@Override
    public String readString() throws IOException
    {
        final int length = readRawVarint32();
        if (length < 0)
            throw ProtobufException.negativeSize();

        if (offset + length > limit)
            throw ProtobufException.misreportedSize();

        final int offset = this.offset;

        this.offset += length;

        return STRING.deser(buffer, offset, length);
    }
Read an {@code sint64} field value from the internal buffer.
protostuff-core/src/main/java/io/protostuff/ByteArrayInput.java