Getting started with libmapper

Since libmapper uses GNU autoconf, getting started with the library is the same as any other library on Linux; use ./configure and then make to compile it. You'll need swig available if you want to compile the Python bindings. On Mac OS X, we provide a precompiled Framework bundle for 32- and 64-bit Intel platforms, so using it with XCode should be a matter of including it in your project.

Overview of the API structure

If you take a look at the API documentation, there is a section called "modules". This is divided into the following sections:

For this tutorial, the only sections to pay attention to are Devices and Signals. Networks are reserved for providing custom networking configurations, but in general you don't need to worry about it.

The Database module is used to keep track of what devices, signals and maps are on the network. It is used mainly for creating user interfaces for mapping design and will also not be covered here.

Functions and types from each module are prefixed with mapper_<module>_, in order to avoid namespace clashing.


Creating a device

To create a libmapper device, it is necessary to provide a few parameters to mapper_device_new:

mapper_device mapper_device_new(const char *name_prefix,
                                int initial_port,
                                mapper_network network);

Every device on the network needs a name and port. In fact the requested name and port are only "starting values". There is an initialization period after a device is created where a unique ordinal is chosen to append to the device name. This allows multiple devices with the same name to exist on the network.

Similarly, each device gets a unique port to use for exchanging signal data. The provided port number is the "starting port", but the allocation algorithm will possibly choose another port number close to it if someone else on the network is already using it. We usually use a port number of 9000, and let the system decide if this is appropriate.^[Strictly this is only necessary for devices on the same computer, but port numbers are in abundance so we just allocate one per device to keep things consistent.]

The third parameter of mapper_device_new is an optional network instance. It is not necessary to provide this, but can be used to specify different networking parameters, such as specifying the name of the network interface to use.

An example of creating a device:

mapper_device my_device = mapper_device_new("test", 0, 0);

Polling the device

The device lifecycle looks like this, in terrible ASCII diagram art:

mapper_device_new -> mapper_device_poll +-> mapper_device_free
                        |               |

In other words, after a device is created, it must be continuously polled during its lifetime, and then explicitly freed when it is no longer needed.

The polling is necessary for several reasons: to respond to requests on the admin bus; to check for incoming signals; to update outgoing signals. Therefore even a device that does not have signals must be polled. The user program must organize to have a timer or idle handler which can poll the device often enough. Polling interval is not extremely sensitive, but should be at least 100 ms or less. The faster it is polled, the faster it can handle incoming and outgoing signals.

The mapper_device_poll function can be blocking or non-blocking, depending on how you want your application to behave. It takes a number of milliseconds during which it should do some work, or 0 if it should check for any immediate actions and then return without waiting:

int mapper_device_poll(mapper_device dev, int block_ms);

An example of calling it with non-blocking behaviour:

mapper_device_poll(my_device, 0);

If your polling is in the middle of a processing function or in response to a GUI event for example, non-blocking behaviour is desired. On the other hand if you put it in the middle of a loop which reads incoming data at intervals or steps through a simulation for example, you can use mapper_device_poll as your "sleep" function, so that it will react to network activity while waiting.

It returns the number of messages handled, so optionally you could continue to call it until there are no more messages waiting. Of course, you should be careful doing that without limiting the time it will loop for, since if the incoming stream is fast enough you might never get anything else done!

Note that an important difference between blocking and non-blocking polling is that during the blocking period, messages will be handled immediately as they are received. On the other hand, if you use your own sleep, messages will be queued up until you can call poll(); stated differently, it will "time-quantize" the message handling. This is not necessarily bad, but you should be aware of this effect.

Since there is a delay before the device is completely initialized, it is sometimes useful to be able to determine this using mapper_device_ready. Only when mapper_device_ready returns non-zero is it valid to use the device's name.

Freeing the device

It is necessary to explicitly free the device at the end of your program. This not only frees memory, but also sends some messages to "politely" remove itself from the network.

An example of freeing a device:



Now that we know how to create a device, poll it, and free it, we only need to know how to add signals in order to give our program some input/output functionality. While libmapper enables arbitrary connections between any declared signals, we still find it helpful to distinguish between two type of signals: inputs and outputs.

This can become a bit confusing, since the "reverb" parameter of a sound synthesizer might be updated locally through user interaction with a GUI, however the normal use of this signal is as a destination for control data streams so it should be defined as an input signal. Note that this distinction is to help with GUI organization and user-understanding – libmapper enables connections from input signals and to output signals if desired.

Creating a signal

We'll start with creating a "sender", so we will first talk about how to update output signals. A signal requires a bit more information than a device, much of which is optional:

mapper_signal mapper_device_add_input_signal(mapper_device dev, const char *name,
                                             int length, char type, const char *unit,
                                             void *minimum, void *maximum,
                                             mapper_signal_handler *h,
                                             void *user_data);

mapper_signal mapper_device_add_output_signal(mapper_device dev, const char *name,
                                              int length, char type, const char *unit,
                                              void *minimum, void *maximum);

The only required parameters here are the signal "length", its name, and data type. Signals are assumed to be vectors of values, so for usual single-valued signals, a length of 1 should be specified. Finally, supported types are currently 'i', 'f', or 'd' (specified as characters in C, not strings), for int, float, and double values, respectively.

The other parameters are not strictly required, but the more information you provide, the more the mapper can do some things automatically. For example, if minimum and maximum are provided, it will be possible to create linear-scaled connections very quickly. If unit is provided, the mapper will be able to similarly figure out a linear scaling based on unit conversion (from centimeters to inches for example). Currently automatic unit-based scaling is not a supported feature, but will be added in the future. You can take advantage of this future development by simply providing unit information whenever it is available. It is also helpful documentation for users.

Notice that optional values are provided as void* pointers. This is because a signal can either be int, float, or double, and your maximum and minimum values should correspond in type. So you should pass in a int*, float*, or double* by taking the address of a local variable.

Lastly, it is usually necessary to be informed when input signal values change. This is done by providing a function to be called whenever its value is modified by an incoming message. It is passed in the handler parameter, with context information to be passed to that function during callback in user_data.

An example of creating a "barebones" int scalar output signal with no unit, minimum, or maximum information:

mapper_signal outA = mapper_device_add_output_signal(dev, "outA", 1,
                                                     'i', 0, 0, 0);

An example of a float signal where some more information is provided:

float min = 0.0f;
float max = 5.0f;
mapper_signal s1 = mapper_device_add_output_signal(dev, "sensor1", 1, 'f',
"V", &min, &max);

So far we know how to create a device and to specify an output signal for it. To recap, let's review the code so far:

mapper_device dev = mapper_device_new("my_device", 0, 0);
mapper_signal s1 = mapper_device_add_output_signal(dev, "sensor1", 1, 'f',
                                                   "V", &min, &max);

while (!done) {
    mapper_device_poll(dev, 50);
    ... do stuff ...
    ... update signals ...


Note that although you have a pointer to the mapper_signal structure (which was returned by mapper_device_add_output_signal()), its memory is "owned" by the device. In other words, you should not worry about freeing its memory - this will happen automatically when the device is destroyed. It is possible to retrieve a device's inputs or outputs by name or by index at a later time using the functions mapper_device_signal_by_<name/index>.

Updating signals

We can imagine the above program getting sensor information in a loop. It could be running on an network-enable ARM device and reading the ADC register directly, or it could be running on a computer and reading data from an Arduino over a USB serial port, or it could just be a mouse-controlled GUI slider. However it's getting the data, it must provide it to libmapper so that it will be sent to other devices if that signal is mapped.

This is accomplished by the function mapper_signal_update():

void mapper_signal_update(mapper_signal sig, void *value,
                          int count, mapper_timetag_t timetag);

As you can see, a void* pointer must be provided. This must point to a data structure identified by the signal's length and type. In other words, if the signal is a 10-vector of int, then value should point to the first item in a C array of 10 ints. If it is a scalar float, it should be provided with the address of a float variable. The count argument allows you to specify the number of value samples that are being updated - for now we will set this to 1. Lastly the timetag argument allows you to specify a time associated with the signal update. If your value update was generated locally, or if your program does not have access to upstream timing information (e.g., from a microcontroller sampling sensor values), you can use the macro MAPPER_NOW and libmapper will tag the update with the current time.

To simplify things even further, a short-hand is provided for scalar signals of particular types:

void mapper_signal_update_int(mapper_signal sig, int value);
void mapper_signal_update_float(mapper_signal sig, float value);

So in the "sensor 1" example, assuming in do_stuff we have some code which reads sensor 1's value into a float variable called v1, the loop becomes:

while (!done) {
    mapper_device_poll(my_device, 50);

    // call hypothetical user function that reads a sensor
    float v1 = do_stuff();
    mapper_signal_update_float(sensor1, v1);

This is about all that is needed to expose sensor 1's value to the network as a mappable parameter. The libmapper GUI can now map this value to a receiver, where it could control a synthesizer parameter or change the brightness of an LED, or whatever else you want to do.

Signal conditioning

Most synthesizers of course will not know what to do with the value of sensor1 --it is an electrical property that has nothing to do with sound or music. This is where libmapper really becomes useful.

Scaling or other signal conditioning can be taken care of before exposing the signal, or it can be performed as part of the mapping. Since the end user can demand any mathematical operation be performed on the signal, he can perform whatever mappings between signals as he wishes.

As a developer, it is therefore your job to provide information that will be useful to the end user.

For example, if sensor 1 is a position sensor, instead of publishing "voltage", you could convert it to centimeters or meters based on the known dimensions of the sensor, and publish a "sensor1/position" signal instead, providing the unit information as well.

We call such signals "semantic", because they provide information with more meaning than a relatively uninformative value based on the electrical properties of the sensing technqiue. Some sensors can benefit from low-pass filtering or other measures to reduce noise. Some sensors may need to be combined in order to derive physical meaning. What you choose to expose as outputs of your device is entirely application-dependent.

Best of all, you can publish both "sensor1/position" and "sensor1/voltage" if desired, in order to expose both processed and raw data. Keep in mind that these will not take up significant processing time, and zero network bandwidth, if they are not mapped.

Receiving signals

Now that we know how to create a sender, it would be useful to also know how to receive signals, so that we can create a sender-receiver pair to test out the provided mapping functionality. The current value and timestamp for a signal can be retrieved at any time by calling mapper_signal_value(), however for event-driven applications you may want to be informed of new values as they are received or generated.

As mentioned above, the mapper_device_add_input_signal() function takes an optional handler and user_data. This is a function that will be called whenever the value of that signal changes. To create a receiver for a synthesizer parameter "pulse width" (given as a ratio between 0 and 1), specify a handler when calling mapper_device_add_input_signal(). We'll imagine there is some C++ synthesizer implemented as a class Synthesizer which has functions setPulseWidth() which sets the pulse width in a thread-safe manner, and startAudioInBackground() which sets up the audio thread.

Create the handler function, which is fairly simple,

void pulsewidth_handler(mapper_signal sig, mapper_id instance, void *value,
                        int count, mapper_timetag_t *tt) {
    Synthesizer *s = (Synthesizer*) props->user_data;

First, the pointer to the Synthesizer instance is extracted from the user_data pointer, then it is dereferenced to set the pulse width according to the value pointed to by v.

Then main() will look like,

void main()
    Synthesizer synth;

    float min = 0.0f;
    float max = 1.0f;

    mapper_device synth_dev = mapper_device_new("synth", 0, 0);
    mapper_signal pw = mapper_device_add_input_signal(synth_dev, "pulsewidth",
                                                      1, 'f', 0, &min, &max,
                                                      pulsewidth_handler, &synth);

    while (!done)
        mapper_device_poll(synth_dev, 50);


Working with timetags

libmapper uses the mapper_timetag_t data structure to store NTP timestamps. For example, the handler function called when a signal update is received contains a timetag argument. This argument indicates the time at which the source signal was sampled (in the case of sensor signals) or generated (in the case of sequenced or algorithimically-generated signals).

When updating output signals, using the functions mapper_signal_update_int() or mapper_signal_update_float() will automatically label the outgoing signal update with the current time. In cases where the update should more properly be labeled with another time, this can be accomplished with the function mapper_signal_update(). This timestamp should only be overridden if your program has access to a more accurate measurement of the real time associated with the signal update, for example if you are writing a driver for an outboard sensor system that provides the sampling time. Otherwise the constant MAPPER_NOW can be used as the timetag argument to cause libmapper to provide the current time.

libmapper also provides helper functions for getting the current device-time, setting the value of a mapper_timetag_t from other representations, and comparing or copying timetags. Check the API documentation for more information.

Working with signal instances

libmapper also provides support for signals with multiple instances, for example:

The important qualities of signal instances in libmapper are:

All signals possess one instance by default. If you would like to reserve more instances you can use:

mapper_signal_reserve_instances(mapper_signal sig, int num, mapper_id *ids,
                                void **user_data);

If the ids argument is null libmapper will automatically assign unique ids to the reserved instances.

After reserving instances you can update a specific instance:

mapper_signal_instance_update(mapper_signal sig, mapper_id instance,
                              void *value, int count,
                              mapper_timetag_t timetag);

All of the arguments except one should be familiar from the documentation of mapper_signal_update() presented earlier. The instance argument does not have to be considered as an array index - it can be any integer that is convenient for labelling your instance. libmapper will internally create a map from your id label to one of the preallocated instance structures.

Receiving instances

You might have noticed earlier that the handler function called when a signal update is received has a argument called instance. Here is the function prototype again:

void mapper_signal_update_handler(mapper_signal sig, mapper_id instance,
                                  const void *value, int count,
                                  mapper_timetag_t *tt);

Under normal usage, this argument will have a value (0 <= n <= num_instances) and can be used as an array index. Remember that you will need to reserve instances for your input signal using mapper_signal_reserve_instances() if you want to receive instance updates.

Instance Stealing

For handling cases in which the sender signal has more instances than the receiver signal, the instance allocation mode can be set for an input signal to set an action to take in case all allocated instances are in use and a previously unseen instance id is received. Use the function:

void mapper_signal_set_instance_stealing_mode(mapper_signal sig,
                                              mapper_instance_stealing_type mode);

The argument mode can have one of the following values:

If you want to use another method for determining which active instance to release (e.g. the sound with the lowest volume), you can create an instance_event_handler for the signal and write the method yourself:

void my_handler(mapper_signal sig, mapper_id instance,
                mapper_instance_event event, mapper_timetag_t *tt) {
    // hypothetical user code chooses which instance to release
    mapper_id release_me = choose_instance_to_release(sig);

    mapper_signal_release_instance(sig, release_me, *tt);

    // now an instance is available

For this function to be called when instance stealing is necessary, we need to register it for MAPPER_INSTANCE_OVERFLOW events:

mapper_signal_set_instance_event_callback(sig, my_handler,

Publishing metadata

Things like device names, signal units, and ranges, are examples of metadata --information about the data you are exposing on the network.

libmapper also provides the ability to specify arbitrary extra metadata in the form of name-value pairs. These are not interpreted by libmapper in any way, but can be retrieved over the network. This can be used for instance to give a device X and Y information, or to perhaps give a signal some property like "reliability", or some category like "light", "motor", "shaker", etc.

Some GUI could then use this information to display information about the network in an intelligent manner.

Any time there may be extra knowledge about a signal or device, it is a good idea to represent it by adding such properties, which can be of any OSC-compatible type. (So, numbers and strings, etc.)

The property interface is through the functions,

void mapper_device_set_property(mapper_device dev, const char *property,
                                int length, char type, void *value);

void mapper_signal_set_property(mapper_signal sig, const char *property,
                                int length, char type, void *value);

void mapper_link_set_property(mapper_link lnk, const char *property,
                              int length, char type, void *value);

void mapper_map_set_property(mapper_map map, const char *property,
                             int length, char type, void *value);

The type of the value argument is specified by type: floats are 'f', 32-bit integers are 'i', doubles are 'd', and strings are 's'.

For example, to store a float indicating the X position of a device, you can call it like this:

float x = 12.5;
mapper_device_set_property(my_device, "x", 'f', &x);

char *sensingMethod = "resistive";
mapper_signal_set_property(sensor1, "sensingMethod",
                           's', sensingMethod);

If the parent object (a device and a signal in this case) is local the property change takes place immediately. If the object is remote the property change is only staged and must be pushed out to the network using the functions <object_name>_push():

mapper_device_push(mapper_device dev);
mapper_signal_push(mapper_signal sig);
mapper_link_push(mapper_link lnk);
mapper_map_push(mapper_map mpa);

Reserved keys

You can use any property name not already reserved by libmapper.

Reserved keys for devices

description, host, id, is_local, libversion, name, num_incoming_maps, num_outgoing_maps, num_inputs, num_outputs, port, synced, value, version, user_data

Reserved keys for signals

description, direction, id, is_local, length, max, maximum, min, minimum, name, num_incoming_maps, num_instances, num_outgoing_maps, rate, type, unit, user_data

Reserved keys for links

description, id, is_local, num_maps

Reserved keys for maps

description, expression, id, is_local, mode, muted, num_destinations, num_sources, process_location, ready, status

Reserved keys for map slots

bound_max, bound_min, calibrating, causes_update, direction, length, maximum, minimum, num_instances, use_as_instance, type