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Functions
| GstAdapter * | gst_adapter_new () |
| void | gst_adapter_clear () |
| void | gst_adapter_push () |
| gconstpointer | gst_adapter_map () |
| void | gst_adapter_unmap () |
| void | gst_adapter_copy () |
| void | gst_adapter_flush () |
| gsize | gst_adapter_available () |
| gsize | gst_adapter_available_fast () |
| gpointer | gst_adapter_take () |
| GstBuffer * | gst_adapter_take_buffer () |
| GList * | gst_adapter_take_list () |
| GstClockTime | gst_adapter_prev_pts () |
| GstClockTime | gst_adapter_prev_dts () |
| gssize | gst_adapter_masked_scan_uint32 () |
| gssize | gst_adapter_masked_scan_uint32_peek () |
Description
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_map()/gst_adapter_unmap()
and/or gst_adapter_copy(). After the data has been processed, it is freed
using gst_adapter_unmap().
Other methods such as gst_adapter_take() and gst_adapter_take_buffer()
combine gst_adapter_map() and gst_adapter_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:
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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 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.
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 gst_adapter_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
gst_adapter_prev_pts_at_offset() can be used to determine the last
seen timestamp at a particular offset in the adapter.
A last thing to note is that while GstAdapter 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 gst_adapter_available_fast() are provided to help
speed up such cases should you want to. To avoid repeated memory allocations,
gst_adapter_copy() can be used to copy data into a (statically allocated)
user provided buffer.
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 2009-05-13 (0.10.24).
Functions
gst_adapter_new ()
GstAdapter *
gst_adapter_new (void);
Creates a new GstAdapter. Free with g_object_unref().
gst_adapter_clear ()
void
gst_adapter_clear (GstAdapter *adapter);
Removes all buffers from adapter
.
gst_adapter_push ()
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.
gst_adapter_map ()
gconstpointer gst_adapter_map (GstAdapter *adapter,gsize 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 memory or the potentially more performant
gst_adapter_take_buffer().
Returns NULL if size
bytes are not available.
gst_adapter_unmap ()
void
gst_adapter_unmap (GstAdapter *adapter);
Releases the memory obtained with the last gst_adapter_map().
gst_adapter_copy ()
void gst_adapter_copy (GstAdapter *adapter,gpointer dest,gsize offset,gsize 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.
gst_adapter_flush ()
void gst_adapter_flush (GstAdapter *adapter,gsize 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_map(), gst_adapter_unmap()
gst_adapter_available ()
gsize
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_map() without that function
returning NULL.
gst_adapter_available_fast ()
gsize
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).
gst_adapter_take ()
gpointer gst_adapter_take (GstAdapter *adapter,gsize 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.
Free-function: g_free
Returns
oven-fresh hot data, or NULL if nbytes
bytes are not available.
[transfer full][array length=nbytes][element-type guint8]
gst_adapter_take_buffer ()
GstBuffer * gst_adapter_take_buffer (GstAdapter *adapter,gsize 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. This function will always return a
buffer with a single memory region.
Note that no assumptions should be made as to whether certain buffer flags such as the DISCONT flag are set on the returned buffer, or not. The caller needs to explicitly set or unset flags that should be set or unset.
Caller owns a reference to the returned buffer. gst_buffer_unref() after
usage.
Free-function: gst_buffer_unref
Returns
a GstBuffer containing the first nbytes
of
the adapter, or NULL if nbytes
bytes are not available.
gst_buffer_unref() when no longer needed.
[transfer full]
gst_adapter_take_list ()
GList * gst_adapter_take_list (GstAdapter *adapter,gsize nbytes);
Returns a GList of buffers containing the first nbytes
bytes of the
adapter
. The returned bytes will be flushed from the adapter.
When the caller can deal with individual buffers, this function is more
performant because no memory should be copied.
Caller owns returned list and contained buffers. gst_buffer_unref() each
buffer in the list before freeing the list after usage.
gst_adapter_prev_pts ()
GstClockTime gst_adapter_prev_pts (GstAdapter *adapter,guint64 *distance);
Get the pts that was before the current byte in the adapter. When
distance
is given, the amount of bytes between the pts and the current
position is returned.
The pts is reset to GST_CLOCK_TIME_NONE and the distance is set to 0 when the adapter is first created or when it is cleared. This also means that before the first byte with a pts is removed from the adapter, the pts and distance returned are GST_CLOCK_TIME_NONE and 0 respectively.
gst_adapter_prev_dts ()
GstClockTime gst_adapter_prev_dts (GstAdapter *adapter,guint64 *distance);
Get the dts that was before the current byte in the adapter. When
distance
is given, the amount of bytes between the dts and the current
position is returned.
The dts is reset to GST_CLOCK_TIME_NONE and the distance is set to 0 when the adapter is first created or when it is cleared. This also means that before the first byte with a dts is removed from the adapter, the dts and distance returned are GST_CLOCK_TIME_NONE and 0 respectively.
gst_adapter_masked_scan_uint32 ()
gssize gst_adapter_masked_scan_uint32 (GstAdapter *adapter,guint32 mask,guint32 pattern,gsize offset,gsize size);
Scan for pattern pattern
with applied mask mask
in the adapter data,
starting from offset offset
.
The bytes in pattern
and mask
are interpreted left-to-right, regardless
of endianness. All four bytes of the pattern must be present in the
adapter for it to match, even if the first or last bytes are masked out.
It is an error to call this function without making sure that there is enough data (offset+size bytes) in the adapter.
This function calls gst_adapter_masked_scan_uint32_peek() passing NULL
for value.
Parameters
adapter |
||
mask |
mask to apply to data before matching against |
|
pattern |
pattern to match (after mask is applied) |
|
offset |
offset into the adapter data from which to start scanning, returns the last scanned position. |
|
size |
number of bytes to scan from offset |
Returns
offset of the first match, or -1 if no match was found.
Example:
// Assume the adapter contains 0x00 0x01 0x02 ... 0xfe 0xff gst_adapter_masked_scan_uint32 (adapter, 0xffffffff, 0x00010203, 0, 256); // -> returns 0 gst_adapter_masked_scan_uint32 (adapter, 0xffffffff, 0x00010203, 1, 255); // -> returns -1 gst_adapter_masked_scan_uint32 (adapter, 0xffffffff, 0x01020304, 1, 255); // -> returns 1 gst_adapter_masked_scan_uint32 (adapter, 0xffff, 0x0001, 0, 256); // -> returns -1 gst_adapter_masked_scan_uint32 (adapter, 0xffff, 0x0203, 0, 256); // -> returns 0 gst_adapter_masked_scan_uint32 (adapter, 0xffff0000, 0x02030000, 0, 256); // -> returns 2 gst_adapter_masked_scan_uint32 (adapter, 0xffff0000, 0x02030000, 0, 4); // -> returns -1
gst_adapter_masked_scan_uint32_peek ()
gssize gst_adapter_masked_scan_uint32_peek (GstAdapter *adapter,guint32 mask,guint32 pattern,gsize offset,gsize size,guint32 *value);
Scan for pattern pattern
with applied mask mask
in the adapter data,
starting from offset offset
. If a match is found, the value that matched
is returned through value
, otherwise value
is left untouched.
The bytes in pattern
and mask
are interpreted left-to-right, regardless
of endianness. All four bytes of the pattern must be present in the
adapter for it to match, even if the first or last bytes are masked out.
It is an error to call this function without making sure that there is enough data (offset+size bytes) in the adapter.
Parameters
adapter |
||
mask |
mask to apply to data before matching against |
|
pattern |
pattern to match (after mask is applied) |
|
offset |
offset into the adapter data from which to start scanning, returns the last scanned position. |
|
size |
number of bytes to scan from offset |
|
value |
pointer to uint32 to return matching data |