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 Adapter. It can be freed again with
unref.
The theory of operation is like this: All buffers received are put into the adapter using
push and the data is then read back in chunks of the desired size using
map/gst_adapter_unmap() and/or
copy. After the data has been processed, it is freed using unmap.
Other methods such as take and
take_buffer combine map
and unmap in one method and are potentially more convenient for some use
cases.
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, GstObject *parent, GstBuffer *buffer)
{
MyElement *this;
GstAdapter *adapter;
GstFlowReturn ret = GST_FLOW_OK;
this = MY_ELEMENT (parent);
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) {
const guint8 *data = gst_adapter_map (adapter, 512);
// use flowreturn as an error value
ret = my_library_foo (data);
gst_adapter_unmap (adapter);
gst_adapter_flush (adapter, 512);
}
return ret;
}
For another example, a simple element inside GStreamer that uses Adapter is the libvisual element.
An element using Adapter in its sink pad chain function should ensure that when the FLUSH_STOP event is
received, that any queued data is cleared using clear. Data should also be
cleared or processed on EOS and when changing state from PAUSED to READY.
Also check the GST_BUFFER_FLAG_DISCONT flag on the buffer. Some elements might need to clear the adapter after a discontinuity.
The adapter will keep track of the timestamps of the buffers that were pushed. The last seen timestamp before the current position can be
queried with prev_pts. This function can optionally return the number of bytes
between the start of the buffer that carried the timestamp and the current adapter position. The distance is useful when dealing with, for
example, raw audio samples because it allows you to calculate the timestamp of the current adapter position by using the last seen
timestamp and the amount of bytes since. Additionally, the
prev_pts_at_offset can be used to determine the last seen timestamp at a particular offset in the adapter.
The adapter will also keep track of the offset of the buffers (#GST_BUFFER_OFFSET) that were pushed. The last seen offset before the
current position can be queried with prev_offset. This function can
optionally return the number of bytes between the start of the buffer that carried the offset and the current adapter position.
Additionally the adapter also keeps track of the PTS, DTS and buffer offset at the last discontinuity, which can be retrieved with
pts_at_discont,
dts_at_discont and
offset_at_discont. The number of bytes that were consumed since then
can be queried with distance_from_discont.
A last thing to note is that while Adapter is pretty optimized, merging buffers still might be an operation
that requires a `malloc()` and `memcpy()` operation, and these operations are not the fastest. Because of this, some functions like
available_fast are provided to help speed up such cases should you want
to. To avoid repeated memory allocations, copy can be used to copy data into a
(statically allocated) user provided buffer.
Adapter 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 Adapter is inside one pad's chain function, in which case access is
serialized via the pad's STREAM_LOCK.
Note that 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.
- public size_t available ()
Gets the maximum amount of bytes available, that is it returns the
maximum value that can be supplied to map without that function returning
null.
- public size_t available_fast ()
Gets the maximum number of bytes that are immediately available
without requiring any expensive operations (like copying the data into a temporary buffer).
- public void clear ()
Removes all buffers from this.
- public void copy (uint8[] dest, size_t offset)
Copies dest.length bytes of data starting at offset
out of the buffers contained in Adapter into an array dest provided by the caller.
- public Bytes copy_bytes (size_t offset, size_t size)
Similar to gst_adapter_copy, but more suitable for language bindings.
- public uint64 distance_from_discont ()
Get the distance in bytes since the last buffer with the
DISCONT flag.
- public ClockTime dts_at_discont ()
Get the DTS that was on the last buffer with the
GST_BUFFER_FLAG_DISCONT flag, or GST_CLOCK_TIME_NONE.
- public void flush (size_t flush)
Flushes the first flush bytes in the
this.
- public Buffer? get_buffer (size_t nbytes)
Returns a Buffer containing the first
nbytes of the this, but does not flush them from the adapter.
- public Buffer? get_buffer_fast (size_t nbytes)
Returns a Buffer containing the first
nbytes of the this, but does not flush them from the adapter.
- public BufferList? get_buffer_list (size_t nbytes)
Returns a BufferList of buffers containing
the first nbytes bytes of the this but does not flush them from the adapter.
- public List<Buffer>? get_list (size_t nbytes)
Returns a List of buffers containing the
first nbytes bytes of the this, but does not flush them from the adapter.
- public unowned uint8[]? map (size_t size)
Gets the first return.length
bytes stored in the this.
- public ssize_t masked_scan_uint32 (uint32 mask, uint32 pattern, size_t offset, size_t size)
Scan for pattern pattern with applied mask mask
in the adapter data, starting from offset offset.
- public ssize_t masked_scan_uint32_peek (uint32 mask, uint32 pattern, size_t offset, size_t size, out uint32 value)
Scan for pattern pattern with applied mask mask
in the adapter data, starting from offset offset.
- public uint64 offset_at_discont ()
Get the offset that was on the last buffer with the
GST_BUFFER_FLAG_DISCONT flag, or GST_BUFFER_OFFSET_NONE.
- public ClockTime prev_dts (out uint64 distance)
Get the dts that was before the current byte in the adapter.
- public ClockTime prev_dts_at_offset (size_t offset, out uint64 distance)
Get the dts that was before the byte at offset offset in
the adapter.
- public uint64 prev_offset (out uint64 distance)
Get the offset that was before the current byte in the adapter.
- public ClockTime prev_pts (out uint64 distance)
Get the pts that was before the current byte in the adapter.
- public ClockTime prev_pts_at_offset (size_t offset, out uint64 distance)
Get the pts that was before the byte at offset offset in
the adapter.
- public ClockTime pts_at_discont ()
Get the PTS that was on the last buffer with the
GST_BUFFER_FLAG_DISCONT flag, or GST_CLOCK_TIME_NONE.
- public void push (owned Buffer buf)
Adds the data from buf to the data stored inside
this and takes ownership of the buffer.
- public uint8[]? take (size_t nbytes)
Returns a freshly allocated buffer containing the first
return.length bytes of the this.
- public Buffer? take_buffer (size_t nbytes)
Returns a Buffer containing the first
nbytes bytes of the this.
- public Buffer? take_buffer_fast (size_t nbytes)
Returns a Buffer containing the first
nbytes of the this.
- public BufferList? take_buffer_list (size_t nbytes)
Returns a BufferList of buffers containing
the first nbytes bytes of the this.
- public List<Buffer>? take_list (size_t nbytes)
Returns a List of buffers containing the
first nbytes bytes of the this.
- public void unmap ()
Releases the memory obtained with the last
map.