[GStreamer] 内存管理

最新推荐文章于 2025-04-02 18:00:26 发布

SleepyPat

最新推荐文章于 2025-04-02 18:00:26 发布

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分类专栏： # gstreamer 文章标签： gstreamer

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GstMemory：管理一小块实际的虚拟内存
GstBuffer：一个对象，是element、plugin、application互相信息交互的最小数据单位，内部包含GstMemory、时间信息和一些其他信息。
GstMeta：附加在GstBuffer上，用来提供一些描述内存的额外信息。
GstBufferPool：buffer池，但是只能分配尺寸固定的buffer。

GstMemory：

/**
 * GstMemory:
 * @mini_object: parent structure
 * @allocator: pointer to the #GstAllocator
 * @parent: parent memory block
 * @maxsize: the maximum size allocated
 * @align: the alignment of the memory
 * @offset: the offset where valid data starts
 * @size: the size of valid data
 *
 * Base structure for memory implementations. Custom memory will put this structure
 * as the first member of their structure.
 */
struct _GstMemory {
  GstMiniObject   mini_object;

  GstAllocator   *allocator;

  GstMemory      *parent;
  gsize           maxsize;
  gsize           align;
  gsize           offset;
  gsize           size;
};

GstMemory由GstAllocator创建。

GstAllocator：

/**
 * GstAllocator:
 * @mem_map: the implementation of the GstMemoryMapFunction
 * @mem_unmap: the implementation of the GstMemoryUnmapFunction
 * @mem_copy: the implementation of the GstMemoryCopyFunction
 * @mem_share: the implementation of the GstMemoryShareFunction
 * @mem_is_span: the implementation of the GstMemoryIsSpanFunction
 * @mem_map_full: the implementation of the GstMemoryMapFullFunction.
 *      Will be used instead of @mem_map if present. (Since: 1.6)
 * @mem_unmap_full: the implementation of the GstMemoryUnmapFullFunction.
 *      Will be used instead of @mem_unmap if present. (Since: 1.6)
 *
 * The #GstAllocator is used to create new memory.
 */
struct _GstAllocator
{
  GstObject  object;

  const gchar               *mem_type;

  /*< public >*/
  GstMemoryMapFunction       mem_map;
  GstMemoryUnmapFunction     mem_unmap;

  GstMemoryCopyFunction      mem_copy;
  GstMemoryShareFunction     mem_share;
  GstMemoryIsSpanFunction    mem_is_span;

  GstMemoryMapFullFunction   mem_map_full;
  GstMemoryUnmapFullFunction mem_unmap_full;

  /*< private >*/
  gpointer _gst_reserved[GST_PADDING - 2];

  GstAllocatorPrivate *priv;
};

内存分配器，是一个接口层，如果自行管理内存分配，可以自己实现这个接口。

内存分配和使用：

使用 gst_allocator_alloc 分配内存，此函数返回的是一个 GstMemory 指针，GstMemory内的数据必须通过 gst_memory_map 保护才可以访问，然后通过 gst_memory_unmap 释放保护后别人才可以访问，类似于加锁解锁。

[...]

  GstMemory *mem;
  GstMapInfo info;
  gint i;

  /* allocate 100 bytes */
  mem = gst_allocator_alloc (NULL, 100, NULL);

  /* get access to the memory in write mode */
  gst_memory_map (mem, &info, GST_MAP_WRITE);

  /* fill with pattern */
  for (i = 0; i < info.size; i++)
    info.data[i] = i;

  /* release memory */
  gst_memory_unmap (mem, &info);

[...]

GstBuffer ：

GstBuffer内部包含了n个GstMemory，同时也包含了一些meta data：
dts 、pts：用来进行同步。
duration：buffer内数据总时长，不是整个流的总时长。
offset ：buffer存视频时，表示当前buffer内第一帧数据在整个流中是第多少个帧；buffer存音频时，???
offset_end：buffer存视频时，表示当前buffer内最后一帧数据在整个流中是第多少个帧；buffer存音频时，？？？

/**
 * GstBuffer:
 * @mini_object: the parent structure
 * @pool: pointer to the pool owner of the buffer
 * @pts: presentation timestamp of the buffer, can be #GST_CLOCK_TIME_NONE when the
 *     pts is not known or relevant. The pts contains the timestamp when the
 *     media should be presented to the user.
 * @dts: decoding timestamp of the buffer, can be #GST_CLOCK_TIME_NONE when the
 *     dts is not known or relevant. The dts contains the timestamp when the
 *     media should be processed.
 * @duration: duration in time of the buffer data, can be #GST_CLOCK_TIME_NONE
 *     when the duration is not known or relevant.
 * @offset: a media specific offset for the buffer data.
 *     For video frames, this is the frame number of this buffer.
 *     For audio samples, this is the offset of the first sample in this buffer.
 *     For file data or compressed data this is the byte offset of the first
 *       byte in this buffer.
 * @offset_end: the last offset contained in this buffer. It has the same
 *     format as @offset.
 *
 * The structure of a #GstBuffer. Use the associated macros to access the public
 * variables.
 */
struct _GstBuffer {
  GstMiniObject          mini_object;

  /*< public >*/ /* with COW */
  GstBufferPool         *pool;

  /* timestamp */
  GstClockTime           pts;
  GstClockTime           dts;
  GstClockTime           duration;

  /* media specific offset */
  guint64                offset;
  guint64                offset_end;
};

默认情况下GstBuffer是不可写的，需要先通过gst_buffer_make_writable函数处理，然后才可以修改里面的内容，比如dts、pts、duration、offset等等。

三种创建GstBuffer的方法:

gst_buffer_new 创建GstBuffer 对象，但是创建的对象不包含 GstMemory，需要手动创建GstMemory再进行绑定；
gst_buffer_new_allocate 同时创建GstBuffer 和指定数量的GstMemory，然后还会自动完成绑定动作；
gst_buffer_new_wrapped_full 为现有GstMemory创建一个GstBuffer。

[...]
  GstBuffer *buffer;
  GstMemory *mem;
  GstMapInfo info;

  /* make empty buffer */
  buffer = gst_buffer_new ();

  /* make memory holding 100 bytes */
  mem = gst_allocator_alloc (NULL, 100, NULL);

  /* add the buffer */
  gst_buffer_append_memory (buffer, mem);

[...]

  /* get WRITE access to the memory and fill with 0xff */
  gst_buffer_map (buffer, &info, GST_MAP_WRITE);
  memset (info.data, 0xff, info.size);
  gst_buffer_unmap (buffer, &info);

[...]

  /* free the buffer */
  gst_buffer_unref (buffer);

[...]

GstMeta：

/**
 * GstMeta:
 * @flags: extra flags for the metadata
 * @info: pointer to the #GstMetaInfo
 *
 * Base structure for metadata. Custom metadata will put this structure
 * as the first member of their structure.
 */
struct _GstMeta {
  GstMetaFlags       flags;
  const GstMetaInfo *info;
};

GstMeta可以随意地附加到一个GstBuffer中。

#include <gst/video/gstvideometa.h>

[...]
  GstVideoCropMeta *meta;    //GstMeta的一个子类

  /* buffer points to a video frame, add some cropping metadata */
  meta = gst_buffer_add_video_crop_meta (buffer);

  /* configure the cropping metadata */
  meta->x = 8;
  meta->y = 8;
  meta->width = 120;
  meta->height = 80;
[...]

#include <gst/video/gstvideometa.h>

[...]
  GstVideoCropMeta *meta;

  /* buffer points to a video frame, get the cropping metadata */
  meta = gst_buffer_get_video_crop_meta (buffer);

  if (meta) {
    /* render frame with cropping */
    _render_frame_cropped (buffer, meta->x, meta->y, meta->width, meta->height);
  } else {
    /* render frame */
    _render_frame (buffer);
  }
[...]

如何实现自定义的 GstMeta ：Memory allocation

GstBufferPool：

由内存池分配的 GstBuffer 具备完全相同的 meta data，唯一不一样的就是指向的内存区域。

GstBufferPool 以 GstBuffer 为管理单位，当且仅当 GstBufferPool 为 inactive 状态时，可以配置 GstBuffer 的meta data；当且仅当 GstBufferPool 为 active 状态时，可以分配和销毁GstBuffer。

  GstStructure *config;

[...]

  /* get config structure */
  config = gst_buffer_pool_get_config (pool);

  /* set caps, size, minimum and maximum buffers in the pool */
  gst_buffer_pool_config_set_params (config, caps, size, min, max);

  /* configure allocator and parameters */
  gst_buffer_pool_config_set_allocator (config, allocator, &params);

  /* store the updated configuration again */
  gst_buffer_pool_set_config (pool, config);

[...]

The following options can be configured on a GstBufferPool:

The caps of the buffers to allocate.

The size of the buffers. This is the suggested size of the buffers in the pool. The pool might decide to allocate larger buffers to add padding.

The minimum and maximum amount of buffers in the pool. When minimum is set to \> 0, the bufferpool will pre-allocate this amount of buffers. When maximum is not 0, the bufferpool will allocate up to maximum amount of buffers.

The allocator and parameters to use. Some bufferpools might ignore the allocator and use its internal one.

Other arbitrary bufferpool options identified with a string. a bufferpool lists the supported options with gst_buffer_pool_get_options() and you can ask if an option is supported with gst_buffer_pool_has_option(). The option can be enabled by adding it to the configuration structure with gst_buffer_pool_config_add_option (). These options are used to enable things like letting the pool set metadata on the buffers or to add extra configuration options for padding, for example.

gst_buffer_pool_set_active(pool, TRUE)用来激活内存池，之后用gst_buffer_pool_acquire_buffer ()来获取内存池对象。

  [...]

  GstFlowReturn ret;
  GstBuffer *buffer;

  ret = gst_buffer_pool_acquire_buffer (pool, &buffer, NULL);
  if (G_UNLIKELY (ret != GST_FLOW_OK))
    goto pool_failed;

  [...]