Reading raw bytes from file java (endian issue)

Question

Ok so I have a file that contains exactly 8 bytes:

hexdump /tmp/temp_session_TGAyUSfICJgY.txt
0000000 b21b 113c bf3a 4a92                    
0000008

When I cat the file I see gobbly-gook which is normal and expected (you might see real characters depending on your encoding)

cat /tmp/temp_session_TGAyUSfICJgY.txt 
�<:��J

Now in java when I try to read the bytes, they come out backwards. My code is as follows:

            InputStream input = new FileInputStream(session_file_param);
            int a = 0;
            int i=0;
            while(a != -1) {
                    a = input.read();
                    System.out.println(a);
                    if(a != -1) {
                            pw[i] = (byte)a;
                    }
                    i++;
            }

            System.out.println("String representation of session pw is " + pw.toString());

My output is (added the =HEX for readability):

27 = 1b
178 = b2
60 = 3c
17 = 11
58 = 3a
191 = bf
146 =92
74 = 4a
-1
String representation of pw is [B@7f971afc

If I am reading a file RAW, byte by byte, shouldn't the bytes come out in order? Basically each two byte block is flipped.

EDIT:

You are right, sorry for the alarm. I made the following to test:

#include <stdio.h>
#include <stdlib.h>

int main() {
        FILE *fp = fopen("/tmp/temp_session_TGAyUSfICJgY.txt", "r");
        char byte;
        while (!feof(fp)) {
                fread(&byte,1,1, fp);
                printf("%x\n", byte);
        }
}

and output:

1b
ffffffb2
3c
11
3a
ffffffbf
ffffff92
4a

I believe your problem is actually hexdump (though can't be certain without knowing which version/platform you're using). The default display is normally 16-bit (two byte) combined, which would make sense given the output you're seeing. Try running hexdump in a single-byte mode (such as with -b), and I expect you'll see the output bytes in the order you expect (or at least, in the order Java is reading them). — user1676075
– user1676075, Commented Jul 16, 2013 at 18:50

joev · Accepted Answer · 2013-07-16 18:47:41Z

1

It appears that hexdump, with its defaults, is outputting your file in two byte chunks, reversing them.

Try using

hexdump -C /tmp/temp_session_TGAyUSfICJgY.txt

or

xxd /tmp/temp_session_TGAyUSfICJgY.txt

to see the bytes displayed in the order they appear in the file.

answered Jul 16, 2013 at 18:47

joev

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Alexander Pogrebnyak · Accepted Answer · 2013-07-16 18:45:00Z

0

Use this variant of hexdump:

hexdump -C /tmp/temp_session_TGAyUSfICJgY.txt

You will see the bytes in the same order that your Java program is producing.

I think, by default the hexdump does big-endian shorts.

answered Jul 16, 2013 at 18:45

Alexander Pogrebnyak

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Community · Accepted Answer · 2017-05-23 11:49:13Z

0

You really need to know what was written to the file (and how) to determine if you are reading it correctly. You can control the ByteOrder once you know how the file was written. See this question. Export ByteBuffer as Little Endian File in Java

edited May 23, 2017 at 11:49

CommunityBot

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answered Jul 16, 2013 at 19:00

kheld

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azzurroverde · Accepted Answer · 2013-07-16 19:07:13Z

0

Try using:

int read(byte[] b) Reads some number of bytes from the input stream and stores them into the buffer array b.

instead of int read()

answered Jul 16, 2013 at 19:07

azzurroverde

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