Bison Syntax Error easy file - syntax-error

Question

i'm trying to run this .y file
%{
#include <stdlib.h>
#include <stdio.h>
int yylex();
int yyerror();
%}
%start BEGIN
%%
BEGIN: 'a' | BEGIN 'a'
%%
int yylex(){
return getchar();
}
int yyerror(char* s){
fprintf(stderr, "*** ERROR: %s\n", s);
return 0;
}
int main(int argn, char **argv){
yyparse();
return 0;
}
It's a simple program in bison, the syntax seems to me correct, but always get the Syntax error problem ...
Thanks for your help.

Answer 1

The lexer function yylex needs to return 0 to indicate the end of the input. However, your implementation simply passes through the value returned by getchar, which will be EOF (normally -1).
Also, your input is almost certain to include a newline character, which will also be passed through to the parser.
Since the parser recognizes neither \n nor EOF, it produces an error when it receives one of them.
At a minimum, you would need to modify yylex to correctly respond to end of input:
int yylex(void) {
int ch = getchar();
return (ch == EOF) ? 0 : ch;
}
But you will still have to deal with newline charactets, either by handling them in your lexer (possibly ignoring them or possibly returning an end of input imdication), or by handling them in your grammar.
Note that bison/yacc-generated parsers always parse the entire input stream, not just the longest sequence satisfying the grammar. That can be adjusted with some work -- see the documentation for the YYACCEPT special action -- but the standard behaviour is usually what is desired when parsing.
By the way, please use standard style conventions in your bison/yacc grammars, in order to avoid problems and in order to avoid confusing readers. Normally we reserve UPPER_CASE for terminal symbols, since those are also used as compile-time constants in the lexer. Non-terminals are usually written in lower_case although some prefer to use CamelCase. For the terminals, you need to avoid the use of names reserved by the standard library (such as EOF) or by (f)lex (BEGIN) or bison/yacc (END). There are lists of reserved names in the manuals.