本文主要分析gsbuffer怎么读写内存，怎么添加删除meta，仅供参考。
GstMeta
typedef struct _GstMeta GstMeta;
struct _GstMeta {
? GstMetaFlags ? ? ? flags;
? const GstMetaInfo *info;
};
typedef struct _GstMetaItem GstMetaItem;
struct _GstMetaItem {
? GstMetaItem *next;
? guint64 seq_num;? ?//编号
? GstMeta meta;
};
可以看到GstMetaItem是一个GstMeta的链表。
GstMemory
struct _GstMemory {
? GstMiniObject ? mini_object;
? GstAllocator ? *allocator;
? GstMemory ? ? ?*parent;
? gsize ? ? ? ? ? maxsize;
? gsize ? ? ? ? ? align;
? gsize ? ? ? ? ? offset;
? gsize ? ? ? ? ? size;
};
看起来_GstMemory包含了内存的大小size，以及分配者allocator。
/* default memory implementation */
typedef struct
{
? GstMemory mem;
? gsize slice_size;
? guint8 *data;
? gpointer user_data;
? GDestroyNotify notify;
} GstMemorySystem;
GstMemorySystem会对GstMemory再进行封装，真正的包含了内存的指针data，可以参考_sysmem_new。
GstMemory相关的重要函数有：
Getting access to the data of the memory is performed with?gst_memory_map. The call will return a pointer to offset bytes into the region of memory. After the memory access is completed,?gst_memory_unmap?should be called.
gst_buffer_append_memory (GstBuffer * buffer, GstMemory * mem)->gst_buffer_insert_memory->_memory_add-> GST_BUFFER_MEM_PTR (buffer, idx) = mem;
GstMemory 的例子,?gst_rtp_rtx_buffer_new的片段：
? /* copy extension if any */
? if (rtp.size[1]) {
? ? mem = gst_allocator_alloc (NULL, rtp.size[1], NULL);
? ? gst_memory_map (mem, &map, GST_MAP_WRITE);? ?//得到GstMemory的内存指针
? ? memcpy (map.data, rtp.data[1], rtp.size[1]);
? ? gst_memory_unmap (mem, &map);
? ? gst_buffer_append_memory (new_buffer, mem);? //写完，关闭
? }
gst_allocator_alloc的定义在gstallocator.c中，如果第一个参数为空，会调用_default_allocator类的alloc函数，那_default_allocator从哪里来呢？gstallocator.c中定义了一个类GstAllocatorSysmem，G_DEFINE_TYPE (GstAllocatorSysmem, gst_allocator_sysmem, GST_TYPE_ALLOCATOR);
在初始化函数_priv_gst_allocator_initialize (void)中，创建了GstAllocatorSysmem的对象_sysmem_allocator（?_default_allocator = gst_object_ref (_sysmem_allocator);），然后赋给了_default_allocator，所以_default_allocator是GstAllocatorSysmem对象的指针。GstAllocatorSysmem类的alloc函数是default_alloc，最终调用的顺序是：
GstMemory * gst_allocator_alloc(gstallocator.c)->default_alloc->_sysmem_new_block->g_slice_alloc(gslice.c)->调g_malloc，slab
gstBuffer
typedef struct _GstBuffer GstBuffer;
struct _GstBuffer {
? GstMiniObject ? ? ? ? ?mini_object;
? /* timestamp */
? GstClockTime ? ? ? ? ? pts;? ? //时间戳
? GstClockTime ? ? ? ? ? dts;
.....
};
typedef struct
{
? GstBuffer buffer;
? gsize slice_size;
? /* the memory blocks */
? guint len;
? GstMemory *mem[GST_BUFFER_MEM_MAX];? ?//存储数据
? /* memory of the buffer when allocated from 1 chunk */
? GstMemory *bufmem;
? /* FIXME, make metadata allocation more efficient by using part of the
? ?* GstBufferImpl */
? GstMetaItem *item;? ? ? ? ? ?//头指针
? GstMetaItem *tail_item;? ? //尾指针
} GstBufferImpl;
可以看到GstBuffer包含时间戳等成员。GstBufferImpl继承自GstBuffer，包含GstMemory，?GstMetaItem等成员，GstMetaItem是个链表，可以为GstBuffer添加，删除GstMeta。
这里有一些操作的宏：
#define GST_BUFFER_SLICE_SIZE(b) ? (((GstBufferImpl *)(b))->slice_size)
#define GST_BUFFER_MEM_LEN(b) ? ? ?(((GstBufferImpl *)(b))->len)
#define GST_BUFFER_MEM_ARRAY(b) ? ?(((GstBufferImpl *)(b))->mem)
#define GST_BUFFER_MEM_PTR(b,i) ? ?(((GstBufferImpl *)(b))->mem[i])
#define GST_BUFFER_BUFMEM(b) ? ? ? (((GstBufferImpl *)(b))->bufmem)
#define GST_BUFFER_META(b) ? ? ? ? (((GstBufferImpl *)(b))->item)
#define GST_BUFFER_TAIL_META(b) ? ?(((GstBufferImpl *)(b))->tail_item)
gst_buffer_new
GstBuffer *? ?gst_buffer_new (void)
{
? GstBufferImpl *newbuf;
? newbuf = g_slice_new (GstBufferImpl);
? GST_CAT_LOG (GST_CAT_BUFFER, "new %p", newbuf);
? gst_buffer_init (newbuf, sizeof (GstBufferImpl));
? return GST_BUFFER_CAST (newbuf);
}
可以看到创建一个GstBuffer，就是创建一个GstBufferImpl。
gst_buffer_add_meta
Arbitrary extra metadata can be set on a buffer with?gst_buffer_add_meta. Metadata can be retrieved with?gst_buffer_get_meta. See also?GstMeta.
GstMeta *gst_buffer_add_meta (GstBuffer * buffer, const GstMetaInfo * info,??gpointer params)
{
? GstMetaItem *item;
? GstMeta *result = NULL;
? gsize size;
? g_return_val_if_fail (buffer != NULL, NULL);
? g_return_val_if_fail (info != NULL, NULL);
? g_return_val_if_fail (gst_buffer_is_writable (buffer), NULL);
? /* create a new slice */
? size = ITEM_SIZE (info);
? /* We warn in gst_meta_register() about metas without
? ?* init function but let's play safe here and prevent
? ?* uninitialized memory
? ?*/
? if (!info->init_func)
? ? item = g_slice_alloc0 (size);
? else
? ? item = g_slice_alloc (size);
? result = &item->meta;
? result->info = info;
? result->flags = GST_META_FLAG_NONE;
。。。。。
? /* call the init_func when needed */
? if (info->init_func)
? ? if (!info->init_func (result, params, buffer))
? ? ? goto init_failed;
? item->seq_num = gst_atomic_int64_inc (&meta_seq);
? item->next = NULL;
? if (!GST_BUFFER_META (buffer)) {? ? //插入到链表尾部
? ? GST_BUFFER_META (buffer) = item;
? ? GST_BUFFER_TAIL_META (buffer) = item;
? } else {
? ? GST_BUFFER_TAIL_META (buffer)->next = item;
? ? GST_BUFFER_TAIL_META (buffer) = item;
? }
。。。。。。
}
关键是第二个参数，传入GstMetaInfo类型，返回GstMetaInfo对应meta，例如注册一个GstParentBufferMeta名称的的GstMetaInfo。
const GstMetaInfo *
gst_parent_buffer_meta_get_info (void) {
? static const GstMetaInfo *meta_info = NULL;
? if (g_once_init_enter ((GstMetaInfo **) & meta_info)) {
? ? const GstMetaInfo *meta =
? ? ? ? gst_meta_register (gst_parent_buffer_meta_api_get_type (),
? ? ? ? "GstParentBufferMeta",
? ? ? ? sizeof (GstParentBufferMeta),
? ? ? ? (GstMetaInitFunction) _gst_parent_buffer_meta_init,
? ? ? ? (GstMetaFreeFunction) _gst_parent_buffer_meta_free,
? ? ? ? _gst_parent_buffer_meta_transform);
? ? g_once_init_leave ((GstMetaInfo **) & meta_info, (GstMetaInfo *) meta);
? }
? return meta_info;
}
GstParentBufferMeta *
gst_buffer_add_parent_buffer_meta (GstBuffer * buffer, GstBuffer * ref)
{
? GstParentBufferMeta *meta;
? g_return_val_if_fail (GST_IS_BUFFER (ref), NULL);
? meta =
? ? ? (GstParentBufferMeta *) gst_buffer_add_meta (buffer,
? ? ? GST_PARENT_BUFFER_META_INFO, NULL);
? if (!meta)
? ? return NULL;
? meta->buffer = gst_buffer_ref (ref);
? return meta;
}
第三方库的应用（deepstream）
deepstream用于AI推理，比如输入交通上的视频，推理得出汽车的位置，车牌的位置等，这些推理后的数据都存放在meta当中。deepstream有个streammux插件，在mux之前，meta绑定在GstBuffer上，如用户加一些自定义的meta ，streammux之后，所有数据打成一个batch，之前所有的meta数据都会挂到batch meta的链表下。以deepstream_gst_metadata.c为例，pipeline是?file-source -> h264-parser -> nvh264-decoder -> nvinfer -> nvvidconv -> nvosd -> video-renderer，
用户添加的meta:
/* nvdecoder_src_pad_buffer_probe() will attach decoder metadata to gstreamer
?* buffer on src pad. The decoder can not attach to NvDsBatchMeta metadata because
?* batch level metadata is created by nvstreammux component. The decoder
?* component is present is before nvstreammmux. So it attached the metadata
?* using gstnvdsmeta API's.?*/
static GstPadProbeReturn
nvdecoder_src_pad_buffer_probe (GstPad * pad, GstPadProbeInfo * info,
? ? gpointer u_data)
{
? GstBuffer *buf = (GstBuffer *) info->data;
? NvDsMeta *meta = NULL;
? NvDecoderMeta *decoder_meta = (NvDecoderMeta *)g_malloc0(sizeof(NvDecoderMeta));
? if(decoder_meta == NULL)
? {
? ? return GST_FLOW_ERROR;
? }
? /* Add dummy metadata */
? decoder_meta->frame_type = (frame_number % 3);
? decoder_meta->frame_num = frame_number++;
? decoder_meta->dec_err = ((frame_number % 4) / 3);
? /* Attach decoder metadata to gst buffer using gst_buffer_add_nvds_meta() */
? meta = gst_buffer_add_nvds_meta (buf, decoder_meta, NULL,
? ? ? decoder_meta_copy_func, decoder_meta_release_func);
? /* Set metadata type */
? meta->meta_type = (GstNvDsMetaType)NVDS_DECODER_GST_META_EXAMPLE;
? /* Set transform function to transform decoder metadata from Gst meta to
? ?* nvds meta */
? meta->gst_to_nvds_meta_transform_func = decoder_gst_to_nvds_meta_transform_func;
需要设置转换函数，streammux之后，meta会转到bachmeta下的链表下。
? /* Set release function to release the transformed nvds metadata */
? meta->gst_to_nvds_meta_release_func = decoder_gst_nvds_meta_release_func;
? g_print("GST Dec Meta attached with gst decoder output buffer for Frame_Num = %d\n",
? ? ? decoder_meta->frame_num);
? g_print("Attached decoder Metadata: frame type = %d, frame_num = %d decode_error_status = %d\n\n",
? ? ? decoder_meta->frame_type, decoder_meta->frame_num,
? ? ? decoder_meta->dec_err);
? return GST_PAD_PROBE_OK;
}
infer过后，读取用户之前添加的meta,
/* nvinfer_src_pad_buffer_probe() will extract the metadata received on nvinfer
?* src pad.
?* It explains the mechanism to extract the decoder metadata (which is attached
?* using gstnvdsmeta API's in nvdecoder_src_pad_buffer_probe()),
?* now transformed into nvdsmeta. Decoder meta, attached to gst buffer
?* is set as user data at NvDsFrameMeta level
?* It also extracts metadata attached on src pad of h264parse element.?*/
static GstPadProbeReturn
nvinfer_src_pad_buffer_probe (GstPad * pad, GstPadProbeInfo * info,
? ? gpointer u_data)
{
? GstBuffer *buf = (GstBuffer *) info->data;
? NvDsMetaList * l_frame = NULL;
? NvDsUserMeta *user_meta = NULL;
? NvDecoderMeta * decoder_meta = NULL;
? H264parseMeta * h264parse_meta = NULL;
? NvDsMetaList * l_user_meta = NULL;
? NvDsBatchMeta *batch_meta = gst_buffer_get_nvds_batch_meta (buf);
? ? for (l_frame = batch_meta->frame_meta_list; l_frame != NULL;
? ? ? l_frame = l_frame->next) {
? ? ? ? NvDsFrameMeta *frame_meta = (NvDsFrameMeta *) (l_frame->data);
? ? ? ? for (l_user_meta = frame_meta->frame_user_meta_list; l_user_meta != NULL;
? ? ? ? ? ? l_user_meta = l_user_meta->next) {
? ? ? ? ? user_meta = (NvDsUserMeta *) (l_user_meta->data);
? ? ? ? ? if(user_meta->base_meta.meta_type == NVDS_DECODER_GST_META_EXAMPLE)
? ? ? ? ? {
? ? ? ? ? ? decoder_meta = (NvDecoderMeta *)user_meta->user_meta_data;
? ? ? ? ? ? g_print("Dec Meta retrieved as NVDS USER METADTA For Frame_Num = %d ?\n",
? ? ? ? ? ? ? ? decoder_meta->frame_num);
? ? ? ? ? ? g_print("Retrieved Decoder Metadata: frame type = %d, frame_num = %d decode_error_status = %d\n\n",
? ? ? ? ? ? ? ? decoder_meta->frame_type, decoder_meta->frame_num,
? ? ? ? ? ? ? ? decoder_meta->dec_err);
? ? ? ? ? }
? ? ? ? }
? ? }
? ? return GST_PAD_PROBE_OK;
}
