CODEWITHSUNDEEP
javacmatlabbinary
skyuuka
skyuuka

Reputation: 675

How to read binary file created by C/Matlab using JAVA

I have created a binary file using the following matlab code:

x is an array of int32 numbers
n is the length of x

fid = fopen("binary_file.dat", "wb");
fwrite(fid, n, 'int32');
fwrite(fid, x, 'int32');
fclose(fid);

I can use the following C code to read this file:

fp = fopen("binary_file.dat", "rb");
int n;
fread(&n, 4, 1, fp);//read 4 bytes
int *x = new int[n];
for (int i = 0; i < n; i++)
{
int t;
fread(&t,4, 1,fp);//read 4 bytes
x[i] = t;
}
......

The above C code can read correct results. However, I now want to read such binary file in JAVA. My code is shown as follows:

DataInputStream data_in = new DataInputStream(
             new BufferedInputStream(
                    new FileInputStream(
                new File("binary_file.dat"))));
while(true)
{
   try {
      int t = data_in.readInt();//read 4 bytes
      System.out.println(t);
   } catch (EOFException eof) {
    break;
   }
}
data_in.close();

It DOES terminates after n+1 loops, but the results are not correct. Can anybody help me out. Thanks very much!

Upvotes: 4

Views: 4327

Answers (1)

Curd
Curd

Reputation: 12437

As I was guessing it is an endianness issue, i.e. your binary file is written as little-endian integers (probably, because you are using a Intel or similar CPU).

The Java code, however, is reading big-endian integers, no matter what CPU it is running on.

To show the problem the following code will read your data and display the integers as hex number before and after endianness conversion.

import java.io.*;

class TestBinaryFileReading {

  static public void main(String[] args) throws IOException {  
    DataInputStream data_in = new DataInputStream(
        new BufferedInputStream(
            new FileInputStream(new File("binary_file.dat"))));
    while(true) {
      try {
        int t = data_in.readInt();//read 4 bytes

        System.out.printf("%08X ",t); 

        // change endianness "manually":
        t = (0x000000ff & (t>>24)) | 
            (0x0000ff00 & (t>> 8)) | 
            (0x00ff0000 & (t<< 8)) | 
            (0xff000000 & (t<<24));
        System.out.printf("%08X",t); 
        System.out.println();
      } 
      catch (java.io.EOFException eof) {
        break;
      }
    } 
    data_in.close();
  }
}

If you don't want to do change endianness "manually", see answers to this question:
convert little Endian file into big Endian

Upvotes: 4

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