使用ffmpeg打开音视频文件，解封状，即做Demuxer时，需要用到函数avformat_open_input，该函数有两种使用方式。 第一种是传入文件名，由该API创建AVFormatContext，如下：
int main() {
AVFormatContext* format_context = nullptr;
if (avformat_open_input(&format_context, argv[1], NULL, NULL) != 0) {
return -1;
}
return 0;
}
另一种是提供自定义的字节流IO，即提供AVIOContext结构，下面的例子来源chromium的media模块。 先看一下创建AVIOContext的avio_alloc_context函数声明。
AVIOContext *avio_alloc_context(
unsigned char *buffer,
int buffer_size,
int write_flag,
void *opaque,
int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),
int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
int64_t (*seek)(void *opaque, int64_t offset, int whence));`
可以看到需要提供read，write，seek这三个操作，opaque是自定义的数据。其中write不是必须的，一般解封装也不需要写入。在chromium里面，定义了FFmpegURLProtocol接口来抽象各种输入。
class MEDIA_EXPORT FFmpegURLProtocol {
public:
// Read the given amount of bytes into data, returns the number of bytes read
// if successful, kReadError otherwise.
virtual int Read(int size, uint8_t* data) = 0;
// Returns true and the current file position for this file, false if the
// file position could not be retrieved.
virtual bool GetPosition(int64_t* position_out) = 0;
// Returns true if the file position could be set, false otherwise.
virtual bool SetPosition(int64_t position) = 0;
// Returns true and the file size, false if the file size could not be
// retrieved.
virtual bool GetSize(int64_t* size_out) = 0;
// Returns false if this protocol supports random seeking.
virtual bool IsStreaming() = 0;
};
基于此再实现avio_alloc_context需要的read、seek函数，见下：
// Internal buffer size used by AVIO for reading.
// TODO(dalecurtis): Experiment with this buffer size and measure impact on
// performance. Currently we want to use 32kb to preserve existing behavior
// with the previous URLProtocol based approach.
enum { kBufferSize = 32 * 1024 };
static int AVIOReadOperation(void* opaque, uint8_t* buf, int buf_size) {
return reinterpret_cast<FFmpegURLProtocol*>(opaque)->Read(buf_size, buf);
}
static int64_t AVIOSeekOperation(void* opaque, int64_t offset, int whence) {
FFmpegURLProtocol* protocol = reinterpret_cast<FFmpegURLProtocol*>(opaque);
int64_t new_offset = AVERROR(EIO);
switch (whence) {
case SEEK_SET:
if (protocol->SetPosition(offset))
protocol->GetPosition(&new_offset);
break;
case SEEK_CUR:
int64_t pos;
if (!protocol->GetPosition(&pos))
break;
if (protocol->SetPosition(pos + offset))
protocol->GetPosition(&new_offset);
break;
case SEEK_END:
int64_t size;
if (!protocol->GetSize(&size))
break;
if (protocol->SetPosition(size + offset))
protocol->GetPosition(&new_offset);
break;
case AVSEEK_SIZE:
protocol->GetSize(&new_offset);
break;
default:
NOTREACHED();
}
return new_offset;
}
使用时，先用avformat_alloc_context创建AVFormatContext，再用avio_alloc_context创建AVIOContext，设置使用AVFMT_FLAG_CUSTOM_IO的参数并赋值AVIOContext实例给AVFormatContext。之后调用avformat_open_input时，提供自定义的AVFormatConext参数并且不提供第二个文件名参数。
FFmpegGlue::FFmpegGlue(FFmpegURLProtocol* protocol) {
// Initialize an AVIOContext using our custom read and seek operations. Don't
// keep pointers to the buffer since FFmpeg may reallocate it on the fly. It
// will be cleaned up
format_context_ = avformat_alloc_context();
avio_context_.reset(avio_alloc_context(
static_cast<unsigned char*>(av_malloc(kBufferSize)), kBufferSize, 0,
protocol, &AVIOReadOperation, nullptr, &AVIOSeekOperation));
// Ensure FFmpeg only tries to seek on resources we know to be seekable.
avio_context_->seekable =
protocol->IsStreaming() ? 0 : AVIO_SEEKABLE_NORMAL;
// Ensure writing is disabled.
avio_context_->write_flag = 0;
// Tell the format context about our custom IO context. avformat_open_input()
// will set the AVFMT_FLAG_CUSTOM_IO flag for us, but do so here to ensure an
// early error state doesn't cause FFmpeg to free our resources in error.
format_context_->flags |= AVFMT_FLAG_CUSTOM_IO;
// Enable fast, but inaccurate seeks for MP3.
format_context_->flags |= AVFMT_FLAG_FAST_SEEK;
// Ensures we can read out various metadata bits like vp8 alpha.
format_context_->flags |= AVFMT_FLAG_KEEP_SIDE_DATA;
// Ensures format parsing errors will bail out. From an audit on 11/2017, all
// instances were real failures. Solves bugs like http://crbug.com/710791.
format_context_->error_recognition |= AV_EF_EXPLODE;
format_context_->pb = avio_context_.get();
}
bool FFmpegGlue::OpenContext(bool is_local_file) {
DCHECK(!open_called_) << "OpenContext() shouldn't be called twice.";
// If avformat_open_input() is called we have to take a slightly different
// destruction path to avoid double frees.
open_called_ = true;
// By passing nullptr for the filename (second parameter) we are telling
// FFmpeg to use the AVIO context we setup from the AVFormatContext structure.
const int ret =
avformat_open_input(&format_context_, nullptr, nullptr, nullptr);
// 下面省略....
}