code which is generated defaults to stdin and stdout for input and
>output, respectively.  i'd like to redefine these defaults w/o having 
>to hack on the intermediate c-code, since this is a live production 
>project; i'd like to be able to update and modify the program simply by 
>saying "make". 

There are two approaches to this.  You can either close stdin and
reopen it to point to your input file or #define yyinput to be
something which returns the character from your file.  Then, when
lex.yy.c is compiled, instead of calling the yyinput function your
#define is called instead.  E.g.

#define yyinput my_yyinput
int my_yyinput()
  {
    /* get the character you want and return it */
  }
  
You'll also have to redefine yyunput(c)  if you want to do this.
  
>---------------------------------------------------------------------
>2.)  how can one get the automagically-defined #defines, which can
>normally be created from yacc with the -d flag, to come out when you
>use a makefile?  i.e. suppose i have lex.l and yacc.y lex and yacc
>source files, respectively, and i have object files defined in my makefile 
>called lex.o and yacc.o such that "make" follows default rules to create 
>these from the aforementioned source files.   

You'll need to specify an explicit rule to do this.  Or, at the
expense of some processor time you might want to run:

y.tab.h: yacc.y
	yacc yacc.y
	rm y.tab.c

(This shouldn't take too long, yacc is *fast* compared with the cc stage)

>---------------------------------------------------------------------
>3.)  if i have a yacc construct such as:
> 
>line3	: 	A B C
>		{  yacc action sequence }
>
>
>which indicates that the construct line3 is composed of the 3 tokens
>A B and C, in that order ...
> 
>how can i now assign the values of A, B, and C into local vars of my
>choice?  the problem lies in the fact that each of A B and C represent
>three calls to lex, and if i pass back a pointer to yytext[] from lex, 
>i only retain the value of the last token in the sequence, in this case C, 
>when i get to the action sequence in my yacc code.  what if i want to 
>be able to select the EXACT ascii tokens for each of A B and C above in 
>my yacc code.  how do i do that?

line3: 
	A {atext=strdup(yytext);}
	B {btext=strdup(yytext);}
	C {ctext=strdup(yytxet);}
	
Note: if you're grammar is more comlex than this you can lead to
all sorts of comflicts in the compiler - when the parser executes an
action it is `committed' to that branch of the parse tree and cannot
backtrack to resolve any ambiguity that might occur (the classic
problem here is if ... then ... else in programming languages).


Hope this information helps.

AndyC

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