expected identifier or '(' before '{' token in Flex - c

Question

I am trying to use flex and yacc to parse 'C' source code. Unfortunately I am getting the error "expected identifier or '(' before '{' token" on lines 1,12,13,14... . Any ideas why?
This is my flex file (called mini.l):
%{
%}
digit [0-9]
letter [a-zA-Z]
number (digit)+
id (letter|_)(letter|digit|_)*
integer (int)
character (char)
comma [,]
%%
{integer} {return INT;}
{character} {return CHAR;}
{number} {return NUM;}
{id} {return IDENTIFIER;}
{comma} {return ',';}
[-+*/] {return *yytext;}
. {}
%%
main()
{
yylex();
}
The corresponding yacc file (called my_yacc.y) is as shown below:
%{
#include <ctype.h>
#include <stdio.h>
/* #include "myhead.h" */
#include "mini.l"
#define YYSTYPE double
# undef fprintf
%}
%token INT
%token CHAR
%token IDENTIFIER
%token NUM
%token ','
%left '+' '-'
%left '*' '/'
%right UMINUS
%%
lines:lines expr '\n' {printf("%g\n",$2);}
|lines '\n'
|D
|
;
expr :expr '*' expr {$$=$1*$3;}
|expr '/' expr {$$=$1/$3;}
|expr '+' expr {$$=$1+$3;}
|expr '-' expr {$$=$1+$3;}
|'(' expr ')' {$$=$2;}
|'-' expr %prec UMINUS {$$=-$2;}
|IDENTIFIER {}
|NUM {}
;
T :INT {}
|CHAR {}
;
L :L ',' IDENTIFIER {}
|IDENTIFIER {}
;
D :T L {printf("T is %g, L is %g",$1,$2);}
;
%%
/*void yyerror (char *s)
{
fprintf (stderr, "%s\n", s);
}
*/
I am compiling the generated code using the commands:
flex mini.l
yacc my_yacc.y
gcc y.tab.c -ly

Answer 1

The errors you are seeing are coming from the C compiler and are caused by you misunderstanding of how flex and yacc work together. When I run it though the same tools as you, I get the following errors, as you also do, and as noted by #KeithThompson:
In file included from my_yacc.y:5:0:
mini.l:1:1: error: expected identifier or '(' before '%' token
%{
^
mini.l:5:1: error: expected '=', ',', ';', 'asm' or '__attribute__' before' letter'
letter [a-zA-Z]
^
mini.l:12:11: error: expected identifier or '(' before '{' token
{integer} {return INT;}
^
... elided the rest ...
Although you have used the commands flex, yacc and gcc in the correct sequence, you have included the file mini.l in your bison input. This is incorrect. You should be including the output of the flex that was created from mini.l. This file is called lex.yy.c. It is also necessary to include this at the end on the yacc input file. (This is because if you don't you get the error found by #flolo). If you make the necessary changes to your yacc file, you will have this:
%{
#include <ctype.h>
#include <stdio.h>
/* #include "myhead.h" */
/* #include "mini.l" */
#define YYSTYPE double
# undef fprintf
%}
%token INT
%token CHAR
%token IDENTIFIER
%token NUM
%token ','
%left '+' '-'
%left '*' '/'
%right UMINUS
%%
lines:lines expr '\n' {printf("%g\n",$2);}
|lines '\n'
|D
|
;
expr :expr '*' expr {$$=$1*$3;}
|expr '/' expr {$$=$1/$3;}
|expr '+' expr {$$=$1+$3;}
|expr '-' expr {$$=$1+$3;}
|'(' expr ')' {$$=$2;}
|'-' expr %prec UMINUS {$$=-$2;}
|IDENTIFIER {}
|NUM {}
;
T :INT {}
|CHAR {}
;
L :L ',' IDENTIFIER {}
|IDENTIFIER {}
;
D :T L {printf("T is %g, L is %g",$1,$2);}
;
%%
void yyerror (char *s)
{
fprintf (stderr, "%s\n", s);
}
#include "lex.yy.c"
If you run your command sequence on this now, you will find it both compiles and runs, and also process C language input correctly.
The mistake you made is a common error made by new users to the tools flex and yacc.