ffmpeg 内存池AVBufferPool

1、首先参考：FFmpeg5.0源码阅读——内存池AVBufferPool
2、链表是怎么串起来的？
看下面三个核心函数：

static void pool_release_buffer(void *opaque, uint8_t *data)
{BufferPoolEntry *buf = opaque;AVBufferPool *pool = buf->pool;if(CONFIG_MEMORY_POISONING)memset(buf->data, FF_MEMORY_POISON, pool->size);ff_mutex_lock(&pool->mutex);buf->next = pool->pool;pool->pool = buf;ff_mutex_unlock(&pool->mutex);if (atomic_fetch_sub_explicit(&pool->refcount, 1, memory_order_acq_rel) == 1)buffer_pool_free(pool);
}/* allocate a new buffer and override its free() callback so that* it is returned to the pool on free */
static AVBufferRef *pool_alloc_buffer(AVBufferPool *pool)
{BufferPoolEntry *buf;AVBufferRef     *ret;av_assert0(pool->alloc || pool->alloc2);ret = pool->alloc2 ? pool->alloc2(pool->opaque, pool->size) :pool->alloc(pool->size);if (!ret)return NULL;buf = av_mallocz(sizeof(*buf));if (!buf) {av_buffer_unref(&ret);return NULL;}buf->data   = ret->buffer->data;buf->opaque = ret->buffer->opaque;buf->free   = ret->buffer->free;buf->pool   = pool;ret->buffer->opaque = buf;ret->buffer->free   = pool_release_buffer;return ret;
}AVBufferRef *av_buffer_pool_get(AVBufferPool *pool)
{AVBufferRef *ret;BufferPoolEntry *buf;ff_mutex_lock(&pool->mutex);buf = pool->pool;if (buf) {ret = av_buffer_create(buf->data, pool->size, pool_release_buffer,buf, 0);if (ret) {pool->pool = buf->next;buf->next = NULL;}} else {ret = pool_alloc_buffer(pool);}ff_mutex_unlock(&pool->mutex);if (ret)atomic_fetch_add_explicit(&pool->refcount, 1, memory_order_relaxed);return ret;
}

其中用户调用av_buffer_pool_get() -> pool_alloc_buffer()
如果是第一次创建的时候：

int av_hwframe_ctx_init(AVBufferRef *ref)
{AVHWFramesContext *ctx = (AVHWFramesContext*)ref->data;const enum AVPixelFormat *pix_fmt;int ret;if (ctx->internal->source_frames) {/* A derived frame context is already initialised. */return 0;}/* validate the pixel format */for (pix_fmt = ctx->internal->hw_type->pix_fmts; *pix_fmt != AV_PIX_FMT_NONE; pix_fmt++) {if (*pix_fmt == ctx->format)break;}if (*pix_fmt == AV_PIX_FMT_NONE) {av_log(ctx, AV_LOG_ERROR,"The hardware pixel format '%s' is not supported by the device type '%s'\\n",av_get_pix_fmt_name(ctx->format), ctx->internal->hw_type->name);return AVERROR(ENOSYS);}/* validate the dimensions */ret = av_image_check_size(ctx->width, ctx->height, 0, ctx);if (ret < 0)return ret;/* format-specific init */if (ctx->internal->hw_type->frames_init) {ret = ctx->internal->hw_type->frames_init(ctx);if (ret < 0)goto fail;}if (ctx->internal->pool_internal && !ctx->pool)ctx->pool = ctx->internal->pool_internal;/* preallocate the frames in the pool, if requested */if (ctx->initial_pool_size > 0) {ret = hwframe_pool_prealloc(ref);if (ret < 0)goto fail;}return 0;
fail:if (ctx->internal->hw_type->frames_uninit)ctx->internal->hw_type->frames_uninit(ctx);return ret;
}

hwframe_pool_prealloc（）是第一次创建内存池。

static int hwframe_pool_prealloc(AVBufferRef *ref)
{AVHWFramesContext *ctx = (AVHWFramesContext*)ref->data;AVFrame **frames;int i, ret = 0;frames = av_mallocz_array(ctx->initial_pool_size, sizeof(*frames));if (!frames)return AVERROR(ENOMEM);for (i = 0; i < ctx->initial_pool_size; i++) {frames[i] = av_frame_alloc();if (!frames[i])goto fail;ret = av_hwframe_get_buffer(ref, frames[i], 0);if (ret < 0)goto fail;}fail:for (i = 0; i < ctx->initial_pool_size; i++)av_frame_free(&frames[i]);av_freep(&frames);return ret;
}

可以看到for循环创建的过程。av_hwframe_get_buffer()

int av_hwframe_get_buffer(AVBufferRef *hwframe_ref, AVFrame *frame, int flags)
{AVHWFramesContext *ctx = (AVHWFramesContext*)hwframe_ref->data;int ret;if (ctx->internal->source_frames) {// This is a derived frame context, so we allocate in the source// and map the frame immediately.AVFrame *src_frame;frame->format = ctx->format;frame->hw_frames_ctx = av_buffer_ref(hwframe_ref);if (!frame->hw_frames_ctx)return AVERROR(ENOMEM);src_frame = av_frame_alloc();if (!src_frame)return AVERROR(ENOMEM);ret = av_hwframe_get_buffer(ctx->internal->source_frames,src_frame, 0);if (ret < 0) {av_frame_free(&src_frame);return ret;}ret = av_hwframe_map(frame, src_frame,ctx->internal->source_allocation_map_flags);if (ret) {av_log(ctx, AV_LOG_ERROR, "Failed to map frame into derived ""frame context: %d.\\n", ret);av_frame_free(&src_frame);return ret;}// Free the source frame immediately - the mapped frame still// contains a reference to it.av_frame_free(&src_frame);return 0;}if (!ctx->internal->hw_type->frames_get_buffer)return AVERROR(ENOSYS);if (!ctx->pool)return AVERROR(EINVAL);frame->hw_frames_ctx = av_buffer_ref(hwframe_ref);if (!frame->hw_frames_ctx)return AVERROR(ENOMEM);ret = ctx->internal->hw_type->frames_get_buffer(ctx, frame);if (ret < 0) {av_buffer_unref(&frame->hw_frames_ctx);return ret;}frame->extended_data = frame->data;return 0;
}

其中会调用用户设置的一个回调函数：
hw_type->frames_get_buffe(),这里我们看英伟达的解码器：

static int cuda_get_buffer(AVHWFramesContext *ctx, AVFrame *frame)
{CUDAFramesContext *priv = ctx->internal->priv;int res;frame->buf[0] = av_buffer_pool_get(ctx->pool);if (!frame->buf[0])return AVERROR(ENOMEM);res = av_image_fill_arrays(frame->data, frame->linesize, frame->buf[0]->data,ctx->sw_format, ctx->width, ctx->height, priv->tex_alignment);if (res < 0)return res;// YUV420P is a special case.// Nvenc expects the U/V planes in swapped order from how ffmpeg expects them, also chroma is half-alignedif (ctx->sw_format == AV_PIX_FMT_YUV420P) {frame->linesize[1] = frame->linesize[2] = frame->linesize[0] / 2;frame->data[2]     = frame->data[1];frame->data[1]     = frame->data[2] + frame->linesize[2] * (ctx->height / 2);}frame->format = AV_PIX_FMT_CUDA;frame->width  = ctx->width;frame->height = ctx->height;return 0;
}

所以真正调用的是av_buffer_pool_get()，这里就和我们最开始的代码一致了。
av_buffer_pool_get（）在第一次创建了一个BufferPoolEntry，在pool_release_buffer()的时候串起来，注意，这里是个单链表：

static void pool_release_buffer(void *opaque, uint8_t *data)
{BufferPoolEntry *buf = opaque;AVBufferPool *pool = buf->pool;if(CONFIG_MEMORY_POISONING)memset(buf->data, FF_MEMORY_POISON, pool->size);ff_mutex_lock(&pool->mutex);buf->next = pool->pool;//这里是链表头，也就是最后一个release的是头pool->pool = buf;ff_mutex_unlock(&pool->mutex);if (atomic_fetch_sub_explicit(&pool->refcount, 1, memory_order_acq_rel) == 1)buffer_pool_free(pool);
}