A_guide_to_the_Jack1_source_code

Robin Gareus edited this page Feb 11, 2013 · 2 revisions

This document should act as a link between the JACK design documentation and the source code. It is not supposed to be helpful if you do not look at the source code in parallel.

Directory structure

The JACK source code tree contains the following subdirectories:

  • jack: API headers
  • jackd: Code of the main JACK server
  • libjack: Client library --- implementation of the API headers

The remaining subdirectories tools, drivers, doc, man, config and example-clients should be self-explanatory.

Shared memory

The shared memory interface of JACK is defined by jack/shm.h. It is structured as follows: There is one central registry, which is a virtually contiguous chunk of shared memory and consists of one jack_shm_header_t followed by MAX_SHM_ID jack_shm_registry_t entries. These entries act as pointers to all of the shared memory elements of the JACK system containing the content data. The header holds an array of all the registered servers (jack_shm_server_t) including their names and their PIDs, the information which kind of shared memory is being used (POSIX or System V), the size of the whole registry, the size of one entry and the size of this header. One jack_shm_registry_t entry holds its index (If you have the pointer jack_shm_header_t * p to the header, you add 1 to it and then cast p to jack_shm_registry_t *, you then can directly navigate through the registry entries by p[index]), the PID of the allocator which created the shared segment, the size of the shared memory element it refers to and the ID of the shared memory element the registry element is representing. This ID is API specific: On System V it is an integer value, on POSIX it is a string, which starts with a slash. For now, we assume to have the POSIX interface.

There is another shared memory structure, which is valid only for a given address space, jack_shm_info_t. It has two members: an index into the registry and a pointer the shared memory segment is attached at in that address space. jack_shmalloc() takes such a jack_shm_info_t structure as a parameter together with the desired size, and it fills this structure. After the shared memory segment was created by jack_shmalloc()---in the case of POSIX, it is named "/JACK-%d", d = registry->id---it can be mapped to the local address space by jack_attach_shm(), which enters a valid value into attached_at of the jack_shm_info_t element.

The registry itself can be locked by a call to jack_shm_lock_registry which decrements the registry's semaphore; it can be unlocked by jack_shm_unlock_registry. These locks only refer to the information stored in the registry, but not to the other shared memory elements themselves.

Data structures

The main data structure of the JACK engine is called jack_engine_t (jack/internal.h) alias struct _jack_engine (jack/engine.h). Its singleton instance is located in the address space of the JACK daemon process (jackd). It has an element control typed as jack_control_t (jack/internal.h). This is the part of the engine structure, which is located in shared memory (no functions, only data); its corresponding jack_shm_info_t member is called control_shm. In jack_engine_t the six function pointers set_buffer_size, set_sample_rate, run_cycle, delay, transport_cycle_start and driver_exit are located, which are set to jack_set_sample_rate (jackd/transengine.c), jack_driver_buffer_size (jackd/engine.c), jack_run_cycle (jackd/engine.c), jack_engine_delay (jackd/engine.c), jack_engine_driver_exit (jackd/engine.c) and jack_transport_cycle_start (jackd/engine.c) by jack_engine_new. Especially jack_run_cycle is important, as this is the entry point for the driver to activate a new cycle.

In engine->clients the engine keeps a JSList (jack/jlist.h), a singly linked list of jack_client_internal_t structures (jack/internal.h). The type jack_client_internal_t is not the structure for internal clients, but the information about every client the engine keeps in its local memory. It holds the server side request file descriptor and the event file descriptor of the private communication channels with the respective client.

There is also a truefeeds and a sortfeeds list. What they mean, is explained in a comment in engine.c:

Each client has a sortfeeds list of clients indicating which clients it should be considered as feeding for the purposes of sorting the graph. This list differs from the clients it actually feeds in the following way:

  1. Connections from a client to itself are disregarded
  2. Connections to a driver client are disregarded
  3. If a connection from A to B is a feedback connection (ie there was already a path from B to A when the connection was made) then instead of B appearing on A's sortfeeds list, A will appear on B's sortfeeds list.

If client A is on client B's sortfeeds list, client A must come after client B in the execution order. The above 3 rules ensure that the sortfeeds relation is always acyclic so that all ordering constraints can actually be met.

Each client also has a truefeeds list which is the same as sortfeeds except that feedback connections appear normally instead of reversed. This is used to detect whether the graph has become acyclic.

The sortfeeds and truefeeds lists are set during every port (dis-)connection.

The pointer to the shared memory part of the client jack_client_control_t is, again, called control; its jack_shm_info_t structure is called control_shm. In jack_client_control_t all the callbacks are put, notably the JackProcessCallback and the JackSyncCallback. There can also be found some information about the client ID, the client name, the clients state (jack_client_state_t, jack/internal.h), whether the client is the timebase master and whether it is a slow sync client or not.

On client side each client has its own representation of itself, called struct _jack_client (libjack/local.h) alias jack_client_t (jack/types.h). It holds the request file descriptor and the event file descriptor (event_fd is #defined as pollfd[EVENT_POLL_INDEX].fd in libjack/client.c.) It has a pointer to the shared memory engine structure (jack_control_t) called engine and a pointer to its own shared memory part jack_client_control_t, called control. The associated jack_shm_info_t member of the shared memory structure is called engine_shm and control_shm, respectively.

jackd and drivers, part 1

The main function can be found in jackd/jackd.c. After the standard GNU getopt_long parameter demarshalling---at least a driver has to be given (seen_driver), we start with loading the descriptions of all the available drivers into a the global variable static JSList * drivers:

      drivers = jack_drivers_load ();

The function jack_drivers_load crawls the JACK drivers directory, which is given in the environment variable JACK_DRIVER_DIR. Whenever a driver is found, it calls jack_drivers_get_descriptor, which extracts the driver description from the shared object file. All drivers in JACK are specific shared object files defined by jack_driver_t or jack_driver_nt_t in jack/driver.h. Therefore, jack_drivers_get_descriptor dynamically loads the driver file via dlopen and calls driver_get_descriptor(), which every driver has to implement. We end up with a JSList of the descriptors of all the drivers found. The driver description type jack_driver_desc_t can be found in jack/driver_interface.h.

IPC

There are two bidirectional channels between each external client and the server: An event socket and a request socket. On the request socket, the client can ask the server to do some work for him, for example register a port, connect a port, activate the client and so on. Via the event socket the server can notify its clients about occurring events, such as a (dis-)connected port, a buffer size or sample rate change or a graph reordering. These channels need to be bidirectional (and therefore are implemented as sockets) because the other side needs a feedback channel to answer, whether the request or the event delivery, respectively, was successful.

The entry sockets on server side are created during jack_engine_new() (jackd/engine.c) by calling make_sockets() (jackd/engine.c): First the ''master server socket'' is created and its file descriptor saved in engine->fds[0]. Then, the socket is bound to its registration path "%s/jack_0" (where the string variable s is filled by jack_server_dir()) and the socket is advised to listen to connection events. This socket is used to establish the request socket between a client and the server. As we see later: Now a client can create a socket and connect it to the path "%s/jack_0". When then server accepts the connection on the master socket, it receives another file descriptor: A privat request channel between the client and the server with one file descriptor on each side is established. After that, the same procedure is repeated for the client/server event ack socket, which is bound to the path "%s/jack_ack_%d".

The communication with the clients is handled in an extra server thread, implemented in the function jack_server_thread(), (jackd/engine.c) and started at the end of jack_engine_new()} ( jackd/engine.c) with jack_client_create_thread() (libjack/thread.c). The jack server thread implements an infinite server loop. Each cycle begins with setting up the engine->pfd array of pollfd structures containing the file descriptors and the events which should be polled for. The master server socket engine->fds[0] is put to engine->pfd[0].fd, the ''client/server event ack server'' engine->fds[1] to engine->pfd[1].fd. The fd member of engine->pfd[2] holds engine->cleanup_fifo[0]: In addition to the main sockets, there is a so-called cleanup FIFO, which is actually a pipe. It is also set up during jack_engine_new() with cleanup_fifo[1] being the source and cleanup_fifo[0] the sink. This pipe is not used for communication, but only for synchronization and it gives the other threads of the server an opportunity to wake up the server thread (jack_wake_server_thread, jackd/clientengine.c) if some errors occurred during client checkup (jack_check_clients, jackd/clientengine.c). After these three elements, all the valid request file descriptors of the clients are added. Now we poll on engine->pfd and sleep, if an event or many events have occurred, the server thread gets waked up and handles the events.

# Time and transport handling
How the time and transport handling works, is explained in the [JACK Audio Connection Kit reference manual](http://jackaudio.org/files/docs/html/transport-design.html).

# The engine and drivers, part 2

The core engine of the jack daemon is situated in `jack_main` (`jackd/jackd.c`). First it creates a new engine structure via `{jack_engine_new` (`jackd/engine.c`). This function sets up all the engine data structures and the ports, creates the request and event master sockets and the cleanup FIFO. At the end it starts the `jack_server_thread` (`jackd/engine.c`), which handles all the requests delivered from the clients.

Now jack_main calls jack_engine_load_driver (jackd/engine.c). The function loads the driver from disk via jack_load_driver (jackd/engine.c). Afterwards jack_engine_load_driver creates a driver client (for the sinks and sources of the driver): jack_create_driver_client (jackd/clientengine.c). It then calls initialize on the drivers info structure, which lets the driver return its real driver structure jack_driver_t. jack_use_driver puts this driver structure into the engine (engine->driver) and attaches the engine to the driver. During attachment, the driver normally registers the ports for its driver client driver->client and activates the client.

A driver can either be threaded or non-threaded: jack_driver_t or jack_driver_nt_t. Many drivers have their own driver data structure; in case of ALSA this is alsa_driver_t. A threaded drivers structure has to have all the elements of jack_driver_t at its beginning (a non-threaded driver them of jack_driver_nt_t) to allow casting it to jack_driver_t (or jack_driver_nt_t). The structure jack_driver_t in turn is a subset of jack_driver_nt_t, so you can cast any jack_driver_nt_t to jack_driver_t. To make programming easier, the elements of jack_driver_t and jack_driver_nt_t are not directly coded into the structures but via the macros JACK_DRIVER_DECL and JACK_DRIVER_NT_DECL, which also should be used for the own driver structures. The difference between threaded and non-threaded drivers is that threaded drivers have to bring up a thread, which executes the driver loop

      while (...)
        { driver->wait (); driver->engine->run_cycle () }

on their own. Whereas a non-threaded driver normally calls jack_driver_nt_init (libjack/driver.c) during driver_initialize, which sets jack_driver_nt_start as the driver starting function driver->start. In jack_driver_nt_thread the driver loop

      while (...)
        { driver->nt_run_cycle() }

is implemented. Hence, the waiting has to be done inside of nt_run_cycle().

In any case, the driver is responsible for initiating a new engine cycle, which is executed in the context of the driver. The function engine->run_cycle (set to jack_run_cycle, jackd/engine.c) is the callback of the engine---or in other words: it is the entry point into the engine from the driver. The important lines of jack_run_cycle are

      for (left = nframes; left >= b_size; left -= b_size) 
         jack_run_one_cycle (engine, b_size, delayed_usecs)

The function jack_run_one_cycle essentially does the following:

  1. driver->read (driver, nframes)
  2. jack_engine_process (engine, nframes)
  3. driver->write (driver, nframes)

The function jack_engine_process iterates over all the clients in the list engine->clients and, as long as the client is active and not dead, it executes jack_process_internal (jackd/engine.c) if the client is internal, and jack_process_external (jackd/engine.c) otherwise.