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GStreamer 0.10 Library Reference Manual | |
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#include <gst/base/gstadapter.h>
GstAdapter;
GstAdapter* gst_adapter_new (void);
void gst_adapter_clear (GstAdapter *adapter);
void gst_adapter_push (GstAdapter *adapter,
GstBuffer *buf);
const guint8* gst_adapter_peek (GstAdapter *adapter,
guint size);
void gst_adapter_copy (GstAdapter *adapter,
guint8 *dest,
guint offset,
guint size);
void gst_adapter_flush (GstAdapter *adapter,
guint flush);
guint gst_adapter_available (GstAdapter *adapter);
guint gst_adapter_available_fast (GstAdapter *adapter);
guint8* gst_adapter_take (GstAdapter *adapter,
guint nbytes);
GstBuffer* gst_adapter_take_buffer (GstAdapter *adapter,
guint nbytes);
This class is for elements that receive buffers in an undesired size. While for example raw video contains one image per buffer, the same is not true for a lot of other formats, especially those that come directly from a file. So if you have undefined buffer sizes and require a specific size, this object is for you.
An adapter is created with gst_adapter_new(). It can be freed again with
g_object_unref().
The theory of operation is like this: All buffers received are put
into the adapter using gst_adapter_push() and the data is then read back
in chunks of the desired size using gst_adapter_peek(). After the data is
processed, it is freed using gst_adapter_flush().
For example, a sink pad's chain function that needs to pass data to a library in 512-byte chunks could be implemented like this:
static GstFlowReturn
sink_pad_chain (GstPad *pad, GstBuffer *buffer)
{
MyElement *this;
GstAdapter *adapter;
GstFlowReturn ret = GST_FLOW_OK;
// will give the element an extra ref; remember to drop it
this = MY_ELEMENT (gst_pad_get_parent (pad));
adapter = this->adapter;
// put buffer into adapter
gst_adapter_push (adapter, buffer);
// while we can read out 512 bytes, process them
while (gst_adapter_available (adapter) >= 512 && ret == GST_FLOW_OK) {
// use flowreturn as an error value
ret = my_library_foo (gst_adapter_peek (adapter, 512));
gst_adapter_flush (adapter, 512);
}
gst_object_unref (this);
return ret;
}
For another example, a simple element inside GStreamer that uses GstAdapter is the libvisual element.
An element using GstAdapter in its sink pad chain function should ensure that
when the FLUSH_STOP event is received, that any queued data is cleared using
gst_adapter_clear(). Data should also be cleared or processed on EOS and
when changing state from GST_STATE_PAUSED to GST_STATE_READY.
Also check the GST_BUFFER_FLAG_DISCONT flag on the buffer. Some elements might need to clear the adapter after a discontinuity.
A last thing to note is that while GstAdapter is pretty optimized,
merging buffers still might be an operation that requires a memcpy()
operation, and this operation is not the fastest. Because of this, some
functions like gst_adapter_available_fast() are provided to help speed up
such cases should you want to.
GstAdapter is not MT safe. All operations on an adapter must be serialized by the caller. This is not normally a problem, however, as the normal use case of GstAdapter is inside one pad's chain function, in which case access is serialized via the pad's STREAM_LOCK.
Note that gst_adapter_push() takes ownership of the buffer passed. Use
gst_buffer_ref() before pushing it into the adapter if you still want to
access the buffer later. The adapter will never modify the data in the
buffer pushed in it.
Last reviewed on 2006-04-04 (0.10.6).
GstAdapter* gst_adapter_new (void);
Creates a new GstAdapter. Free with g_object_unref().
| Returns : | a new GstAdapter |
void gst_adapter_clear (GstAdapter *adapter);
Removes all buffers from adapter.
adapter : |
a GstAdapter |
void gst_adapter_push (GstAdapter *adapter, GstBuffer *buf);
Adds the data from buf to the data stored inside adapter and takes
ownership of the buffer.
adapter : |
a GstAdapter |
buf : |
a GstBuffer to add to queue in the adapter |
const guint8* gst_adapter_peek (GstAdapter *adapter, guint size);
Gets the first size bytes stored in the adapter. The returned pointer is
valid until the next function is called on the adapter.
Note that setting the returned pointer as the data of a GstBuffer is
incorrect for general-purpose plugins. The reason is that if a downstream
element stores the buffer so that it has access to it outside of the bounds
of its chain function, the buffer will have an invalid data pointer after
your element flushes the bytes. In that case you should use
gst_adapter_take(), which returns a freshly-allocated buffer that you can set
as GstBuffer malloc_data or the potentially more performant
gst_adapter_take_buffer().
Returns NULL if size bytes are not available.
adapter : |
a GstAdapter |
size : |
the number of bytes to peek |
| Returns : | a pointer to the first size bytes of data, or NULL.
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void gst_adapter_copy (GstAdapter *adapter, guint8 *dest, guint offset, guint size);
Copies size bytes of data starting at offset out of the buffers
contained in GstAdapter into an array dest provided by the caller.
The array dest should be large enough to contain size bytes.
The user should check that the adapter has (offset + size) bytes
available before calling this function.
adapter : |
a GstAdapter |
dest : |
the memory where to copy to |
offset : |
the bytes offset in the adapter to start from |
size : |
the number of bytes to copy |
Since 0.10.12
void gst_adapter_flush (GstAdapter *adapter, guint flush);
Flushes the first flush bytes in the adapter. The caller must ensure that
at least this many bytes are available.
See also: gst_adapter_peek().
adapter : |
a GstAdapter |
flush : |
the number of bytes to flush |
guint gst_adapter_available (GstAdapter *adapter);
Gets the maximum amount of bytes available, that is it returns the maximum
value that can be supplied to gst_adapter_peek() without that function
returning NULL.
adapter : |
a GstAdapter |
| Returns : | number of bytes available in adapter
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guint gst_adapter_available_fast (GstAdapter *adapter);
Gets the maximum number of bytes that are immediately available without requiring any expensive operations (like copying the data into a temporary buffer).
adapter : |
a GstAdapter |
| Returns : | number of bytes that are available in adapter without expensive
operations
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guint8* gst_adapter_take (GstAdapter *adapter, guint nbytes);
Returns a freshly allocated buffer containing the first nbytes bytes of the
adapter. The returned bytes will be flushed from the adapter.
Caller owns returned value. g_free after usage.
adapter : |
a GstAdapter |
nbytes : |
the number of bytes to take |
| Returns : | oven-fresh hot data, or NULL if nbytes bytes are not available
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GstBuffer* gst_adapter_take_buffer (GstAdapter *adapter, guint nbytes);
Returns a GstBuffer containing the first nbytes bytes of the
adapter. The returned bytes will be flushed from the adapter.
This function is potentially more performant than gst_adapter_take()
since it can reuse the memory in pushed buffers by subbuffering
or merging.
Caller owns returned value. gst_buffer_unref() after usage.
adapter : |
a GstAdapter |
nbytes : |
the number of bytes to take |
| Returns : | a GstBuffer containing the first nbytes of the adapter,
or NULL if nbytes bytes are not available
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Since 0.10.6