C Client socket data not able to read in Java Server

Question

Java Server socket giving the wrong output when bytearray converted to string. C socket header format

int  tccode 4
int  len    4
char data[4088] 4088

C Struct format

Tc Code :1001   Size : 256  
#ifdef   __DTGPL1T__                                                                                                   
struct DTGPL1T_S                                                                                                    
{                                                                                                   
      // ENTRY ZONE 
      short       por1cur; 
      short       por2cur; 
      short       por1ten;
      short       por2ten;
      short       p1lvim1;
      short       p1lvim2;
      short       p1lvim3; 
      short       p2lvim1;
      short       p2lvim2;
      short       p2lvim3; 
      short       brc1 ;
      short       entrylt;                                                                                     short       enlopcar;                                                                                                    
      short       notchlen;                                                                                                 
      short       poruse;                                                                                                     
      short       p1remain;                                                                                                 
      short       p2remain;                                                                                                 
      short       eloopcap;                                                                                            
      short       elooplen;                                                                                               
      short       untielen;                                                                                                 
      short       entryspd;                                                                                         
      short       st8ten; 
      short       brc2  ; 
      short       scbtns; 
      short       scbel ; 
      short       scb1intr;
      short       scb2intr;
      short       scb3intr;
      short       st3ten; 
      short       brc3  ; 
      short       plettkts; 
      short       deli1ten; 
      short       d1loopcc;
      short       d1loopln; 
      short       d1loopcp;
      short       brc4  ; 
      short       br4ten;
      short       plcspd; 
      short       triwidt; 
      short       sthosingwidth  ; 
      short       trigap1;
      short       trigap2; 
      short       trigap3;
      short       trigap4;
      short       trilap1; 
      short       trilap2; 
      short       trilap3; 
      short       trilap4;
      short       aactwid; 
      short       adevwid;
      short       chopw ; 
      short       chopd ; 
      short       tristat;
      short       trispd; 
      short       trmtoplen; 
      short       deli2ten; 
      short       d2loopcc; 
      short       d2loopcp;
      short       d2loopln;                                                                                                   
      short       br5ten; 
      short       brc5  ; 
      short       wpd1sta;
      short       wpd2sta; 
      short       wpd3sta;
      short       wpd3asta;
      short       wpd4sta;
      short       wpd5sta;
      short       chvsen[2]; 
      short       wbsen[2]; 
      short       etcsen;
      short       wpdpass[6];
      short       wpdlen[6];
      short       pl_cv_next_move;                                                                                                   
      short       ccv1_lock_curr ;   
      short       ccv2_lock_curr ;  
      short       ewb1_lock_curr ; 
      short       ewb2_lock_curr ;
      short       etc_lock_curr  ; 
      short       eskd1_lock_curr;
      short       eskd2_lock_curr; 
      short       ecc1_lock_curr ; 
      short       ecc2_lock_curr ;
      short       st_wid_low_err ;

      short       st_wid_upp_err ; 


      short       trim_width_set;                                                                                           
      short       hadling_time;                                                                                                   

      short       spare01[14]    ;

      short       spare02[16]    ;

      //38 + 16 = 53 

};                                                                                                  
typedef struct DTGPL1T_S       DTGPL1T_T;                                                                                                   
typedef        DTGPL1T_T       *DTGPL1T_P;                                                                                                  
#define        DTGPL1T_LN      sizeof(DTGPL1T_T)                                                                                                    
#endif      // Eof of __DTGPL1T__  

And Java server code

DataInputStream in = new DataInputStream(clientSocket.getInputStream());
     //DataInputStream in = new DataInputStream(new BufferedInputStream(clientSocket.getInputStream()));
     int tcCode = in.readInt();
     int length = in.readInt();
     if(tcCode == 1001) {

            byte[] messageByte = new byte[length];
            boolean end = false;
            StringBuilder dataString = new StringBuilder(length);
            int totalBytesRead = 0;
            while(!end) {

                int currentBytesRead = in.read(messageByte);
                totalBytesRead = currentBytesRead + totalBytesRead;
                if(totalBytesRead <= length) {                      
                    dataString
                      .append(new String(messageByte, 0, length - totalBytesRead + currentBytesRead));

                } else {
                    dataString
                      .append(new String(messageByte, 0, length - totalBytesRead + currentBytesRead));
                }
                if(dataString.length()>=length) {
                    end = true;
                }
            }
            System.out.println("Message1  "+dataString);
            }

MessageBytes getting like [-1, -2, 0, 0, 0, 0, 0, 0, 0, 35, 0, 33, 0, 30, 0, 45, 0, 40, 0, 35, 0, 0, 0, 0, -1, -1, 0, 10, 0, 1, 0, 1, 0, 1, 5, -123, 0, 111, 3, 34, 0, 0, 0, 0, 0, 0, 2, -25, 0, 24, -1, -2, -1, -3, 0, 0, 0, 0, 0, 0, 43, 37, -1, -35, 0, 0, 0, 72, 10, 42, 0, 0, 43, 37, 0, 0, 49, 126, 112, -30, 0, 5, 0, 5, 0, 11, 0, 12, -3, 68, -3, 68, -6, -20, -6, -20, 27, 86, 0, 0, 0, 16, 0, 16, 0, 0, 0, 0, 0, -13, -1, -7, 0, 0, 16, 107, 0, -99, 0, 80, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -1, -20, 4, -73, -1, 74, -1] [Output screenshot] 1

Answer 1

Unlike in C, a Java String is not suitable for holding bytes. Converting bytes to a String decodes the bytes, which means some values may be altered or lost.

Even if no values were altered, your data is not going to represent characters, so printing it as text will display nonsense. For instance, many of your bytes are zero, which does not show at all in output, since character \ is a non-printing control character.

You should avoid Strings and StringBuilders entirely. Instead, store your bytes in the byte array only:

while (totalBytesRead < length) {
    int currentBytesRead = in.read(messageByte,
        totalBytesRead, length - totalBytesRead);
    totalBytesRead = currentBytesRead + totalBytesRead;
}

Your data appears to consist entirely of short values, so, assuming the C client is built to regard short as a 16-bit value, and assuming the C compiler didn't pad your struct members in order to word-align them, you can convert your bytes to a sequence of shorts using ByteBuffer and ShortBuffer :

ByteBuffer buffer = ByteBuffer.wrap(messageByte);
ShortBuffer shortBuffer = buffer.asShortBuffer();

To print them, retrieve them as an array and use Arrays.toString :

short[] values = new short[length / 2];
shortBuffer.get(values);
System.out.println(Arrays.toString(values));