GstAdapter: GStreamer 1.0 Library Reference Manual

GstAdapter

GstAdapter — adapts incoming data on a sink pad into chunks of N bytes

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 ()
GBytes *	gst_adapter_copy_bytes ()
void	gst_adapter_flush ()
gsize	gst_adapter_available ()
gsize	gst_adapter_available_fast ()
gpointer	gst_adapter_take ()
GstBuffer *	gst_adapter_take_buffer ()
GstBuffer *	gst_adapter_take_buffer_fast ()
GList *	gst_adapter_take_list ()
GstBufferList *	gst_adapter_take_buffer_list ()
GstClockTime	gst_adapter_prev_pts ()
GstClockTime	gst_adapter_prev_dts ()
GstClockTime	gst_adapter_prev_pts_at_offset ()
GstClockTime	gst_adapter_prev_dts_at_offset ()
gssize	gst_adapter_masked_scan_uint32 ()
gssize	gst_adapter_masked_scan_uint32_peek ()

Types and Values

GstAdapter

Object Hierarchy

    GObject
    ╰── GstAdapter

Includes

#include <gst/base/gstadapter.h>

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:

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.

Functions

gst_adapter_new ()

GstAdapter *
gst_adapter_new (void);

Creates a new GstAdapter. Free with g_object_unref().

Returns

a new GstAdapter.

[transfer full]

gst_adapter_clear ()

void
gst_adapter_clear (GstAdapter *adapter);

Removes all buffers from adapter .

Parameters

adapter

a GstAdapter

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.

Parameters

adapter	a GstAdapter
buf	a GstBuffer to add to queue in the adapter.	[transfer full]

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.

Parameters

adapter	a GstAdapter
size	the number of bytes to map/peek

Returns

a pointer to the first size bytes of data, or NULL.

[transfer none][array length=size][element-type guint8][nullable]

gst_adapter_unmap ()

void
gst_adapter_unmap (GstAdapter *adapter);

Releases the memory obtained with the last gst_adapter_map().

Parameters

adapter

a GstAdapter

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.

[skip]

Parameters

adapter	a GstAdapter
dest	the memory to copy into.	[out caller-allocates][array length=size][element-type guint8]
offset	the bytes offset in the adapter to start from
size	the number of bytes to copy

gst_adapter_copy_bytes ()

GBytes *
gst_adapter_copy_bytes (GstAdapter *adapter,
                        gsize offset,
                        gsize size);

Similar to gst_adapter_copy, but more suitable for language bindings. size bytes of data starting at offset will be copied out of the buffers contained in adapter and into a new GBytes structure which is returned. Depending on the value of the size argument an empty GBytes structure may be returned.

[rename-to gst_adapter_copy]

Parameters

adapter	a GstAdapter
offset	the bytes offset in the adapter to start from
size	the number of bytes to copy

Returns

A new GBytes structure containing the copied data.

[transfer full]

Since: 1.4

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.

Parameters

adapter	a GstAdapter
flush	the number of bytes to flush

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.

Parameters

adapter

a GstAdapter

Returns

number of bytes available in adapter

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).

Parameters

adapter

a GstAdapter

Returns

number of bytes that are available in adapter without expensive operations

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

Parameters

adapter	a GstAdapter
nbytes	the number of bytes to take

Returns

oven-fresh hot data, or NULL if nbytes bytes are not available.

[transfer full][array length=nbytes][element-type guint8][nullable]

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

Parameters

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. gst_buffer_unref() when no longer needed.

[transfer full][nullable]

gst_adapter_take_buffer_fast ()

GstBuffer *
gst_adapter_take_buffer_fast (GstAdapter *adapter,
                              gsize nbytes);

Returns a GstBuffer containing the first nbytes of the adapter . The returned bytes will be flushed from the adapter. This function is potentially more performant than gst_adapter_take_buffer() since it can reuse the memory in pushed buffers by subbuffering or merging. Unlike gst_adapter_take_buffer(), the returned buffer may be composed of multiple non-contiguous GstMemory objects, no copies are made.

This function can return buffer up to the return value of gst_adapter_available() without making copies if possible.

Caller owns a reference to the returned buffer. gst_buffer_unref() after usage.

Free-function: gst_buffer_unref

Parameters

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. gst_buffer_unref() when no longer needed.

[transfer full][nullable]

Since: 1.2

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.

Parameters

adapter	a GstAdapter
nbytes	the number of bytes to take

Returns

a GList of buffers containing the first nbytes of the adapter, or NULL if nbytes bytes are not available.

[element-type Gst.Buffer][transfer full][nullable]

gst_adapter_take_buffer_list ()

GstBufferList *
gst_adapter_take_buffer_list (GstAdapter *adapter,
                              gsize nbytes);

Returns a GstBufferList 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 the returned list. Call gst_buffer_list_unref() to free the list after usage.

Parameters

adapter	a GstAdapter
nbytes	the number of bytes to take

Returns

a GstBufferList of buffers containing the first nbytes of the adapter, or NULL if nbytes bytes are not available.

[transfer full][nullable]

Since: 1.6

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.

Parameters

adapter	a GstAdapter
distance	pointer to location for distance, or `NULL`.	[out][allow-none]

Returns

The previously seen pts.

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.

Parameters

adapter	a GstAdapter
distance	pointer to location for distance, or `NULL`.	[out][allow-none]

Returns

The previously seen dts.

gst_adapter_prev_pts_at_offset ()

GstClockTime
gst_adapter_prev_pts_at_offset (GstAdapter *adapter,
                                gsize offset,
                                guint64 *distance);

Get the pts that was before the byte at offset offset in the adapter. When distance is given, the amount of bytes between the pts and the current position is returned.

Parameters

adapter	a GstAdapter
offset	the offset in the adapter at which to get timestamp
distance	pointer to location for distance, or `NULL`.	[out][allow-none]

Returns

The previously seen pts at given offset.

Since: 1.2

gst_adapter_prev_dts_at_offset ()

GstClockTime
gst_adapter_prev_dts_at_offset (GstAdapter *adapter,
                                gsize offset,
                                guint64 *distance);

Get the dts that was before the byte at offset offset in the adapter. When distance is given, the amount of bytes between the dts and the current position is returned.

Parameters

adapter	a GstAdapter
offset	the offset in the adapter at which to get timestamp
distance	pointer to location for distance, or `NULL`.	[out][allow-none]

Returns

The previously seen dts at given offset.

Since: 1.2

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	a GstAdapter
mask	mask to apply to data before matching against `pattern`
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.

It is an error to call this function without making sure that there is enough data (offset+size bytes) in the adapter.

Parameters

adapter	a GstAdapter
mask	mask to apply to data before matching against `pattern`
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.	[out][allow-none]

Returns

offset of the first match, or -1 if no match was found.

Types and Values

GstAdapter

typedef struct _GstAdapter GstAdapter;

The opaque GstAdapter data structure.