Question

While I was digging into OpenNTPD source code files, I noticed new keywords and syntaxs that I’ve never seen in any C code before such as }%, %%, %type and %token in a file named parse.y:

%{
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

...

%}

%token  LISTEN ON
%token  SERVER SERVERS SENSOR CORRECTION RTABLE REFID WEIGHT
%token  ERROR
%token  <v.string>      STRING
%token  <v.number>

....

grammar     : /* empty */
        | grammar '\n'
        | grammar main '\n'
        | grammar error '\n'        { file->errors++; }
        ;

main        : LISTEN ON address listen_opts {
            struct listen_addr  *la;
            struct ntp_addr     *h, *next;

            if ($3->a)
...

Most of the file’s contents have the usual C syntax except these keywords. Does someone know what these keywords are and what they are used for?

Answer 1

My guess is that this is Yacc code (i.e. the definition of a grammar), not plain C. This is a notation similar to BNF.