Question

I can’t seem to figure out how to concatenate two string in yacc.

Here is the lex code

%{
#include <stdio.h>
#include "y.tab.h"
%}
%%
[0-9]+ {yylval.intval=atoi(yytext); return NR;}
[a-zA-Z]+ {yylval.strval=yytext; return STR;}
"0exit" {return 0;}
[ \t] ;
\n {return 0;}
. {return yytext[0];}

And here i have the basics to add two strings

%{
#include <stdio.h>
#include <string.h>
%}
%union {
int intval;
char* strval;
}
%token STR NR
%type <intval>NR
%type <strval>STR
%type <strval>operatie
%left '+' '-'
%right '='
%start S
%%
S   : S operatie        {}
    | operatie          {printf("%s\n",$<strval>$);}
    ;

operatie    :   STR '+' STR {   char* s=malloc(sizeof(char)*(strlen($1)+strlen($3)+1));
                                strcpy(s,$1); strcat(s,$3);
                                $$=s;}
            ;
%%
int main(){
 yyparse();
}    

The code works, the problem is that the output is something like this:
If i input

aaaa + bbbb

i get the output

aaaa + bbbbbbbb

Answer 1

The problem is here:

yylval.strval = yytext;

yytext changes with every token and every buffer. Change it to

yylval.strval = strdup(yytext);