How to update contents of an array and updating a JSON file?

[Jan Karlo Camero]

Hi ,

I have a JSON file called "network_sysconfig.cfg".

My aim is to read the JSON file, then update the "interfaces" array and then eventually update the file.

In my code below, json_dump_file() returns -1.

The JSON file contents look like this:

{
    "hostname": "defaulthostname",
    "gateway": "192.168.1.1",
    "interfaces": [
        {
            "description": "Port 1 network settings",
            "port_number": "1",
            "ip_address": "192.168.3.1/24",
            "mac_addr" : "00:00:00:00:00:00"
        },
        {
            "description": "Port 2 network settings",
            "port_number": "2",
            "ip_address": "192.168.4.1/24",
            "mac_addr" : "00:00:00:00:00:01"
        }
    ],
    "version": "1.00"
}

I wish to update "interfaces" array specifically index 1 so I wrote my code like this (index 1 was hardcoded):

//root is pass by reference

int updatearr( const char * input, json_t **root)

{

     //set array

     json_t *interface_element, *portinfo_obj;

     json_t *arr = json_object_get(*root, "interfaces");

    

    if(json_is_array(arr))

    {

    //index 1 is hardcoded for simplicity

    interface_element = json_array_get(arr, 1);

    if(NULL == interface_element)

    {

      fprintf(stderr, "update_sysconfig_attr: Error updating \"ip_address\" in %s \n", "./network_sysconfig.cfg");

      json_decref(arr);

      return -1;

    }

    else

    {

        //now we are inside interfaces array, we select ip_address key.

        portinfo_obj = json_object_get(interface_element, "ip_address");

     

        if(NULL == portinfo_obj)

        {

            fprintf(stderr, "update_sysconfig_attr: Error updating \"ip_address\" in %s \n",

                 "./network_sysconfig.cfg");

         json_decref(interface_element);

         json_decref(arr);

         return -1;

        }

       else

       {

           // here we set our intended value for ip_address

           if( -1 == json_string_set(portinfo_obj, input) )

           {

                fprintf(stderr, "update_sysconfig_attr: Error updating \"ip_address\" in %s \n",

                 cfg_fortiiot_networkconf);

                 json_decref(portinfo_obj);

                 json_decref(interface_element);

                 json_decref(arr);

                return -1;

          }

       }

     }

   }

   json_decref(portinfo_obj);

   json_decref(interface_element);

   json_decref(arr);

   return 0;

}

int main(int argc, char *argv[])

{

    json_error_t json_error;

    json_t *root = json_load_file("./sysconfig.cfg",0,&json_error);  //load JSON file

   updatearr( newipnum, &root);

   if(0 != json_dump_file(root, "./network_sysconfig.cfg", JSON_INDENT(4)))

   {

      fprintf(stderr, "File Error\n");   //I cant properly write to my file. The resulting file is shown below.

   }

   json_decref(root);

  

   return 1;

}

Resulting file:

The resulting file is broken until interfaces and I get a -1 return value to json_dump_file()

{
    "hostname": "defaulthostname",
    "gateway": "192.168.1.1",
    "interfaces": [

Am I updating my array/file incorrectly?

Is it due to the fact that I called json_decref() to my json objects inside my updatearr() function?

I am assuming this is ok since I passed root as pass by reference?

Thank in advance!

[Graeme Smecher]

Hi Jan,

On 2020-01-29 12:34 p.m., Jan Karlo Camero wrote:
> Am I updating my array/file incorrectly?
>
> Is it due to the fact that I called json_decref() to my json objects
> inside my updatearr() function?

Yes. Please take a look at the documentation for json_array_get() and
json_object_get():

https://jansson.readthedocs.io/en/2.8/apiref.html#array
https://jansson.readthedocs.io/en/2.8/apiref.html#object

Both of these calls return a borrowed references, so your json_decref()
calls at the botton of updatearr() are in error. As a result, your JSON
object contains references to free'd pointers and is corrupt when it
arrives at json_dump_file().

As usual, valgrind gives you some useful hints about what the error is,
and where it occurred. I recommend you try it out to gain confidence and
visibility into your reference counts.

> I am assuming this is ok since I passed root as pass by reference?

Jansson's reference-counting mechanism is completely distinct from
pointer management in C. There is no way you can affect Jansson's
reference counts except directly, by using the Jansson API.

best,
Graeme

[Jan Karlo Camero]

Perfect!

I now understand thank you.

The fix worked and I used valgrind with no resulting errors.

Many thanks!