GstAdapter

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

Synopsis


#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_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);


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_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).

Details

GstAdapter

typedef struct {
  GObject	object;
} GstAdapter;

The opaque GstAdapter data structure.

GObject object; the parent object.

gst_adapter_new ()

GstAdapter* gst_adapter_new                 (void);

Creates a new GstAdapter. Free with g_object_unref().

Returns : a new GstAdapter

gst_adapter_clear ()

void        gst_adapter_clear               (GstAdapter *adapter);

Removes all buffers from adapter.

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.

adapter : a GstAdapter
buf : a GstBuffer to add to queue in the adapter

gst_adapter_peek ()

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.

gst_adapter_flush ()

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

gst_adapter_available ()

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

gst_adapter_available_fast ()

guint       gst_adapter_available_fast      (GstAdapter *adapter);

Gets the maximum number of bytes available without the need to do expensive operations (like copying the data into a temporary buffer).

adapter : a GstAdapter
Returns : number of bytes available in adapter without expensive operations

gst_adapter_take ()

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

gst_adapter_take_buffer ()

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 that gst_adapter_take() since it can reuse the memory in the pushed buffer 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

Since 0.10.6