Compilation error (g++, bison, flex) when main is in another file - c++

Question

I have a problem in compiling my code.
It works when main() is in the same file as yacc parser but its not working when I put main() in another file.
This is my Flex file: (lex1.ll)
%{
#include <stdio.h>
#include "yac1.tab.hh"
extern "C"
{
int yylex(void);
}
int line_num = 1;
%}
alpha [A-Za-z]
digit [0-9]
%%
"DELETE ALL" return DELALL;
[ \t] ;
INSERT return INSERT;
DELETE return DELETE;
FIND return FIND;
[0-9]+\.[0-9]+ { yylval.fval = atof(yytext); return FLOAT; }
[0-9]+ { yylval.ival = atoi(yytext); return INT; }
[a-zA-Z0-9_]+ { yylval.sval = strdup(yytext);return STRING; }
\n { ++line_num; return ENDL; }
. ;
%%
This is my Bison file: (yac1.yy)
%{
#include <stdio.h>
#include <stdlib.h>
int intval;
void yyerror(const char *s)
{
fprintf(stderr, "error: %s\n", s);
}
extern "C"
{
int yyparse(void);
int yylex(void);
int yywrap()
{
return 1;
}
}
%}
%token INSERT DELETE DELALL FIND ENDL
%union {
int ival;
float fval;
char *sval;
}
%token <ival> INT
%token <fval> FLOAT
%token <sval> STRING
%%
S:T {printf("INPUT ACCEPTED....\n");exit(0);};
T: INSERT val {printf("hey insert FOUND\n");}
| DELETE val
| DELALL ENDL
| FIND val
;
val : INT ENDL {printf("hey %d\n",$1);intval=$1; }
|
FLOAT ENDL
|
STRING ENDL {printf("hey %s\n",$1);}
;
%%
/*
It works if I uncomment this block of code
int main()
{
while(1){
printf("Enter the string");
yyparse();
}
}
*/
This is my main program: (testlex.cc)
#include <stdio.h>
#include "lexheader.h"
#include "yac1.tab.hh"
extern int intval;
main()
{
/*
char * line = "INSERT 54\n";
YY_BUFFER_STATE bp = yy_scan_string( line );
yy_switch_to_buffer(bp);
yyparse();
yy_delete_buffer(bp);
printf ("hello %d",intval);
*/
printf("Enter the query:");
//while(1)
printf ("%d\n",yyparse());
}
And this is my Makefile
parser: lex1.ll yac1.yy testlex.cc
bison -d yac1.yy
flex --header-file="lexheader.h" lex1.ll
g++ -o parser yac1.tab.cc lex.yy.c testlex.cc -lfl
clean:
rm -rf *.o parser
When I compile I get this error.
bison -d yac1.yy
flex --header-file="lexheader.h" lex1.ll
g++ -o parser yac1.tab.cc lex.yy.c testlex.cc -lfl
testlex.cc: In function ‘int main()’:
testlex.cc:21:24: error: ‘yyparse’ was not declared in this scope
make: *** [parser] Error 1
PS: It is necessary for me to compile with g++.With gcc I have a working code.
Any help in this regard is highly appreciated.
Thanks.

Answer 1

Did you read the documentation of GNU Bison ? It has a chapter about C++ parsers with a complete example (quite similar to yours).
You could explicitly declare yyparse as suggested by this answer, but making a real C++ parser is perhaps better.
Your Makefile is not very good. You could have something like
LEX= flex
YACC= bison
LIBES= -lfl
CXXFLAGS= -Wall
parser: lex1.o yac1.tab.o lex.yy.o testlex.o
$(LINKER.cc) -o $# $^ $(LIBES)
lex1.cc: lex1.ll
$(LEX) --header-file="lexheader.h" $< -o $#
yac1.tag.cc: yac1.yy
$(YACC) -d $<
Take also time to read the documentation of GNU make. You might want to use remake as remake -x to debug your Makefile.

Answer 2

You need to declare the yyparse function in the testlex.cc file:
int yyparse();
This is what is known as a function prototype, and tells the compiler that the function exists and can be called.
After looking a little closer at your source I now know the reason why the existing prototype didn't work: It's because you declared it as extern "C" but compiled the file as a C++ source. The extern "C" told the compiler that the yyparse function was an old C style function but then you continued to compile the source with a C++ compiler. This caused a name mismatch.