callback on gattlib_connect not invoke

[oranelbaz2000]

I'm new to BLE development, and we're using Linux (ubuntu) with the latest version of GattLib (0.7.2).
When we call the gattlib_connect function, the callback (on_device_connect) is only invoked when the function does not succeed, meaning the return value is non-zero.
Why does this happen? How can I interact with the "connection" object if the callback isn't triggered on a successful connection? Any guidance would be appreciated! (start scan and stop scan working fine)

#include
#include <pthread.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/queue.h>
#include "gattlib.h"
#include
#include
#include
#include

typedef void (*ble_discovered_device_t)(const char *addr, const char *name);

gattlib_adapter_t *adapter;
gattlib_connection_t *current_connection;
bool isConnected;

// represent a Bluetooth device
struct Device
{
std::string name;
std::string macID;
};

// list of devices
std::vector deviceList;

void on_device_connect(gattlib_adapter_t *adapter, const char *dst, gattlib_connection_t *connection, int error, void *user_data)
{
current_connection = connection;
}

void ble_discovered_device(gattlib_adapter_t *adapter, const char *addr, const char *name, void *user_data)
{
int ret;

if (name)
{
    printf("Device %s - '%s'\n", addr, name);
}
else
{
    printf("Device %s\n", addr);
}

// insert device info to list
Device newDevice;
newDevice.name = (name != nullptr) ? std::string(name) : "";
newDevice.macID = (addr != nullptr) ? std::string(addr) : "";   
deviceList.push_back(newDevice);

}

void BLEInterface::Init()
{
isConnected = false;
deviceList.clear();

int ret = gattlib_adapter_open(NULL, &adapter);
if (ret)
{
    printf("Failed to open adapter. Error code: %d\n", ret);
}

}

void BLEInterface::StartScaning(BoardSide side, PairType pairingType, uint8_t roomId)
{

std::thread([](){
     
    gattlib_mainloop([](void *arg) -> void * {

        // start scanning
        printf("Start scanning\n");
        int ret = gattlib_adapter_scan_enable(adapter, ble_discovered_device, 0, NULL /* user_data */);
        if (ret)
        {
            printf("Failed to start scan. Error code: %d\n", ret);
        }       
        
        return NULL;
    
    }, NULL);

}).detach();   

}

void BLEInterface::StopScaning()
{
printf("Stop scanning...\n");

int ret = gattlib_adapter_scan_disable(adapter);

if (ret)
{
    printf("Failed to stop scan. Error code: %d\n", ret);
}
else
{
    printf("scanning stopped\n");
}

}

void BLEInterface::Connect(uint8_t *address, uint8_t len)
{

if (IsConnected())
{
    printf("already connected\n");
}
else
{
    printf("connecting to device...\n");

    int ret = gattlib_connect(adapter, (char*)address, GATTLIB_CONNECTION_OPTIONS_NONE, on_device_connect, NULL);
        
    if (ret)
    {
        printf("Failed to connect. Error code: %d\n", ret);
        current_connection = NULL;
    }
    else
    {
        printf("connected\n");
        isConnected = true;
         // in this state the on_device_connect callback is not invoke!!!
    } 
           
}

}

void BLEInterface::Disconnect()
{
if (!IsConnected())
{
printf("not connected\n");
}
else
{
printf("disconnecting from device...\n");

    int ret = gattlib_disconnect(current_connection, true);
    
    if (ret)
    {
        printf("Failed to disconnect Error code: %d\n", ret);
    }
    else
    {
        printf("disconnected\n");
        isConnected = false;
    }
}

}

bool BLEInterface::IsConnected()
{
return isConnected;
}