static int dyn_buf_write(void *opaque, uint8_t *buf, int buf_size)
{
    DynBuffer *d = opaque;
    int new_size, new_allocated_size;

    /* reallocate buffer if needed */
    new_size = d->pos + buf_size;
    new_allocated_size = d->allocated_size;
    if(new_size < d->pos || new_size > INT_MAX/2)
        return -1;
    while (new_size > new_allocated_size) {
        if (!new_allocated_size)
            new_allocated_size = new_size;
        else
            new_allocated_size += new_allocated_size / 2 + 1;
    }

    if (new_allocated_size > d->allocated_size) {
        d->buffer = av_realloc(d->buffer, new_allocated_size);
        if(d->buffer == NULL)
             return -1234;
        d->allocated_size = new_allocated_size;
    }
    memcpy(d->buffer + d->pos, buf, buf_size);
    d->pos = new_size;
    if (d->pos > d->size)
        d->size = d->pos;
    return buf_size;
}