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71 lines
2.4 KiB
Markdown
71 lines
2.4 KiB
Markdown
# Query
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## Purpose
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Queries are used to get information about the stream. A query is started
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on a specific pad and travels up or downstream.
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## Requirements
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- multiple return values, grouped together when they make sense.
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- one pad function to perform the query
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- extensible queries.
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## Implementation
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- `GstQuery` extends `GstMiniObject` and contains a `GstStructure` (see
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`GstMessage`)
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- some standard query types are defined below
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- methods to create and parse the results in the `GstQuery`.
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- define pad
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method:
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``` c
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gboolean (*GstPadQueryFunction) (GstPad *pad,
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GstObject *parent,
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GstQuery *query);
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```
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pad returns result in query structure and TRUE as result or FALSE when query is
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not supported.
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## Query types
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**`GST_QUERY_POSITION`**: get info on current position of the stream in `stream_time`.
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**`GST_QUERY_DURATION`**: get info on the total duration of the stream.
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**`GST_QUERY_LATENCY`**: get amount of latency introduced in the pipeline. (See [latency](design/latency.md))
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**`GST_QUERY_RATE`**: get the current playback rate of the pipeline
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**`GST_QUERY_SEEKING`**: get info on how seeking can be done
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- getrange, with/without offset/size
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- ranges where seeking is efficient (for caching network sources)
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- flags describing seeking behaviour (forward, backward, segments,
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play backwards, ...)
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**`GST_QUERY_SEGMENT`**: get info about the currently configured playback segment.
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**`GST_QUERY_CONVERT`**: convert format/value to another format/value pair.
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**`GST_QUERY_FORMATS`**: return list of supported formats that can be used for `GST_QUERY_CONVERT`.
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**`GST_QUERY_BUFFERING`**: query available media for efficient seeking (See [buffering](design/buffering.md))
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**`GST_QUERY_CUSTOM`**: a custom query, the name of the query defines the properties of the query.
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**`GST_QUERY_URI`**: query the uri of the source or sink element
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**`GST_QUERY_ALLOCATION`**: the buffer allocation properties (See [bufferpool](design/bufferpool.md))
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**`GST_QUERY_SCHEDULING`**: the scheduling properties (See [scheduling](design/scheduling.md))
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**`GST_QUERY_ACCEPT_CAPS`**: check if caps are supported (See [negotiation](design/negotiation.md))
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**`GST_QUERY_CAPS`**: get the possible caps (See [negotiation](design/negotiation.md))
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