GstBuffer
---------

This document describes the design for buffers.

A GstBuffer is the object that is passed from an upstream element to a
downstream element and contains memory and metadata information.

Requirements
~~~~~~~~~~~~

 - It must be fast
    * allocation, free, low fragmentation
 - Must be able to attach multiple memory blocks to the buffer
 - Must be able to attach artibtrary metadata to buffers
 - efficient handling of subbuffer, copy, span, trim

Lifecycle
~~~~~~~~~

GstMemory extends from GstMiniObject and therefore uses its lifecycle
management (See part-miniobject.txt).

Writability
~~~~~~~~~~~

When a Buffers is writable as returned from gst_buffer_is_writable():

 - metadata can be added/removed and the metadata can be changed
 - GstMemory blocks can be added/removed

The individual memory blocks have their own locking and READONLY flags
that might influence their writability.

Buffers can be made writable with gst_buffer_make_writable(). This will copy the
buffer with the metadata and will ref the memory in the buffer. This means that
the memory is not automatically copied when copying buffers.


Managing GstMemory
------------------

A GstBuffer contains an array of pointers to GstMemory objects. 

When the buffer is writable, gst_buffer_insert_memory() can be used to add a
new GstMemory object to the buffer. When the array of memory is full, memory
will be merged to make room for the new memory object.

gst_buffer_n_memory() is used to get the amount of memory blocks on the
GstBuffer.

With gst_buffer_peek_memory(), memory can be retrieved from the memory array.
The desired access pattern for the memory block should be specified so that
appropriate checks can be made and, in case of GST_MAP_WRITE, a writable copy
can be constructed when needed.

gst_buffer_remove_memory_range() and gst_buffer_remove_memory() can be used to
remove memory from the GstBuffer.


Subbuffers
----------

Subbuffers are made by copying only a region of the memory blocks and copying
all of the metadata.


Span
----

Spanning will merge together the data of 2 buffers into a new buffer


Data access
-----------

 Accessing the data of the buffer can happen by retrieving the individual
 GstMemory objects in the GstBuffer or my using the gst_buffer_map() and
 gst_buffer_unmap() function.

 The _map and _unmap function will always return the memory of all blocks as one
 large contiguous region of memory. Using the _map and _unmap function might be
 more convenient than accessing the individual memory blocks at the expense of
 being more expensive because it might perform memcpy operations.

 For buffers with only one GstMemory object (the most common case), _map and
 _unmap have no performance penalty at all.


* Read access with 1 memory block

  The memory block is accessed and mapped for read access.
  The memory block is unmapped after usage

* write access with 1 memory block

  The buffer should be writable or this operation will fail..
  The memory block is accessed. If the memory block is readonly, a copy is made
  and the original memory block is replaced with this copy. then the memory
  block is mapped in write mode.
  The memory block is unmapped after usage.

* Read access with multiple memory blocks

  The memory blocks are combined into one large memory block. If the buffer is
  writable, The memory blocks are replace with this new memory block. If the
  buffer is not writable, the memory is returned as is.
  The memory block is then mapped in read mode.

  When the memory is unmapped after usage and the buffer has multiple memory
  blocks, this means that the map operation was not able to store the combined
  buffer and it thus returned memory that should be freed. Otherwise, the memory
  is unmapped.

* Write access with multiple memory blocks

  The buffer should be writable or the operation fails. The memory blocks are
  combined into one large memory block and the existing blocks are replaced with
  this new block. The memory is then mapped in write mode.
  The memory is unmapped after usage.


Use cases
---------

Generating RTP packets from h264 video
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We receive as input a GstBuffer with an encoded h264 image and we need to
create RTP packets containing this h264 data as the payload. We typically need
to fragment the h264 data into multiple packets, each with their own RTP and
payload specific header.

                       +-------+-------+---------------------------+--------+
  input H264 buffer:   | NALU1 | NALU2 |  .....                    | NALUx  |
                       +-------+-------+---------------------------+--------+
                             |
                             V
  array of             +-+ +-------+  +-+ +-------+            +-+ +-------+
  output buffers:      | | | NALU1 |  | | | NALU2 |   ....     | | | NALUx |
                       +-+ +-------+  +-+ +-------+            +-+ +-------+
                       :           :  :           :
                       \-----------/  \-----------/
                         buffer 1        buffer 2

The output buffer array consists of x buffers consisting of an RTP payload header
and a subbuffer of the original input H264 buffer. Since the rtp headers and
the h264 data don't need to be contiguous in memory, they are added to the buffer
as separate GstMemory blocks and we can avoid to memcpy the h264 data into
contiguous memory.

A typical udpsink will then use something like sendmsg to send the memory regions
on the network inside one UDP packet. This will further avoid having to memcpy
data into contiguous memory.

Using bufferlists, the complete array of output buffers can be pushed in one
operation to the peer element.