How does a Lex & Yacc parser output values? - parsing

Question

So for a project that I'm working on, I am using Lex and Yacc to parse a FTP configuration file. The configuration files look something like this:
global {
num_daemons = 10
etc = /etc/ftpd
};
host "ftp-1.foobar.com" {
ftproot = /var/ftp/server1
max_out_bandwidth = 20.7
};
host "ftp-2.foobar.com" {
ftproot = /var/ftp/server2
exclude = /var/ftp/server2/private
};
host "ftp-3.foobar.com" {
ftproot = /var/ftp/server3
};
Now, my question is, how do I obtain this information in a usable way? Let's say I wanted to put things like the address after the host token into a struct. How would I do that? Also, how would I simply print out the values that I've parsed to the command line? Also, to run it, do I just cat the config file and pipe in the compiled c program? Thanks in advance for any help!
Here is my code:
%{
// tokens.l
#include <stdio.h>
#include <stdlib.h>
#include "y.tab.h"
int yyparse();
%}
%option noyywrap
%x OPTION
%x OPTID
%%
<INITIAL>global { return GLOBAL; }
<INITIAL>host { return HOST; }
<INITIAL>"[a-zA-z1-9./-]+" { return NAME; }
<INITIAL>\{ { return CURLY_OPEN; BEGIN OPTION; }
<INITIAL>\n { return EOLN; }
<INITIAL><<EOF>> { return EOFTOK; }
<OPTION>[a-zA-z1-9./-_]+ { return ID_NAME; BEGIN OPTID; }
<OPTION>[\t] {}
<OPTION>[\};] { return OPTION_CLOSE; BEGIN INITIAL;}
<OPTID>[a-zA-z1-9./-]+ { return ID_STRING; BEGIN OPTION; }
<OPTID>[0-9.]+ { return ID_NUM; BEGIN OPTION; }
<OPTID>[\n] { return EOLN; }
%%
int main(int argc, char **argv) {
// Where I am confused..
}
and my yacc file:
%{
// parse.y
#include <stdio.h>
#include <stdlib.h>
int yyerror(char *);
int yylex(void);
%}
%token ERROR EOLN EOFTOK
%token OPTION_CLOSE GLOBAL HOST NAME ID_NAME ID_STRING ID_NUM CURLY_OPEN
%%
input
: lines EOFTOK { YYACCEPT; }
;
lines
:
| lines line
;
line
: option
| opident
| OPTION_CLOSE
;
option
: GLOBAL CURLY_OPEN
| HOST NAME CURLY_OPEN
;
opident
: ID_NAME '=' ID_STRING
| ID_NAME '=' ID_NUM
;
%%
int yyerror(char *msg) {}

Answer 1

You would generally have variables which were accessible and set up before calling the parser, like a linked list of key/value pairs:
typedef struct sNode {
char *key;
char *val;
struct sNode *next;
} tNode;
tNode *lookupHead = NULL;
Then, in your Yacc code, something like:
opident
: ID_NAME '=' ID_STRING { addLookupStr (lookupHead, $1, $3); }
| ID_NAME '=' ID_NUM { other function call here }
;
This would basically execute that code as the rules are found (replacing the $ variables with the item in the rule, $1 is the value for the ID_NAME token, $2 is the =, and so on).
The function would be something like:
void addLookupStr (char *newkey, char *newval) {
// Check for duplicate keys, then attempt to add. All premature returns
// should also be logging errors and setting error flags as needed.
tNode *curr = lookupHead;
while (curr != NULL) {
if (strcmp (curr->key, newkey) == 0)
return;
curr = curr->next;
}
if ((curr = malloc (sizeof (tNode))) == NULL)
return;
if ((curr->key = strdup (newkey)) == NULL) {
free (curr);
return;
}
if ((curr->val = strdup (newval)) == NULL) {
free (curr->newkey);
free (curr);
return;
}
// All possibly-failing ops complete, insert at head of list.
curr->next = lookupHead;
lookupHead = curr;
}