Parsing JSON with lex and yacc

json.l

%{
    #include<stdio.h>
    #include "json.tab.h"
    char *strclone(char *str);
    /*
    #define STRING "str"
    #define NUMBER "number"
    #define O_BEGIN "o_begin"
    #define O_END "o_end"
    #define A_BEGIN "a_begin"
    #define A_END "a_end"
    #define COMMA "comma"
    */
%}
DIGIT1to9 [1-9]
DIGIT [0-9]
DIGITS {DIGIT}+
INT {DIGIT}|{DIGIT1to9}{DIGITS}|-{DIGIT}|-{DIGIT1to9}{DIGITS}
FRAC [.]{DIGITS}
EXP {E}{DIGITS}
E [eE][+-]?
HEX_DIGIT [0-9a-f]
NUMBER {INT}|{INT}{FRAC}|{INT}{EXP}|{INT}{FRAC}{EXP}
UNESCAPEDCHAR [ -!#-\[\]-~]
ESCAPEDCHAR \\["\\bfnrt/]
UNICODECHAR \\u{HEX_DIGIT}{HEX_DIGIT}{HEX_DIGIT}{HEX_DIGIT}
CHAR {UNESCAPEDCHAR}|{ESCAPEDCHAR}|{UNICODECHAR}
CHARS {CHAR}+
DBL_QUOTE ["]
%%
{DBL_QUOTE}{DBL_QUOTE} |
{DBL_QUOTE}{CHARS}{DBL_QUOTE} {
    yylval=strclone(yytext);
    return STRING;
};
{NUMBER} {
    yylval=strclone(yytext);
    return NUMBER;
}
true {
    return true;
};
false {
    return false;
};
null {
    return null;
};
\{ {
    return O_BEGIN;
};

\} {
    return O_END;
};

\[ {
    return A_BEGIN;
};

\] {
    return A_END;
};

, {
    return COMMA;
};
: {
    return COLON;
};

[ \t\n]+ /* ignore whitespace */;

. {
    printf("Unexpected: %c\nExiting...\n",*yytext);
    exit(0);
}
%%
char *strclone(char *str)
{
    int len = strlen(str);
    char *clone = (char *)malloc(sizeof(char)*(len+1));
    strcpy(clone,str);
    return clone;
}

json.y

%{
    #include<stdio.h>
    #include<string.h>
    #include<stdlib.h>
    #define YYSTYPE char*
    char *strconcat(char *str1, char *str2);
%}
%token NUMBER
%token STRING
%token true false null
%left O_BEGIN O_END A_BEGIN A_END
%left COMMA
%left COLON
%%
START: ARRAY {
    printf("%s",$1);
  }
| OBJECT {
    printf("%s",$1);
  }
;
OBJECT: O_BEGIN O_END {
    $$ = "{}";
  }
| O_BEGIN MEMBERS O_END {
    $$ = (char *)malloc(sizeof(char)*(1+strlen($2)+1+1));
    sprintf($$,"{%s}",$2);
  }
;
MEMBERS: PAIR {
    $$ = $1;
  }
| PAIR COMMA MEMBERS {
    $$ = (char *)malloc(sizeof(char)*(strlen($1)+1+strlen($3)+1));
    sprintf($$,"%s,%s",$1,$3);
  }
;
PAIR: STRING COLON VALUE {
    $$ = (char *)malloc(sizeof(char)*(strlen($1)+1+strlen($3)+1));
    sprintf($$,"%s:%s",$1,$3);
  }
;
ARRAY: A_BEGIN A_END {
    $$ = (char *)malloc(sizeof(char)*(2+1));
    sprintf($$,"[]");
  }
| A_BEGIN ELEMENTS A_END {
    $$ = (char *)malloc(sizeof(char)*(1+strlen($2)+1+1));
    sprintf($$,"[%s]",$2);
}
;
ELEMENTS: VALUE {
    $$ = $1;
  }
| VALUE COMMA ELEMENTS {
    $$ = (char *)malloc(sizeof(char)*(strlen($1)+1+strlen($3)+1));
    sprintf($$,"%s,%s",$1,$3);
  }
;
VALUE: STRING {$$=yylval;}
| NUMBER {$$=yylval;}
| OBJECT {$$=$1;}
| ARRAY {$$=$1;}
| true {$$="true";}
| false {$$="false";}
| null {$$="null";}
;
%%
int main()
{
   printf("\n");
   yyparse();
   printf("\n");
   return 0;
}
int yywrap()
{
   return 1;
}
void yyerror (char const *s) {
   fprintf (stderr, "%s\n", s);
}
char *strconcat(char *str1, char *str2)
{
    int len1 = strlen(str1);
    int len2 = strlen(str2);
    char *str3 = (char *)malloc(sizeof(char)*(len1+len2+1));
    strcpy(str3,str1);
    strcpy(&(str3[len1]),str2);
    return str3;
}

LICENSE

The MIT License (MIT)

Copyright (c) 2015 J Kishore Kumar

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

Makefile

CC=gcc
YACC=bison
LEX=flex

json: json.o
	ln -nfs $< $@

%.o: %.lex.c %.tab.c %.tab.h
	$(CC) -o $@ $^ -lfl

%.lex.c: %.l
	$(LEX) -o $(patsubst %.l,%.lex.c,$<) $<

%.tab.c %.tab.h: %.y
	$(YACC) -d $<

.PHONY: clean

clean:
	rm -f *.lex.c *.tab.c *.tab.h *.o json

Thanks a lot for this.

You could replace strclone with strdup.

Thank you for this. In order to compile on a Mac:
Makefile:
%.o: %.lex.c %.tab.c
$(CC) -Wall -o $@ $^ -ll

In both json.l and json.y add just before the .h includes:

#ifndef YYSTYPE
# define YYSTYPE char*
#endif

in json.y add "int yylex();" in the includes as well.

Sorry I can't submit a pull request on a Gist. So comments it is.

Thanks for sharing this. A couple of comments:

I think whitespace should include carriage return i.e.

[ \t\n\r]+ /* ignore whitespace */;

in json.l

Also, the root should accept VALUE not just OBJECT | ARRAY in json.y (see https://tools.ietf.org/html/rfc7159#section-2).

How should I compile and run in Ubuntu?