flex bison windows introduction

Question

As I'm new to lexer and parser, so I'm trying to read and understand others code.

Here is the code i'm trying to use : https://gist.github.com/justjkk/436828

But it's giving me error. How can I resolve this?

E:\flex_bison_test>gcc lex.yy.c y.tab.c -o json.exe
json.l: In function 'yylex':
json.l:34:11: warning: assignment to 'YYSTYPE' {aka 'int'} from 'char *' makes integer from pointer without a cast [-Wint-conversion]
     yylval=strclone(yytext);
           ^
json.l:38:11: warning: assignment to 'YYSTYPE' {aka 'int'} from 'char *' makes integer from pointer without a cast [-Wint-conversion]
     yylval=strclone(yytext);
           ^
json.l: In function 'strclone':
json.l:82:15: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
     int len = strlen(str);
               ^~~~~~
json.l:82:15: warning: incompatible implicit declaration of built-in function 'strlen'
json.l:82:15: note: include '<string.h>' or provide a declaration of 'strlen'
json.l:79:1:
+#include <string.h>
 %%
json.l:82:15:
     int len = strlen(str);
               ^~~~~~
json.l:84:5: warning: implicit declaration of function 'strcpy' [-Wimplicit-function-declaration]
     strcpy(clone,str);
     ^~~~~~
json.l:84:5: warning: incompatible implicit declaration of built-in function 'strcpy'
json.l:84:5: note: include '<string.h>' or provide a declaration of 'strcpy'
y.tab.c: In function 'yyparse':
y.tab.c:627:16: warning: implicit declaration of function 'yylex' [-Wimplicit-function-declaration]
 # define YYLEX yylex ()
                ^~~~~
y.tab.c:1272:16: note: in expansion of macro 'YYLEX'
       yychar = YYLEX;
                ^~~~~
y.tab.c:1540:7: warning: implicit declaration of function 'yyerror'; did you mean 'yyerrok'? [-Wimplicit-function-declaration]
       yyerror (YY_("syntax error"));
       ^~~~~~~
       yyerrok
json.y: At top level:
json.y:80:6: warning: conflicting types for 'yyerror'
 void yyerror (char const *s) {
      ^~~~~~~
y.tab.c:1540:7: note: previous implicit declaration of 'yyerror' was here
       yyerror (YY_("syntax error"));
       ^~~~~~~

E:\flex_bison_test>

Or these should remain as it is.

All the commands, I have given :

flex json.l
bison -dy json.y
gcc lex.yy.c y.tab.c -o json.exe

Honestly, I think you'd be better off studying the examples in the [flex manual](http://westes.github.io/flex/manual/Simple-Examples.html#Simple-Examples) and the [bison manual](https://www.gnu.org/software/bison/manual/bison.html#Examples). It's true (and unfortunate) that these examples don't show how bison and flex can be used together, but they do show how to use both tools. Most importantly, the manuals *explain* the examples. The code you link here is simply not a good basis for using flex and bison, and the fact that it doesn't compile without errors is just one obvious symptom. — rici, Feb 21 '20 at 14:38

dragosht · Accepted Answer · 2020-02-21T08:18:27.773

2

Simply:

#include <string.h>

in your flex definitions section on top of json.l should fix it for you.

There's also a Makefile in the repository you pointed to. Maybe you should use that. You don't seem to be generating the parser files properly. See comment below.

As for the remaining warnings:

warning: implicit declaration of function 'yyerror';
warning: implicit declaration of function 'yylex';

These can be easily fixed by adding declarations of yylex() and yyerror should be present in the bison prologue section at the top of your json.y:

%{
    int yylex();
    void yyerror(const char *s);
%}

As for these ones:

json.l:34:11: warning: assignment to 'YYSTYPE' {aka 'int'} from 'char *' makes integer from pointer without a cast [-Wint-conversion]
 yylval=strclone(yytext);
       ^
json.l:38:11: warning: assignment to 'YYSTYPE' {aka 'int'} from 'char *' makes integer from pointer without a cast [-Wint-conversion]
 yylval=strclone(yytext);
       ^

They're a bit more subtle. I would suggest have a look here on how to use yylval for correctly passing on strings from the lex's tokens into the parser's actions. The problem now is that yylval is a bare int but it ends up being assigned char pointers for both NUMBER and STRING tokens.

edited Feb 21 '20 at 08:18

answered Feb 21 '20 at 07:23

dragosht

3,237
2
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@Danial The remaining ones are warnings. That `-dy` flag in your bison invocation seems odd - Notice that you're generating `y.tab.{c,h}` but your lexer needs `#include "json.tab.h"` – dragosht Feb 21 '20 at 07:41
1

@dragosht: The warning about `yylval` having the wrong type in the lexer needs to be taken seriously. Also, forward declarations of yylex and yyerror are required in the bison prologue. – rici Feb 21 '20 at 07:47
@rici I was just about to suggest adding `yylex` and `yyerror` into `bison`. As for those `yylval` assignments - I can't really say what all that's about ... – dragosht Feb 21 '20 at 07:51
1

@dragosht: it's from only redefining YYSTYPE in the bison file. But modern bison has a better way to declare semantic type which automatically puts the declaration into the generated header file. See [Defining Language Semantics](https://www.gnu.org/software/bison/manual/bison.html#Semantics) in the manual. – rici Feb 21 '20 at 08:04
@rici I would refer to this post [here](https://stackoverflow.com/questions/1851829/how-to-use-yylval-with-strings-in-yacc/12549501) on the topic of properly using `yylval`. – dragosht Feb 21 '20 at 08:09

flex bison windows introduction

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