extract subtitle stream from mkv file, using libav

e.go

package main

/*
#cgo LDFLAGS: -lavformat -lavutil -lavcodec
#include <libavformat/avformat.h>
#include <libavutil/avutil.h>
*/
import "C"
import (
	"fmt"
	"log"
	"os"
	"unsafe"
)

func main() {
	C.av_register_all()
	var ctx *C.AVFormatContext
	if C.avformat_open_input(&ctx, C.CString(os.Args[1]), nil, nil) != C.int(0) {
		log.Fatalf("cannot open file\n")
	}
	streams := (*[1 << 30]*C.AVStream)(unsafe.Pointer(ctx.streams))
	for i := C.uint(0); i < ctx.nb_streams; i++ {
		stream := streams[i]
		if stream.codec.codec_type != C.AVMEDIA_TYPE_SUBTITLE {
			continue
		}
		codec := C.avcodec_find_decoder(stream.codec.codec_id)
		if C.avcodec_open2(stream.codec, codec, nil) != C.int(0) {
			log.Fatalf("error opening codec\n")
		}
	}

	var packet C.AVPacket
	C.av_init_packet(&packet)
	var gotSub, result C.int
	var subtitle C.AVSubtitle
	var stream *C.AVStream
	var start, end C.int64_t
	var rects *[1 << 31]*C.AVSubtitleRect
	var rect *C.AVSubtitleRect
	var i C.uint
	for C.av_read_frame(ctx, &packet) >= C.int(0) {
		stream = streams[packet.stream_index]
		if stream.codec.codec_type != C.AVMEDIA_TYPE_SUBTITLE {
			continue
		}
		result = C.avcodec_decode_subtitle2(stream.codec, &subtitle, &gotSub, &packet)
		if result >= C.int(0) {
			start = packet.pts*C.int64_t(1000*stream.time_base.num/stream.time_base.den) + C.int64_t(subtitle.start_display_time)
			end = packet.pts*C.int64_t(1000*stream.time_base.num/stream.time_base.den) + C.int64_t(subtitle.end_display_time)
			fmt.Printf("%d %d\n", start, end)
			rects = (*[1 << 31]*C.AVSubtitleRect)(unsafe.Pointer(subtitle.rects))
			for i = C.uint(0); i < subtitle.num_rects; i++ {
				rect = rects[i]
				if rect._type == C.SUBTITLE_ASS {
					fmt.Printf("=>%s<=\n", C.GoString(rect.ass))
				} else if rect._type == C.SUBTITLE_TEXT {
					fmt.Printf("=>%s<=\n", C.GoString(rect.text))
				}
			}
		}
	}
}