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

[Peng Yu]

On Wed, Dec 30, 2009 at 12:54 AM, Marcel Laverdet <address@hidden> wrote:
>
>
> 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:

Do you actually mean '.' matches non newline?

> <*>.|\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).

I made some corrections. Now yylval_string.l becomes the following.
You said yyerror should not be used in the flex file. I'm wondering
what I should use to replace the line '.        { yyerror("mystery character
%c\n", *yytext); }' in order to catch errors.

%option nodefault
%{
# include "yylval_string.tab.h"
%}
%%
[0-9]+ { yylval=atoi(yytext); return NUMBER; }
[[:space:]] { /*SPACE*/ }
.       { yyerror("mystery character %c\n", *yytext); }
%%



> 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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