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

[Marcel Laverdet]

> Do you actually mean '.' matches non newline?

Yes, apologies :)

> 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.

Really, it's up to you. You can replace it with whatever you want. You
could replace it with assert(false) if you want or fprintf(stderr, "oh
no!"), I don't know what you want it's your program. It's just a matter of
what you want to do when something unexpected happens. Bison will generate
error messages for you when something bad happens, but Flex is not in a
position to generate error messages. In my scanners I usually do one of two
things. I either memoize an error message in my yyextra (or just as a
global if you're not using "%option reentrant"), which I can then raise to
the client of my scanner/parser. Or, I do something like "return
UNEXPECTED_CHARACTER;" (you would have to define this token). Then, Bison
would generate an error such as "Error: Unexpected token
UNEXPECTED_CHARACTER on line 5".


On Wed, 30 Dec 2009 09:39:30 -0600, Peng Yu <address@hidden> wrote:
> 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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