Parsing a byte array sent over TCP in Java

Question

I am developing an embedded system which sends some data over TCP. This system is ARM-based and its code is written in C. In C side, I have an array of char (or unsigned byte, i.e. uint8_t) which represents some encoded data:

 char buffer[BUFFER_SIZE] = {0, 11, 34,176,255}; // for example.

This buffer will be sent to a server via TCP/IP protocol, using a popular GPRS module called SIM800. The connection between microcontroller and SIM800 is UART, i.e. standard serial communication. I can send either uint8_t or char array. It doesn't differ in C world.

At the server side, there exist some Java services that receive and parse this array.

The problem is: In C language, the uint8_t and char data types are somehow identical, i.e 0 -> 255 is equal to the whole ASCII table. But as far as I know, this is not true at the server. In Java byte data type is intrinsically signed and its range is from -128 to 127. Moreover the extended ASCII characters, from 128 to 255 are somehow non-standard and differ from system to system.

The Java service receives data as String and then converts to an array of bytes.

I am confused. What will happen if I send the above-mentioned array to server. Can Java service reinterpret it?

Most likely you need to read it as array of char instead of array of byte. Not sure how your Java service is implemented though. — Jai
– Jai, Commented Oct 19, 2018 at 7:09

Mohan · Accepted Answer · 2018-10-19 07:29:50Z

1

You may try the below after reading the bytes from TCP stream

        String str = new String(bytes, 
                                         StandardCharsets.US_ASCII);

answered Oct 19, 2018 at 7:29

Mohan

3342 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

user2786531 · Accepted Answer · 2018-10-19 08:04:51Z

1

You could convert byte array to base64 and send to the java server. Java service then would convert it back to the original byte array.

answered Oct 19, 2018 at 8:04

user2786531

937 bronze badges

Comments

Serge Ballesta · Accepted Answer · 2018-10-19 08:07:21Z

The problem is that conversion between char and byte in java is not straightforward, because it involves a charset. The Latin1 or ISO-8859-1 charset is the direct conversion where the low order byte of the char is the original one, while the high order byte is 0.

So you must ensure (it should be said in the documentation of the Java service) how the service converts the input bytes to a String (what is the used charset) and then use the same charset for the reverse conversion.

The natural way would be to use a Latin1 conversion, in that case, the Java bytes would be the int8_t value of the representation of the uint_t original byte. So all bytes below 128 should be unchanged and bytes starting at 128 will receive original_value - 256. For example 255 will be -1 and 128 will be -128.

Collectives™ on Stack Overflow

Parsing a byte array sent over TCP in Java

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related