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209 lines
7 KiB
Text
209 lines
7 KiB
Text
NB: This document does not represent the current state of CVS but my current plans on how to implement this.
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Basics
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======
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Hierarchy
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---------
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GstData
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GstInstream
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GstBuffer
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GstEventNewMedia
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GstEventDiscontinuous
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GstEventEOS
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GstEventLength
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GstOutOfStream
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GstEventLock
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GstEventUnLock
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GstEventSeek
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GstEventFlush
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GstEventEmpty
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GstData
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=======
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typedef GstData * (*GstDataCopyFunction) (GstData *data);
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typedef void (*GstDataFreeFunction) (GstData *data);
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struct _GstData
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{
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/* is a */
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GstDataClass * klass;
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/* refcounting */
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#ifdef HAVE_ATOMIC_H
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atomic_t refcount;
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#else
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gint refcount;
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GMutex * reflock;
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#endif
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/* flags */
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guint flags;
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};
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struct _GstDataClass
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{
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GstDataType type;
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GstDataCopyFunction copy;
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GstDataFreeFunction free;
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};
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Inheritance
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-----------
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A GstData descandant GstMyData, would look like this:
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struct _GstMyData {
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GstData parent;
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/* more stuff specific to GstMyData */
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}
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You can even enhance the class struct, if you want to. This works just like inheritance in GLib.
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If it can be a parent class, it should implement these three functions publically:
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void gst_my_data_init (GstMyData *data) {
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/* call the parent's init function, eg: */
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gst_data_init (GST_DATA (data));
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/* initialize your data now */
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}
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void gst_my_data_dispose (GstMyData *data) {
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/* free every data, that needs to be freed */
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/* call the parent's dispose function, eg: */
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gst_data_dispose (GST_DATA (data));
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/* do NOT free the data */
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}
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GstData *gst_my_data_do_copy (GstMyData *to, GstMyData *from) {
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/* call the parent's copy function, eg: */
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gst_data_do_copy (GST_DATA (to), GST_DATA (from));
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/* copy relevant stuff from your struct now */
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/* note: the copy function must return a writable object, you may not set GST_DATA_READONLY here */
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}
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If GstMyData should be instantiable, you should do this:
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Get a class struct, something like this:
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static GstDataClass my_data_class = { GST_TYPE_MY_DATA,
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gst_my_data_copy_func,
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gst_my_data_free_func };
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FIXME: At the moment all types need to be specified in a big enum in gstdata.h.
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We might want to change that when we support event plugins.
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The two functions above should look something like this:
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GstData *gst_my_data_copy_func (GstData *from) {
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/* allocate memory */
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GstMyData *copy = g_new (GstMyData, 1);
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/* copy relevant variables or initialize them */
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gst_my_data_copy (copy, GST_MY_DATA (from));
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return copy;
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}
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void gst_my_data_free_func (GstData *data) {
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/* first dispose all data */
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gst_my_data_dispose (GST_MY_DATA (data));
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/* now free the struct */
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g_free (data);
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}
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Now you just need a function that can be called from the real world:
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GstMyData *gst_my_data_new (void) {
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/* allocate memory */
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GstMyData *my_data = g_new (GstMyData, 1);
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/* initialize the variables */
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gst_my_data_init (my_data);
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/* set the right type */
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GST_DATA (my_data)->type = &my_data_class;
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return my_data;
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}
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summary of inheritance:
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- define structs like GObject inheritance
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- inheritance works by calling the functions of the parent when creating/copying/freeing an object
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- type recognition is done by the type field in the class struct
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- memory allocation is specific to the struct.
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Refcounting
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-----------
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GstData provides threadsafe refcounting. If you create a new object - either by copying or explicit creation - the refcount is initialized to 1.
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This reference is owned by the creator of the object.
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If the reference count reaches 0, the object is freed. The free function of the class is called for that purpose.
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In accordance with GLib, that uses g_object_(un)ref for everything, gst_data_(un)ref is used and no wrapper macros are created.
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MT safety
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---------
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Manipulating data inside an object is not threadsafe unless otherwise noted.
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If an object has a reference count of 1 it is assumed that the reference holder is the only user of that object and he may modify it the way he likes.
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If the reference count is greater than 1, the object may not be modified. If you need to modify it, you have to copy the object and use that copy instead.
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NB: Object creation and refcounting are threadsafe - or must be implemented that way.
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Flags
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-----
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Flags work and can be used like the GstObject flags.
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GstData defines only one flag: GST_DATA_READONLY. If this flag is set, you are not allowed to modify the contents of a struct, even if the refcount is 1.
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GBoxed
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------
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GstInstream
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===========
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GstInstream is the base class for events and buffers that are passed inside the stream.
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It enhances the GstData struct by
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guint64 offset[GST_OFFSET_TYPES];
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This field describes the offset in the current stream in various different ways:
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GST_OFFSET_BYTES: The number of bytes from the beginning of the stream.
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GST_OFFSET_TIME: The timestamp in microseconds. The beginning of the stream equals timestamp 0. In buffers the timestamp should match the beginning of the data.
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GST_OFFSET_FRAMES: This type is specific to the stream and should be defined there. (video will probably use it for frame numbers, audio to count samples)
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If an offset can't be specified, it is set to GST_OFFSET_INVALID, which equals (guint64) -1. The byte offset always has to be specified. It is an error if it is invalid.
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A plugin playing data from an "infinite" source (eg a shoutcast stream from the internet) it should start with byteoffset 0.
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GstBuffer
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=========
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The buffer enhances the GstInstream struct by including a data and a size field.
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Memory allocation
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-----------------
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Memory is allocated via a special memchunk implementation, that is threadsafe. The default implementation uses a mutex and GMemChunks.
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FIXME: This is not true, we use g_malloc/g_free for now.
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GstBufferClass
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--------------
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GstBufferClasses (note the plural) replace bufferpools. The default class uses g_free/g_malloc for storing data. However, you are free to write your own if
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you need buffers that store data in another way.
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Note however that the copy function needs to supply a writable copy of your buffer.
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Subbuffers
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----------
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Subbuffers have been replaced by a special kind of GstBufferClass.
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Instream events
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===============
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GstEventNewMedia
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----------------
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Signals the start of a new stream. This must be send before any buffer of a new stream can be send.
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FIXME: The "must be send" can only be enforced if all elements are event aware. And this is necessary if we don't want to get parts of stream 1 inside stream 2.
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GstEventDiscontinuous
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---------------------
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This must be send between buffers, that don't have continuous data. This is necessary for example after seeking or when data is dropped for speed.
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GstEventEOS
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-----------
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Signals the end of a stream. Must be send after all data is finished. If you want to start a new stream, don't send this event, send a GstEventNewMedia instead.
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After having processed this event, elements in the pipeline switch their state to paused.
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GstEventLength
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--------------
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Specifies the length of the stream.
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FIXME: Write more, when sure how to do this.
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Upstream events
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===============
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These will be discussed in a seperate doc.
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