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Read a c STRUCT from a socket connection

Posted on 2003-11-21
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Last Modified: 2012-05-04

I have a existing  application that consists of several proceses that cumunicate with each other using TCP/IP sockets. The application runs on HP-UX and was written in c.

I want to redevelop one of the processes in Java. The process must be able to read the c struct from the socket and write back return messages aswell.

the struct i sdefined as

#define EVENT_MESSAGE struct event_message_struct
EVENT_MESSAGE {
     char id[8];
     char event_code[4];
     char request_handle[12];
     char filename[40];
};

I can connect to the socket
but using a DataInputstream and readline() results in carbage.

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Question by:pfgrobler
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13 Comments
 
LVL 35

Assisted Solution

by:girionis
girionis earned 60 total points
ID: 9795702
 What jdk are you using?

  From the javadocs:

"Deprecated. This method does not properly convert bytes to characters. As of JDK 1.1, the preferred way to read lines of text is via the BufferedReader.readLine() method. Programs that use the DataInputStream class to read lines can be converted to use the BufferedReader class by replacing code of the form:

     DataInputStream d = new DataInputStream(in);


with:

     BufferedReader d
          = new BufferedReader(new InputStreamReader(in));"

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LVL 35

Expert Comment

by:girionis
ID: 9795729
 Also note that a character in Java is 16 bits while in C it is 8...
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LVL 5

Expert Comment

by:lwinkenb
ID: 9795887
Try sending string representations of the data that is in the struct.  Then if you read them on the java side using a BufferedReader, you dont have to worry about the 16 bit vs 8 bit thing.
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LVL 86

Accepted Solution

by:
CEHJ earned 65 total points
ID: 9796209
In order not to break the sending app, 'take it as its word':

byte[] id = new byte[8];
byte[] event_code = new byte[4];
byte[] request_handle = new byte[12];
byte[] filename = new byte[40];
DataInputStream dis = DataInputStream(in);
dis.read(id);
dis.read(event_code);
dis.read(request_handle);
dis.read(filename);
dis.close();


then you can convert those byte arrays into something more usable. Let us know if you need help doing that once it's working.
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LVL 86

Expert Comment

by:CEHJ
ID: 9796224
Or, better:

int id = dis.readInt();
char event_code = dis.readChar();
dis.read(request_handle);
String _filename = new String(dis.read(filename));
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LVL 86

Expert Comment

by:CEHJ
ID: 9796231
...and yet another improvement ;-)

int event_code = dis.readUnsignedShort();
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LVL 7

Expert Comment

by:Igor Bazarny
ID: 9796661
CEHJ, are you sure about the byte order? Java definitely uses small-endian, but I personally don't know HP-UX byte order. If it's different, your initial variant with arrays might be better.
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Expert Comment

by:CEHJ
ID: 9796832
I would think it should be OK. Applications pushing out data over sockets should use network byte order (little-endian)
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LVL 7

Expert Comment

by:Igor Bazarny
ID: 9796950
If structure is defined as in the intitial post, bytes won't be reordered--fields declared as byte arrays, not multibyte integers. The question remains whether that fields mean multibyte integers. What applications are pushing, IMO often depends on developer expectations and lazyness. If it is known that other end will run on the same platform, byte ordering qustion may seem non-issue. Until someone forgets initial assumption and tries to start inter-platform communication.
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Expert Comment

by:CEHJ
ID: 9800163
Well - the ordering should be apparent pretty soon ;-)
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Author Comment

by:pfgrobler
ID: 9816396
I have some sucsess  the datatype was CHAr and not byte .
But using :

    char[] id = new char[8];
    char[] event_code = new char[4];
    char[] request_handle = new char[12];
    char[] filename = new char[40];


   System.out.println("START");
   System.out.println(d.read(id)  + " : " + String.valueOf(id));
   System.out.println(d.read(event_code) + " : " + String.valueOf(event_code) );
   System.out.println(d.read(request_handle)+ " : " + String.valueOf(request_handle) );
   System.out.println(d.read(filename)+ " : " + String.valueOf(filename) );

The result that I get is :
8 : GMD
4 : 00  
12 : 760952@
40 : 00760952.msgè@`+ è@

id and eventcode is fine but request_handle and  filename includes some garbage at the end. I asume that is because the data is shorter than the actual space allocated to it . How can I insure only the relavant data is read ?
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LVL 7

Expert Comment

by:Igor Bazarny
ID: 9816456
Note that java char is 2-byte, while C char is one byte. You have to read bytes. It seems that your char arrays get implicitly casted to byte arrays--there are no methods accepting char arrays in DataInputStream. Garbage at the end caused by not interpreting \0 as terminator--java straings can have embedded \0.
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LVL 86

Expert Comment

by:CEHJ
ID: 9817522
Yes, read it into byte arrays. The handle field is numeric is it not? For the filename, i'd use a similar approach to the following:

            byte[] filename = { 'a', 'b', 'c', '\0', '\1', '\2', '\3' };
            boolean zeroOut = false;
            for(int i = 0;i < filename.length;i++) {
                  if (filename[i] == '\0') {
                        zeroOut = true;
                  }
                  if (zeroOut) {
                        filename[i] = 0;
                  }
            }
            System.out.println(new String(filename));
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