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79 lines
2.8 KiB
Markdown
79 lines
2.8 KiB
Markdown
---
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title: Buffers and Events
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...
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# Buffers and Events
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The data flowing through a pipeline consists of a combination of buffers
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and events. Buffers contain the actual media data. Events contain
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control information, such as seeking information and end-of-stream
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notifiers. All this will flow through the pipeline automatically when
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it's running. This chapter is mostly meant to explain the concept to
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you; you don't need to do anything for this.
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## Buffers
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Buffers contain the data that will flow through the pipeline you have
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created. A source element will typically create a new buffer and pass it
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through a pad to the next element in the chain. When using the GStreamer
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infrastructure to create a media pipeline you will not have to deal with
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buffers yourself; the elements will do that for you.
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A buffer consists, amongst others, of:
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- Pointers to memory objects. Memory objects encapsulate a region in
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the memory.
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- A timestamp for the buffer.
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- A refcount that indicates how many elements are using this buffer.
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This refcount will be used to destroy the buffer when no element has
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a reference to it.
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- Buffer flags.
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The simple case is that a buffer is created, memory allocated, data put
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in it, and passed to the next element. That element reads the data, does
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something (like creating a new buffer and decoding into it), and
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unreferences the buffer. This causes the data to be free'ed and the
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buffer to be destroyed. A typical video or audio decoder works like
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this.
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There are more complex scenarios, though. Elements can modify buffers
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in-place, i.e. without allocating a new one. Elements can also write to
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hardware memory (such as from video-capture sources) or memory allocated
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from the X-server (using XShm). Buffers can be read-only, and so on.
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## Events
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Events are control particles that are sent both up- and downstream in a
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pipeline along with buffers. Downstream events notify fellow elements of
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stream states. Possible events include seeking, flushes, end-of-stream
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notifications and so on. Upstream events are used both in
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application-element interaction as well as element-element interaction
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to request changes in stream state, such as seeks. For applications,
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only upstream events are important. Downstream events are just explained
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to get a more complete picture of the data concept.
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Since most applications seek in time units, our example below does so
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too:
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``` c
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static void
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seek_to_time (GstElement *element,
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guint64 time_ns)
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{
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GstEvent *event;
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event = gst_event_new_seek (1.0, GST_FORMAT_TIME,
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GST_SEEK_FLAG_NONE,
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GST_SEEK_METHOD_SET, time_ns,
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GST_SEEK_TYPE_NONE, G_GUINT64_CONSTANT (0));
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gst_element_send_event (element, event);
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}
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```
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The function `gst_element_seek ()` is a shortcut for this. This is
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mostly just to show how it all works.
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