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adv_ble {#mainpage}

[TOC]

Usage in a Nutshell

int main(void)
{
    int dev = ble_adv_open();
    if (dev == -1) {
        perror("ble_adv_open() failed");
        return EXIT_FAILURE;
    }

    if (ble_adv_scan(dev, BLE_ADV_SCAN_FLAG_ENABLED)) {
        perror("ble_adv_scan() failed");
        return EXIT_FAILURE;
    }

    while (1) {
        struct ble_adv advertisement;
        if (ble_adv_read(dev, &advertisement)) {
            perror("ble_adv_read() failed");
            /* disable scanning on failure */
            ble_adv_scan(dev, 0);
            return EXIT_FAILURE;
        }
        /* work on data */
    }
}

What Does This Library Provide

This tiny C library that provides a convenience wrapper over BlueZ to allow scanning for Bluetooth Low Energy (BLE) Advertisements with as little effort as possible. In addition, it parses the most important entries of BLE advertisements into a structure, so that users don't need to worry about the encoding.

What Does This Library Not Provide

Only a subset of the specified EIR data types are implemented. It should however be relatively straight forward to extend the parse_eir() function to support additional fields. Search for the "Supplement to the Bluetooth Core Specification" and jump to "Part A: Data Types Specification".

It is worth noting that only the service data with a 16 bit UUID are supported, while also 32 and 128 bit flavors are specified.

Motivation

Even as a senior (ahem ahem) C developer I found it surprisingly hard to get started with BlueZ. Due to the lack of documentation, resources, and even third party resources on this matter, I ended up studying the source code of BlueZ to be able to interface with it. The goal of this library is to get C developers to receive BLE advertisements with as little pain in the ass as possible.

Permissions and Security

Enabling/disabling scanning for BLE advertisements and configuring the scan parameters cannot be done as regular user. Still, running programs written in C (and, hence, without any guarantees on memory safety) which parse (somewhat) complex data formats of messages received from untrusted sources as root is ranked pretty high on the list of things I'd personally avoid doing. Luckily, the situation is not as bad as it initially sounds:

  1. Only two capabilities are actually needed, not full root access. Just run sudo setcap 'cap_net_raw,cap_net_admin+eip' <program> on your executable to get it running.
  2. You could implement your program in a way that it drops its capabilities right after it enabled scanning for BLE advertisements but before reading the first advertisements. E.g. with libcap-ng this is straight forward. However, your program won't be able to disable scanning after it has completed its task.
  3. You could fork in your program and let e.g. the parent process keep the capabilities, while the child process gives them up. The parent will only enable and disable BLE scanning, but leave the parsing of the child process. This way, the process doing the risky part runs without extra capabilities.