I'm a firm believer that a maintainer should be, at least in some part, a consumer of the thing they're maintaining. I built GGK for a personal project. That project's communication needs have grown considerably, beyond the point where Bluetooth LE is a viable option and I was forced to make the switch to an IP-based solution. As much as I've enjoyed building and using GGK, I no longer have a use-case for Bluetooh LE or a test-case for GGK.
If you are a GGK user and would like to become an advocate for its future development, please contact me.
This author has deciced that this software should be free. Furthermore, this author's choice should not limit the freedoms of other authors by restricting their choices. As a result, Gobbledegook is now licensed under the New BSD License.
Gobbledegook's license changed from GPL to LGPL in hopes that it would be found useful to more developers.
Gobbledegook is a C/C++ standalone Linux Bluetooth LE GATT server using BlueZ over D-Bus with Bluetooth Management API support built in. That's a lot of words, so I shortened it to Gobbledegook. Then I shortened it again to GGK because let's be honest, it's a pain to type.
For the impatient folks in a hurry (or really just have to pee) skip down to the Quick-start for the impatient section at the bottom of this document.
- DSL-like creation of BLE services makes creating services trivial
- Automated D-Bus object hierarchy introspection generation
- Automated D-Bus ObjectManager implementation
- Automated BlueZ GATT application registration
- Support for the Bluetooth Management API
- Timer events allow services to perform periodic updates
- Application-managed server data
- Comment blocks at the top of each source file with deep explanations of critical concepts
- Written in C++14 (gcc & clang) with a standard C public interface
- Tested on Ubuntu 16.04 on x86 and Raspberry Pi
A server description is the meat of your server. It is a collection one or more GATT Services. Each GATT Service contains a collection of one or more characteristics, which in turn may contain zero or more descriptors. The server description is declared in the Server constructor found in Server.cpp.
The following sections explain how to build a server description.
New to Bluetooth terminology?
Bluetooth Low Energy (or BLE) is the marketing term for Bluetooth 4.0. A BLE server will offer up one or more GATT Services.
GATT stands for Generic Attribute Profile (apparently the P is silent.) This is the standard that defines how BLE devices share data. Specifically, it defines the structure of services, characteristics and descriptors.
A GATT Service can be thought of as a discreet unit of functionality. Examples would be a time service that returns the current local time and timezone or a battery service that returns the current battery level and temperature. A GATT Service serves up information in the form of a collection of one or more GATT Characteristics.
A GATT Characteristic is one unit of data from a GATT Service. For example, our fictitious battery service would have two GATT Characteristics: (1) the current battery level and (2) the temperature. The battery level characteristic might be defined as a single byte value in the range of 0-100 representing the percentage of battery remaining. In addition to their data, a GATT Characteristic may also contain zero or more optional GATT Descriptors.
A GATT Descriptor contains additional information and metadata about a GATT Characteristic. They can be used to describe the characteristic's features or to control certain behaviors of the characteristic. Extending our battery service example further, the temperature characteristic could be a 16-bit value representing a temperature, while the GATT Descriptor further defines how to interpret that data (as degrees Fahrenheit, degrees Celsius or 10th of degrees Kelvin.)
Below is a complete custom GATT Service as defined within the GGK framework. It is a simple service that uses the asctime() function to return the current time as a string:
// Create a service
.gattServiceBegin("ascii_time", "00000001-1E3D-FAD4-74E2-97A033F1BFEE")
// Add a characteristic to the service with the 'read' flag set
.gattCharacteristicBegin("string", "00000002-1E3D-FAD4-74E2-97A033F1BFEE", {"read"})
// Handle the characteristic's "ReadValue" method
.onReadValue(CHARACTERISTIC_METHOD_CALLBACK_LAMBDA
{
time_t timeVal = time(nullptr);
struct tm *pTimeStruct = localtime(&timeVal);
std::string timeString = asctime(pTimeStruct);
self.methodReturnValue(pInvocation, timeString, true);
})
.gattCharacteristicEnd()
.gattServiceEnd()The first thing you may notice about this example is that many of the lines begin with a dot. This is because we're chaining methods together. Each method returns the appropriate type to provide context. Internally the gattServiceBegin() method returns a reference to a GattService object which provides the proper context to create a characteristic within that service. Similarly, the gattCharacteristicBegin() method returns a reference to a GattCharacteristic object which provides the proper context for responding to the onReadValue() method or adding descriptors to the characteristic.
You may also have noticed that we're using lambdas to include our implementation inline. The code to generate the time string is wrapped up in a CHARACTERISTIC_METHOD_CALLBACK_LAMBDA which is just a convenience macro that declares the lambda properly for us. You can use the raw lambda declaration if you wish, but then you're being anti-macroist and that's just not cool, bruh. And if you don't like these new-fangled lambdas, you can just stick a good ol' function pointer in its place.
A compiled GGK library provides a public interface that is compatible with standard C, but you'll need a modern compiler to build a GGK library because the internals are written using features of c++11.
Let's take a look at a more complex example. Here's an implementation of the Bluetooth standard's Current Time Service. We'll even toss a few extras in to keep things interesting:
// Current Time Service (0x1805)
.gattServiceBegin("time", "1805")
.gattCharacteristicBegin("current", "2A2B", {"read", "notify"})
.onReadValue(CHARACTERISTIC_METHOD_CALLBACK_LAMBDA
{
self.methodReturnVariant(pInvocation, ServerUtils::gvariantCurrentTime(), true);
})
.gattDescriptorBegin("description", "2901", {"read"})
.onReadValue(DESCRIPTOR_METHOD_CALLBACK_LAMBDA
{
self.methodReturnValue(pInvocation, "Current server local time", true);
})
.gattDescriptorEnd()
.onEvent(60, nullptr, CHARACTERISTIC_EVENT_CALLBACK_LAMBDA
{
self.sendChangeNotificationVariant(pConnection, ServerUtils::gvariantCurrentTime());
})
.gattCharacteristicEnd()
.gattCharacteristicBegin("local", "2A0F", {"read"})
.onReadValue(CHARACTERISTIC_METHOD_CALLBACK_LAMBDA
{
self.methodReturnVariant(pInvocation, ServerUtils::gvariantLocalTime(), true);
})
.gattDescriptorBegin("description", "2901", {"read"})
.onReadValue(DESCRIPTOR_METHOD_CALLBACK_LAMBDA
{
self.methodReturnValue(pInvocation, "Local time data", true);
})
.gattDescriptorEnd()
.gattCharacteristicEnd()
.gattServiceEnd()If you're already familiar with BLE, then hopefully the expansion to multiple characteristics and the addition of descriptors needs no further explanation. If that's true, then you're probably amazed by that. Maybe a more modest level of amazement than it's-bigger-on-the-inside amazement levels, but you should still be sure to catch your breath before trying to read further. Safety first.
Did you notice the bonus call to onEvent()? The event (a TickEvent to be specific) is not part of the Bluetooth standard. It works similar to a typical GUI timer event. In this example, we're using it to send out a change notification (a "PropertiesChanged" notification in the standard parlance). Any client that has subscribed to that characteristic will receive an updated time every 60 ticks (seconds.)
Working in hierarchical contexts of services, characteristics, descriptors and methods simplifies the process building a server description because each context has a limited set of available tools to work with. For example, within a service context the only tools available are gattCharacteristicBegin() and gattServiceEnd(). This isn't a limitation on your flexibility; anything else would be a deviation from the specification.
A GGK context is a synonym for the C term scope.
The Service context is another way of saying the
GattServiceobject scope. Similarly, Characteristics and Descriptors are scopes of theGattCharacteristicandGattDescriptorobjects.Methods differ slightly in that they are scoped to their lambdas. However, Methods also contain a
selfparameter which is a reference to the containing scope. In other words, a Method within a Descriptor will have aselfreference to theGattDescriptorobject where that method is declared. More on this in the Lambda reference section.
GATT Services are chained together to form a complete server description. So let's focus on how a GATT Service is built.
Below is a template for a GATT Service with all available components. Function parameters are shown by name and indentation denotes context.
Note that a service may contain more than one characteristic, in which case they would follow each other sequentially within the service description. The same holds true for Descriptors, except that a characteristic may have no descriptors. In addition, each of the four methods found in the characteristics and descriptors (.onEvent(), .onReadValue(), .onWriteValue() and .onUpdateValue()) would only be present if needed.
.gattServiceBegin(name, uuid)
.gattCharacteristicBegin(name, uuid, flags[])
.onEvent(tickFrequency, userData, CHARACTERISTIC_EVENT_CALLBACK_LAMBDA
{
[...your code here...]
})
.onReadValue(CHARACTERISTIC_METHOD_CALLBACK_LAMBDA
{
[...your code here...]
})
.onWriteValue(CHARACTERISTIC_METHOD_CALLBACK_LAMBDA
{
[...your code here...]
})
.onUpdatedValue(CHARACTERISTIC_UPDATED_VALUE_CALLBACK_LAMBDA
{
[...your code here...]
})
.gattDescriptorBegin(name, uuid, flags[])
.onEvent(tickFrequency, userData, DESCRIPTOR_EVENT_CALLBACK_LAMBDA
{
[...your code here...]
})
.onReadValue(DESCRIPTOR_METHOD_CALLBACK_LAMBDA
{
[...your code here...]
})
.onWriteValue(DESCRIPTOR_METHOD_CALLBACK_LAMBDA
{
[...your code here...]
})
.onUpdatedValue(DESCRIPTOR_UPDATED_VALUE_CALLBACK_LAMBDA
{
[...your code here...]
})
.gattDescriptorEnd()
.gattCharacteristicEnd()
.gattServiceEnd()
The following methods are available within the context of either a characteristic or descriptor.
Register a lambda or callback that is called whenever a Bluetooth client reads the value of a characteristic or descriptor. It is tied to the ReadValue method described in the BlueZ D-Bus GATT API.
Register a lambda or callback that is called whenever a Bluetooth client writes to the value of a characteristic or descriptor. It is tied to the WriteValue method described in the BlueZ D-Bus GATT API.
Register a lambda or callback that is called after tickFrequency ticks of the periodic timer. Tick events work similar to timer events found in modern GUIs.
Events can be used to update server data, send notifications or perform any other general periodic work. This is a convenience method of GGK and is not part of the Bluetooth standard or BlueZ D-Bus GATT API.
Register a lambda or callback that is called when data is updated internally by the application.
As an application generates or updates its own data, it may notify the server of those updates using the public interface methods ggkNofifyUpdatedCharacteristic() and ggkNofifyUpdatedDescriptor(). The server will then call the appropriate onUpdateValue lambda or callback for the characteristic or descriptor receiving the update. This is a convenience method of GGK and is not part of the Bluetooth standard or BlueZ D-Bus GATT API.
Aside from the application performing data updates, a characteristic or descriptor may modify its own data from within a lambda and trigger this call. For details, see self.callOnUpdatedValue() method in the Lambda reference section below.
Within the context of a lambda there is a self parameter that references the parent context (the characteristic or descriptor under which the lambda is registered.)
The following methods are available to through that self parameter.
Gets a named value from the server data (see the section Server data for details on how this data is managed.) If name is not found, default is returned. This is a templated function to allow non-pointer data of any type to be retrieved. For pointer data, see self.getDataPointer().
Gets a named pointer from the server data (see the section Server data for details on how this data is managed.) If name is not found, default is returned. This is a templated function to allow pointer data of any type to be retrieved. Note that T is a pointer type. For non-pointer values, see self.getDataValue().
Sets the named value in the server data (see the section Server data for details on how this data is managed.) This is a templated function to allow non-pointer data of any type to be set. For pointer data, see self.setDataPointer().
Sets the named pointer in the server data (see the section Server data for details on how this data is managed.) This is a templated function to allow pointer data of any type to be set. Note that T is a pointer type. For non-pointer values, see self.setDataValue().
Call the parent context's onUpdatedValue() method.
This is a convenience method for situations where a lambda modifies the data of its parent characteristic or descriptor and wishes to notify it's own parent of the change. This simplifies reuse of code by placing all data update management code in the onUpdatedValue() registered lambda.
Send a value to a Bluetooth client in response to a method call (such as ReadValue).
Unlike normal return values from methods within a program, return values in response to Bluetooth method calls must be explicitly sent.
This is a templated function to allow common data types to be automatically placed into a GVariant array of bytes ("ay") with the option of wrapping the response in a tuple ("(ay)"). For a generic GVariant * version of this method, see methodReturnVariant().
For information on GVariants, see the GLib reference manual.
Send a GVariant to a Bluetooth client in response to a method call (such as ReadValue).
Unlike normal return values from methods within a program, return values in response to Bluetooth method calls must be explicitly sent.
If wrapInTuple is set to true, the pVariant is automatically wrapped in a tuple before sending. A convenience function is available for responding with common data types (see methodReturnValue()).
For information on GVariants, see the GLib reference manual.
Sends a change notification (in BlueZ parlance, a "PropertiesChanged" signal) with an updated GVariant * value to subscribers to this characteristic.
This is a generalized method that accepts a GVariant *. A templated version is available that supports common types called sendChangeNotificationValue().
For information on GVariants, see the GLib reference manual.
NOTE: This method is only available to characteristics.
Sends a change notification (in BlueZ parlance, a "PropertiesChanged" signal) to subscribers to this characteristic.
This is a helper method that accepts common types. For custom types, there is a form that accepts a GVariant *, called sendChangeNotificationVariant().
NOTE: This method is only available to characteristics.
Server data is maintained by the application. When the application starts the GGK server, it calls ggkStart() with two delegates: a data getter and a data setter. These methods are used by the server to retrieve and store server data.
For details on these delegates and their usage, see the comment blocks in Gobbledegook.h under the section heading SERVER DATA.
When we build a server description, what we're really doing is building a hierarchical structure of D-Bus objects that conforms to BlueZ's standards for GATT services. The *Begin() and *End() calls are the building blocks for this hierarchy.
GGK uses this server description to build an XML introspection used to register the object hierarchy with D-Bus. You can view the generated XML for your services by launching the standalone application with the -d parameter.
GGK also uses the server description to implement the D-Bus ObjectManager interface which is used to describe our services. When we eventually register our application with BlueZ, BlueZ will use our ObjectManager interface to enumerate our services. GGK manages this for us automatically.
The following sections may help users with their system configurations.
My distribution didn't have the latest version of BlueZ with proper support for their D-Bus API (it was still considered experimental up until v5.42.) I had to download and build it for myself. I grabbed BlueZ 5.45.
Here's how I built it - this will probably work for most or all Ubuntu users:
./configure --prefix=/usr --libexecdir=/usr/lib \
--sysconfdir=/etc --localstatedir=/var --enable-test \
--enable-manpages --enable-testing --enable-library \
--enable-maintainer-mode --enable-experimental \
--enable-deprecated
make
service bluetooth stop
sudo make install # Helpful for development
systemctl daemon-reload # Helpful for development
service bluetooth start # Helpful for development
If you're on a Raspberry Pi, you should probably also run sudo rpi-update followed by a reboot.
The configuration file is located at /etc/bluetooth/main.conf. However, you should be fine with defaults here because this server can configure the settings as needed.
Edit the file /lib/systemd/system/bluetooth.service and set the ExecStart line to look like:
ExecStart=/usr/lib/bluetooth/bluetoothd -d --noplugin=hostname
-d - it's a good idea to leave this enabled while you are setting up your services in GGK (you can remove it later). This will allow you to tail -f /var/log/syslog | grep bluetoothd to get some helpful debug information.
--noplugin=hostname - this prevents BlueZ from running the 'hostname' plugin, which will rename your device to whatever its hostname is (ignoring your configurations.)
Once you've done this, run the following commands to reload these settings and restart the service:
sudo systemctl daemon-reload
sudo service bluetooth stop
sudo service bluetooth start
In order for our application to communicate over D-Bus, we'll need to ask D-Bus for an owned name, which will in effect be our address on D-Bus. D-Bus must be configured to grant us permissions to do this. We'll grant these permissions to user root.
You'll need to locate the D-Bus permissions on your box. Likely, you'll find a set of files for this in the directory /etc/dbus-1/system.d. Create the file /etc/dbus-1/system.d/gobbledegook.conf and give it the contents:
<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN" "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>
<policy user="root">
<allow own="com.gobbledegook"/>
<allow send_destination="com.gobbledegook"/>
<allow send_destination="org.bluez"/>
</policy>
<policy at_console="true">
<allow own="com.gobbledegook"/>
<allow send_destination="com.gobbledegook"/>
<allow send_destination="org.bluez"/>
</policy>
<policy context="default">
<deny send_destination="com.gobbledegook"/>
</policy>
</busconfig>
Note the com.gobbledegook entries in your new gobbledegook.conf file. This must match the service name (the first parameter sent to ggkStart() in standalone.cpp). If you change the service name from gobbledegook to clownface in that call to ggkStart(), then you'll need to edit the gobbledegook.conf file and change all occurrances of com.gobbledegook to com.clownface.
You don't need to do anything. this server will automatically power on the adapter, enable LE with advertisement.
However, if you want to do this manually, here are a few helpful commands you might try:
sudo btmgmt -i 0 power off
sudo btmgmt -i 0 le on
sudo btmgmt -i 0 connectable on
sudo btmgmt -i 0 advertising on
sudo btmgmt -i 0 power on
GGK uses the standard autotools build process:
./configure && make
This will build libggk.a then compile standalone.cpp into a program that links with libggk.a. There is no make install as there is nothing to install.
Then run with:
sudo src/standalone -d
GGK requires super-user privileges when run due to privileges required for D-Bus and HCI sockets. A system can be configured to allow a user to run a GGK server without sudo, but that's beyond the scope of this document.
During development, I tend to run these three commands, each in their own terminal:
sudo tail -f /var/log/syslog | grep bluetoothd
sudo dbus-monitor --system
sudo ./src/standalone -d
With no parameters, standalone will output only service level output (starting stopping errors, etc.) Additional output parameters are:
`-q` Quiet - errors only
`-v` Verbose - include info log levels
`-d` Debug - include debug log levels
If you don't already have some kind of test harness, you'll probably want something. I've had luck with a free Android app called nRF Connect.
Think of Gobbledegook as a template BLE library. You're expected to modify Server.cpp and replace the example services with your own. Once you've customized your services, the rest is easy.
Just link against libggk.a and include include/Gobbledegook.h to access the public API in your app. You may notice that the public interface isn't documented here. Instead, it is documented in include/Gobbledegook.h.
You can use standalone.cpp as a reference on how to get things setup in your code.
If you decide to dig into the codebase, you'll want to be familiar with D-Bus. If you've never messed with D-Bus, this short Introduction to D-Bus will help a lot. GGK uses GLib's GIO for all D-Bus work, which is fully documented in the GIO Reference Manual. And finally, for the real nitty-gritty on D-Bus be sure to visit the D-Bus Specification.
The BlueZ D-Bus APIs are fully documented. They're brief, but complete. Pay special attention to the BlueZ D-Bus GATT API description, which describes how GATT services are implemented over D-Bus.
The GLib Reference Manual covers many topics used in GGK, most notably the use of GVariant objects for passing data around over D-Bus. Pay special attention to the GVariant Format Strings page.
The following is the output from a reference tool used to connect to the standalone server running with the sample services. The output shows data read from the server over BLE. Note that the hieararchy may not match 1:1 with that of the samples, since some features are automatically provided by BlueZ. In addition, the reference tool periodically writes updates to the Text string service (hence the appended text, "(updated: 3 times)".)
Connected to Gobbledook
* Service: Device Information
> Characteristic: Manufacturer Name String
+ Value (String): 'Acme Inc.'
+ Flags: {read}
> Characteristic: Model Number String
+ Value (String): 'Marvin-PA'
+ Flags: {read}
* Service: (Example) CPU Info
> Characteristic: CPU Count
+ Value (UInt16): 4
+ Flags: {read}
- Descriptor: Characteristic user description
- Value (String): 'This might represent the number of CPUs in the system'
> Characteristic: CPU Model
+ Value (String): 'ARMv7 Processor rev 4 (v7l)'
+ Flags: {read}
- Descriptor: Characteristic user description
- Value (String): 'Possibly the model of the CPU in the system'
* Service: Battery Information
> Characteristic: Battery Level
+ Value (Percent): 75%
+ Flags: {read, notify}
- Descriptor: Client characteristic configuration
- Value (Flags): Notifications enabled[false], Indications enabled[false]
* Service: (Example) asctime()
> Characteristic: Ascii Time
+ Value (String): 'Thu Aug 24 18:43:10 2017'
+ Flags: {read}
- Descriptor: Characteristic user description
- Value (String): 'Returns the local time (as reported by POSIX asctime()) each time it is read'
* Service: (Example) Text string
> Characteristic: String value
+ Value (String): 'Hello, world (updated: 3 times)'
+ Flags: {read, write, notify}
- Descriptor: Client characteristic configuration
- Value (Flags): Notifications enabled[false], Indications enabled[false]
- Descriptor: Characteristic user description
- Value (String): 'A mutable text string used for testing. Read and write to me, it tickles!'
* Service: Current Time
> Characteristic: Current Time
+ Value (DateTime): Thu, 2017/08/24 18:43:29.00
+ Flags: {read, notify}
- Descriptor: Client characteristic configuration
- Value (Flags): Notifications enabled[false], Indications enabled[false]
> Characteristic: Local Time Information
+ Value (LocTime): utcOffset: -5.0 hours, DST (+1h)
+ Flags: {read}
Build with ./configure && make. This will build libggk.a then compile standalone.cpp into a program that links with libggk.a.
Run sudo ./src/standalone -d. This will require your machine to be setup correctly with BlueZ and working BLE hardware. Don't forget the sudo; you'll need it unless you configure the appropriate permissions for your user to access to D-Bus and the HCI socket. Options are: -e (errors-only output), -v (verbose output), -d (debug output)
Coding your own BLE services is actually very easy once you become a little familiar with things. Think of Gobbledegook as a template BLE library. You're expected to modify Server.cpp and replace the example services with your own. Do yourself a favor and scan through the Implementing services with GGK section. It's your quickest path to understanding the foundation.
Integrating into your app is easy: just link against libggk.a and include include/Gobbledegook.h where needed. Use standalone.cpp as a template for getting things setup in your code.