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 arbitrary metadata to buffers
efficient handling of subbuffer, copy, span, trim

Lifecycle

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

Writability

When a GstBuffer is writable as returned by 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 by using the gst_buffer_map() and gst_buffer_unmap() functions.

The _map() and _unmap() functions will always return the memory of all blocks as one large contiguous region. Using these functions 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 and 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 replaced with this new combined 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 and 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.

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