Access a struct variable when struct is passed as a double pointer-CodePudding

I have a function as follows

int check_inband_status(Port **ePort, Port **wPort, InbandPort *inbandPort)
{

                std::ifstream ring_config_file(RING_CONFIG_FILE);

                Json::Value ring_config;
                ring_config_file >> ring_config;
                (*ePort)->port_id = ring_config["east_port"]["port_id"].asInt();
                (*ePort)->port_type = ring_config["east_port"]["port_type"].asString();
                (*wPort)->port_id = ring_config["west_port"]["port_id"].asInt();
                (*ePort)->port_type = ring_config["west_port"]["port_type"].asString();

                ring_config_file.close();
}

I have a json file and I am reading it and assigning the values to ePort and wPort. This is the variable ePort and wPort

class InbandPort : public Port
{
    public:
        uint32_t                vid;
        uint32_t                nicid;
        uint32_t                intf_id;
        uint32_t                number_of_nni;
        bool                    is_lag;
        olt_intf_type_t         intftype;
        Port                    *port1;
        Port                    *port2;
        SubportList             sPorts;
        ERPSPort                *erpsPort;
        bool                    is_active;
        bool                    is_dhcp_done; /* to mark the dhcp is done or not is static dhcp case by default value                                                   is true*/
        bool                    is_ring_configured;
        InbandPort() : Port()
        {
            vid             = INVALID_ID;
            nicid           = INVALID_ID;
            intf_id         = INVALID_ID;
            port1           = NULL;
            port2           = NULL;
            erpsPort        = NULL;
            is_lag          = false;
            is_dhcp_done    = false;
            is_ring_configured = false;
            port_id         = 0;
            number_of_nni   = 0;
        }
.....

Here is the json file

{
   "east_port" : {
      "port_id" : 2,
      "port_type" : "nni"
   },
   "west_port" : {
      "port_id" : 4,
      "port_type" : "nni"
   }
}

It is crashing at

(*ePort)->port_id = ring_config["east_port"]["port_id"].asInt();

I know I am making some mistake in accessing/assigning double pointer. Can someone please point me to it?

Edit: I am passing it as a double pointer because I want to access the same value from other function as well. And how I am passing is

Port                       *ePort  = NULL;
Port                       *wPort = NULL;
check_inband_status(&ePort, &wPort, inbandPort);

CodePudding user response：

It looks like a typical C interface for a setup or init function to me.

Double pointers (object_type**) is used so the object can be allocated on the heap and the address can be returned to the caller of the function. And I fear, that's what you are missing. You try to write to an object, that doesn't exist.

If the caller of the function was benign, they initialized the pointer to nullptr before passing it (or let's better say, its address) to the function and you can verify, that it has to be created first. But since nullptr initialization is not default, you cannot count on it.

Just a few days ago, CppCon channel on YouTube posted a nice talk of last years event on the basics of pointers (https://www.youtube.com/watch?v=0zd8eznWv4k).

CodePudding user response：

You're passing the addresses of null pointers, so both *ePort and *wPort are null.
And then you dereference them, and it goes boom.

The function is probably supposed to (dynamically) create new objects and assign their addresses to *ePort and *wPort.

It's not obvious what type of objects to create, but I would expect it to look something like this (but with some error handling):

int check_inband_status(Port **ePort, Port **wPort, InbandPort *inbandPort)
{
    std::ifstream ring_config_file(RING_CONFIG_FILE);
    Json::Value ring_config;
    ring_config_file >> ring_config;
    Port* east = new Port;
    east->port_id = ring_config["east_port"]["port_id"].asInt();
    east->port_type = ring_config["east_port"]["port_type"].asString();
    Port* west = new Port;
    west->port_id = ring_config["west_port"]["port_id"].asInt();
    west->port_type = ring_config["west_port"]["port_type"].asString();
    *ePort = east;
    *wPort = west;
    return something_meaningful;
}

Another option is that it's not supposed to create objects dynamically but just modify them.
In that case, you need just one level of pointer (or a reference):

int check_inband_status(Port *east, Port *west, InbandPort *inbandPort)
{
    std::ifstream ring_config_file(RING_CONFIG_FILE);
    Json::Value ring_config;
    ring_config_file >> ring_config;
    east->port_id = ring_config["east_port"]["port_id"].asInt();
    east->port_type = ring_config["east_port"]["port_type"].asString();
    west->port_id = ring_config["west_port"]["port_id"].asInt();
    west->port_type = ring_config["west_port"]["port_type"].asString();
    return something_meaningful;
}

// ...

Port ePort;
Port wPort;
check_inband_status(&ePort, &wPort, inbandPort);