ffmpegの詳細分析を整理する.

int ffio_fdopen(AVIOContext **s, URLContext *h)
{
    uint8_t *buffer;
    int buffer_size, max_packet_size;

    max_packet_size = h->max_packet_size;
    if (max_packet_size) {
        buffer_size = max_packet_size; /* no need to bufferize more than one packet */
    } else {
        buffer_size = IO_BUFFER_SIZE;
    }
    buffer = av_malloc(buffer_size);
    if (!buffer)
        return AVERROR(ENOMEM);

    *s = avio_alloc_context(buffer, buffer_size, h->flags & AVIO_FLAG_WRITE, h,
                            (void*)ffurl_read, (void*)ffurl_write, (void*)ffurl_seek);
    if (!*s) {
        av_free(buffer);
        return AVERROR(ENOMEM);
    }
    (*s)->direct = h->flags & AVIO_FLAG_DIRECT;
    (*s)->seekable = h->is_streamed ? 0 : AVIO_SEEKABLE_NORMAL;
    (*s)->max_packet_size = max_packet_size;
    if(h->prot) {
        (*s)->read_pause = (int (*)(void *, int))h->prot->url_read_pause;
        (*s)->read_seek  = (int64_t (*)(void *, int, int64_t, int))h->prot->url_read_seek;
    }
    (*s)->av_class = &ffio_url_class;
    return 0;
}

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))
{
    AVIOContext *s = av_mallocz(sizeof(AVIOContext));
    if (!s)
        return NULL;
    ffio_init_context(s, buffer, buffer_size, write_flag, opaque,
                  read_packet, write_packet, seek);
    return s;
}

int ffio_init_context(AVIOContext *s,
                  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))
{
  <strong><span style="color:#3366ff;">  s->buffer      = buffer;
    s->orig_buffer_size =
    s->buffer_size = buffer_size;
    s->buf_ptr     = buffer;
    s->opaque      = opaque;</span></strong>
    s->direct      = 0;

    url_resetbuf(s, write_flag ? AVIO_FLAG_WRITE : AVIO_FLAG_READ);

    s->write_packet    = write_packet;
    s->read_packet     = read_packet;
    s->seek            = seek;
   <span style="color:#3366ff;"><strong> s->pos             = 0;</strong></span>
    s->must_flush      = 0;
    s->eof_reached     = 0;
    s->error           = 0;
    s->seekable        = seek ? AVIO_SEEKABLE_NORMAL : 0;
    s->max_packet_size = 0;
    s->update_checksum = NULL;

    if (!read_packet && !write_flag) {
        s->pos     = buffer_size;
        s->buf_end = s->buffer + buffer_size;
    }
    s->read_pause = NULL;
    s->read_seek  = NULL;

    return 0;
}

int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
{
    int64_t offset1;
    int64_t pos;
    int force = whence & AVSEEK_FORCE;
    int buffer_size;
    whence &= ~AVSEEK_FORCE;

    if(!s)
        return AVERROR(EINVAL);

    buffer_size = s->buf_end - s->buffer;
    pos = s->pos - (s->write_flag ? 0 : buffer_size);

    if (whence != SEEK_CUR && whence != SEEK_SET)
        return AVERROR(EINVAL);

    if (whence == SEEK_CUR) {
        offset1 = pos + (s->buf_ptr - s->buffer);
        if (offset == 0)
            return offset1;
        offset += offset1;
    }
    offset1 = offset - pos;
    if (!s->must_flush && (!s->direct || !s->seek) &&
        offset1 >= 0 && offset1 <= buffer_size) {
        /* can do the seek inside the buffer */
        s->buf_ptr = s->buffer + offset1;
    } else if ((!s->seekable ||
               offset1 <= s->buf_end + SHORT_SEEK_THRESHOLD - s->buffer) &&
               !s->write_flag && offset1 >= 0 &&
               (!s->direct || !s->seek) &&
              (whence != SEEK_END || force)) {
        while(s->pos < offset && !s->eof_reached)
            fill_buffer(s);
        if (s->eof_reached)
            return AVERROR_EOF;
        s->buf_ptr = s->buf_end + offset - s->pos;
    } else if(!s->write_flag && offset1 < 0 && -offset1 < buffer_size>>1 && s->seek && offset > 0) {
        int64_t res;

        pos -= FFMIN(buffer_size>>1, pos);
        if ((res = s->seek(s->opaque, pos, SEEK_SET)) < 0)
            return res;
        s->buf_end =
        s->buf_ptr = s->buffer;
        s->pos = pos;
        s->eof_reached = 0;
        fill_buffer(s);
        return avio_seek(s, offset, SEEK_SET | force);
    } else {
        int64_t res;
        if (s->write_flag) {
            flush_buffer(s);
            s->must_flush = 1;
        }
        if (!s->seek)
            return AVERROR(EPIPE);
        if ((res = s->seek(s->opaque, offset, SEEK_SET)) < 0)
            return res;
        s->seek_count ++;
        if (!s->write_flag)
            s->buf_end = s->buffer;
        s->buf_ptr = s->buffer;
        s->pos = offset;
    }
    s->eof_reached = 0;
    return offset;
}