## 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_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);

## Object Hierarchy

      GObject
       +----GstAdapter

## 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 _GstAdapter GstAdapter;

The opaque GstAdapter data structure.

### 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_copy ()

    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

### 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 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

### 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 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

Since 0.10.6