Re: [Flex-help] How to debug bison/flex program? usage of yyerror()

[Marcel Laverdet]

1) [:space:] is a character class expression. If you want one or more
spaces you would do [[:space:]]+. What your scanner is looking for right
now is one of either ":, s, p, a, c, or e". Does that make sense? Just wrap
it in another set of []'s

2) . only matches newline, the documentation is not lying :). The message
you're seeing is from the spaces, since your [:space:] does not match them
correctly. Additionally, you're hitting the default rule, whose default
action is to print the matching character. Pretend that this always exists
at the bottom of your file:

<*>.|\n fprintf(strerr, "%c", *yytext);

You can (and should) override this using "%option nodefault" which will
make it so that your scanner fails to build if you don't handle all inputs.
This is a much more reasonable behavior than just spitting out to strerr.

3) yyerror is a macro defined by bison, not flex. There should be no
yyerror in your lex file. bison's definition of yyerror is not ("%s", ...)
it's something different (which depends on your options to bison).

On Tue, 29 Dec 2009 23:02:33 -0600, Peng Yu <address@hidden> wrote:
> I have the source files listed at the end of the message. I basically
> want to parse a file with only numbers (separated by spaces) and print
> the numbers out. It is an overkill to use bison/flex. But I just want
> to try how to use bison/flex.
> 
> I need to understand how to debug the program. Could somebody help me
> with the following three questions?
> 
> 1. I don't understand why the error message is printed. Shouldn't the
> regexes [0-9]+ and [:space:] match all the strings in 'test.txt'.
> 
> 2. I suspect that '.' is matched to EOF. I'm not sure if I'm correct.
> But it seem that EOF can not be printed (is it why '%c' is printed
> literally?).
> 
> 3. Why yyerror() in the .l file has two arguments but it has one
> argument in the .y file? Are they the same function or two different
> functions?
> 
> $ make
> bison -d yylval_string.y
> flex yylval_string.l
> cc -o yylval_string yylval_string.tab.c lex.yy.c -lfl
> $./yylval_string< test.txt
> NUMBER = 133
> error: mystery character %c
> 
> NUMBER = 7
> error: mystery character %c
> 
> NUMBER = 33
> error: mystery character %c
> 
> NUMBER = 76
> 
> NUMBER = 35
> 
> 
> --------------------------------source files listed
> below---------------------------
> 
> $cat yylval_string.y %{
> #  include <stdio.h>
> %}
> 
> %token NUMBER
> 
> %%
> 
> numbers:
>        NUMBER { printf("NUMBER = %d\n", $1); }
>        | numbers NUMBER { printf("NUMBER = %d\n", $2); }
>  ;
> 
> %%
> main()
> {
>   yyparse();
> }
> 
> yyerror(char *s)
> {
>   fprintf(stderr, "error: %s\n", s);
> }
> 
> 
> $cat yylval_string.l
> %{
> # include "yylval_string.tab.h"
> %}
> 
> %%
> [0-9]+ { yylval=atoi(yytext); return NUMBER; }
> [:space:] { /*SPACE*/ }
> .     { yyerror("mystery character %c\n", *yytext); }
> %%
> 
> $cat Makefile
>  .PHONY: all
> 
> all: yylval_string
> 
> yylval_string:        yylval_string.l yylval_string.y
>       bison -d yylval_string.y
>       flex yylval_string.l
>       cc -o $@ yylval_string.tab.c lex.yy.c -lfl
> 
> $cat test.txt
> 133 7 33 76
> 35
> 
>
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