json_type changes during inter process communication.

[boddumanohar]

Hi, I am working on two programs that communicate with each other using IPC. when I am putting json_object in the IPC buffer, I checked the type as 4(json_type_object). But when I check the object type in receiver side, then I get the type as 0 (json_type_null).

/// server.c   
// compiled with gcc server.c -o server -ljson-c
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <json-c/json.h>

#define MAXSIZE 40 

void die(char *s)
{
    perror(s);
	printf("die");
    exit(1);
}

int main()
{
    char c;
    int shmid;
    key_t key;
    json_object *shm;

    key = 56789;

    if ((shmid = shmget(key, MAXSIZE, IPC_CREAT | 0666)) < 0)
        die("shmget");

    if ((shm = shmat(shmid, NULL, 0)) == (json_object *) -1)
        die("shmat");


	char *string = "{\"ame\" : \"joys of programming\"}";
	json_object * jobj = json_tokener_parse(string);
	enum json_type type = json_object_get_type(jobj);
	shm = jobj;
	printf("type %d \n", type);

    while (1) {}
}

// client.c
// compiled with gcc client.c -o client -ljson-c
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include<stdio.h>
#include<stdlib.h>
#include<json-c/json.h>
#define MAXSIZE 40 

void die(char *s) {
	perror(s);
	exit(0);
}

int main() {
	int shmid;
	key_t key;
	json_object *shm;
	key = 56789;
	if ((shmid = shmget(key, MAXSIZE, 0666)) < 0)
			die("shmget");
	if ((shm = shmat(shmid, NULL, 0)) == (json_object *) -1)
			die("shmat");

json_object * jobj = shm;
enum json_type type = json_object_get_type(jobj);
	printf("type %d \n", type);
}

First run the server and then run the client in an other terminal tab/window.
is this a problem with libjson-c?

[ploxiln]

Nope. This does not work at all: shm = jobj; - shm is the address of a chunk of memory which you have to copy data into.

Trying to put a tree of json_object structs with pointers between them into this chunk of memory is very difficult, I wouldn't try it. What you could do is copy the serialized json string into the shm memory chunk and decode it on the other side. But using shm is a whole separate topic.

[boddumanohar]

Thankyou very much this solves my problem.

[hawicz]

One way to do this, if you really wanted to, would be to replace the malloc functions with ones that can use a custom memory region instead of the heap, then have a way to only have those enabled during json-c calls. It'd be pretty hacky though.