Endian swapping (byte-order reversal) on a file

This is a discussion on Endian swapping (byte-order reversal) on a file within the C++ Programming forums, part of the General Programming Boards category; OK, so I found the C programming posts by a few people about byte-order swapping, and I've got that working ...

  1. #1
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    Endian swapping (byte-order reversal) on a file

    OK, so I found the C programming posts by a few people about byte-order swapping, and I've got that working just fine, but I'm a little confused as to how to grab those informations from a file, convert it, and then save that to a different file.

    Code:
    #include <cstdio>
    #include <cstdlib>
    #include <climits>
    #include <iostream>
    #include <stdint.h>
    
    using namespace std;
    
    main(int argc, char *argv[])
    {
        FILE* originalSave;
        FILE* newSave;
        long fSize;
        uint32_t *buffer;
        int x;
        
        uint32_t input = 0, j, result = 0, temp, final = 0, k;
        
        originalSave = fopen(argv[1], "rb");
        if (originalSave == NULL) {fputs("File error",stderr); exit(1);}
        
        newSave = fopen("newSave.fla", "wb+");
        
        fseek(originalSave, 0, SEEK_END);
        fSize = ftell(originalSave);
        rewind(originalSave);
        
        buffer = (char*) malloc(sizeof(char)*fSize);
        if (buffer == NULL)
        {
            fputs("Memory error",stderr); exit(2);
        }
            
        fread(buffer, 1, 4, originalSave);
        
        result = ((buffer << 24) & 0xFF000000) | 
            ((buffer << 8) & 0x00FF0000) |
            ((buffer >> 8) & 0x0000FF00) |
            ((buffer >> 24) & 0X000000FF);
        
        printf("Original hex: %X\n;Swapped hex: %X\n", buffer, result);
        
        //fwrite(buffer, fSize/4, 4, newSave);
        //fclose(newSave);
        //fclose(originalSave);
        //free(buffer);
        
        
        
        cout << "Press ENTER to continue...";
        cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );
    
      	
      return 0;
    }
    I don't even know if my code is functional (I get an error about invalid char or int to binary "operator>>" or whatever about the swap lines), but the structure should work, right? I'm completely confused by this.

    Oh, and eventually, I want to be able to do this with a file of up to 128KB size, so...

  2. #2
    Algorithm Dissector iMalc's Avatar
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    You're trying to endian-swap the pointer itself, instead of the integer it points to!
    My homepage
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  3. #3
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    OK, so I put in the pointer reference (&), and I still get those errors... Here's what they are exactly:

    Code:
    main.cpp invalid operands of types `uint32_t**' and `int' to binary `operator>>'
    That's for all four lines with the bit shifts in them.

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    So & is soooo not "the pointer reference". If you want what the pointer points to, you need to dereference the pointer, using *.

  5. #5
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    OK, so I dereference it. That works. It makes it compile, and so I run it (drag and drop) with a file that has hex values of 043E7A4F. But my output confuses me... I get:
    Code:
    Original hex: 3E24E0
    ;Swapped hex: 43E7A4F
    First of all, how do I make it register the leading 0's in the values and not drop them, and second, is my file not really 043E7A4F if my program reads it as 3E24E0? Or am I having it read it wrong?

    EDIT: OK, that's odd... I change two parts of the program that shouldn't have anything to do with the output, and I sort of get it. (P.S. I changed the fwrite portion at the end to have it be "fwrite(buffer, 1, 4..." instead, and I took the "+" off of the "wb" in the newSave fopen). So now I've got:
    Code:
    Original hex: 4F7A3E04
    ;Swapped hex:43E7A4F
    But that's backwards... The original hex is actually the swapped one, and vice versa...

    Oh, and I've been leaving the last fwrite/fclose/fclose/free section uncommented, but it's also still not creating the new file...

    EDIT 2: Using fwrite on the original save stream doesn't even work...
    Last edited by Molokai; 10-16-2009 at 08:23 AM.

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