Hey there, let's tackle your Bluetooth development project on Ubuntu—covering device discovery, connection, communication, and that tricky RSSI-based distance check you need. You mentioned struggling with RSSI in BlueZ, so here are the three approaches you requested:

1. BlueZ Alternative: Use GIO's Bluetooth API (GTK-Friendly) #

Since you're already using GTK, GIO's Bluetooth subsystem is a natural fit—it's part of the GNOME ecosystem, integrates smoothly with GTK, and has built-in RSSI support that's easier to access than raw BlueZ.

• Core Feature Coverage:
  • Device discovery: Use GBluetoothManager to scan for nearby devices, which emits signals when new devices are detected.
  • Device connection: Call g_bluetooth_device_connect() on a discovered GBluetoothDevice instance to establish a link.
  • Inter-device communication: Use GBluetoothSocket to handle RFCOMM/L2CAP connections (works with standard profiles like SPP for data transfer).
• RSSI Retrieval:
  Once you have a GBluetoothDevice object, you can fetch the current RSSI with g_bluetooth_device_get_rssi(). This value updates automatically as signal strength changes—connect to the notify::rssi signal to get real-time updates for your distance check.
• C++ Integration:
  Use gtkmm (the C++ binding for GTK/GIO) to write cleaner, idiomatic C++ code instead of raw C. Most GIO Bluetooth classes have direct C++ equivalents in gtkmm.

2. Extend BlueZ or Request Upstream RSSI Support #

If you want to stick with BlueZ, you have two actionable paths here:

Option A: Extend BlueZ Yourself to Expose RSSI #

BlueZ does have low-level RSSI capabilities—you just need to surface them via its D-Bus API (the standard way apps interact with BlueZ):

• Low-level foundation: The hci_read_rssi() function in BlueZ's HCI library can retrieve RSSI for connected devices.
• D-Bus extension: Modify BlueZ's source code (specifically the device module) to add a new D-Bus method like GetRSSI that calls hci_read_rssi() and returns the value. Recompile BlueZ and install your modified version to use the new functionality.
• Tip: Look at how existing device properties (like battery level) are exposed in BlueZ's device.c file—mirror that pattern to add RSSI support.

Option B: Request RSSI Support Upstream #

If maintaining a custom BlueZ build isn't feasible, push for official support:

• File a feature request on the BlueZ project's official issue tracker.
• Clearly explain your use case (rough distance estimation in a GTK/C++ app) and reference the existing hci_read_rssi() function to show the capability already exists—it just needs a D-Bus wrapper for easy access.

3. Quick Fix: Linux Commands + C++ for Distance Threshold Check #

If you need a fast, low-effort solution without modifying libraries or APIs, you can call Linux Bluetooth commands from your C++ code and parse their output to check if a device is within ~10 feet.

Key Notes on RSSI vs Distance #

First, a rough environment-dependent rule of thumb:

• RSSI values greater than -60 dBm (e.g., -50, -40) usually mean the device is within 10 feet.
• Values lower than -70 dBm (e.g., -80, -90) are likely beyond 10 feet.
  Remember: Walls, interference, and device hardware can skew these numbers—this is only a rough estimate.

C++ Implementation Example #

Instead of using system() (which doesn't capture output), use popen() to run commands and read their results:

#include <iostream>
#include <string>
#include <cstdio>
#include <algorithm>

bool isDeviceWithin10Feet(const std::string& bd_addr) {
    // Use bluetoothctl to fetch device info (includes RSSI for connected devices)
    std::string cmd = "bluetoothctl info " + bd_addr;
    FILE* pipe = popen(cmd.c_str(), "r");
    if (!pipe) return false;

    char buffer[128];
    int rssi = -100; // Default to "far away"
    while (fgets(buffer, sizeof(buffer), pipe) != nullptr) {
        std::string line(buffer);
        auto rssi_pos = line.find("RSSI");
        if (rssi_pos != std::string::npos) {
            // Parse RSSI value (format: "RSSI: -55")
            size_t value_start = line.find(":") + 2;
            rssi = std::stoi(line.substr(value_start));
            break;
        }
    }
    pclose(pipe);

    // Check against our 10-foot threshold
    return rssi >= -60;
}

int main() {
    std::string device_addr = "AA:BB:CC:DD:EE:FF"; // Replace with your device's BD address
    if (isDeviceWithin10Feet(device_addr)) {
        std::cout << "Device is within ~10 feet\n";
    } else {
        std::cout << "Device is beyond ~10 feet\n";
    }
    return 0;
}

• Alternative Commands: For unconnected devices, use hcitool rssi <bd_addr> (requires root) or btmgmt find to get RSSI during discovery.
• Security Tip: Avoid running commands with root privileges unless necessary—bluetoothctl works without root for connected devices.

内容的提问来源于stack exchange，提问作者Sandeep