Thread: Conver long to 8 byte array and back to long

  1. #1
    Registered User
    Join Date
    Mar 2009

    Conver long to 8 byte array and back to long

    I have these two functions to convert one long to a 8 byte array and to convert a byte array to a long, but somethings isn't right..
    I think that is my shifts, but I'm not certain.

    unsigned char* longToByteArray(long value){
      unsigned char *b;
      b=(unsigned char*)malloc(sizeof(unsigned char)*8);
      int i;
      for(i = 0; i < 8;i++){
        int offset = (8 - 1 - i) * 8;
        b[i] = (unsigned char) ((value  >> offset) & 0xFFFFFFFL);
      return b;
    long byteArrayToLong(unsigned char* b, int offset) {
      long value = 0;
      int i;
      for (i = 0; i <8; i++) {
        int shift = (8 - 1 - i) * 8;
        value += (b[i + offset] & 0xFFFFFFFL) << shift  ;
      return value;
    When I convert long x =1111, the bytes are
    b[0]=0x0, b[1]=0x0, b[2]=0x4, b[3]=0x57,
    b[4]=0x0, b[5]=0x0, b[6]=0x4, b[7]=0x57
    and should be:
    b[0]=0x0, b[1]=0x0, b[2]=0x0, b[3]=0x0,
    b[4]=0x0, b[5]=0x0, b[6]=0x4, b[7]=0x57

    If someone can help me, I would be grateful.
    Thanks in advance.

  2. #2
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    Be lazy and use a union:
        union bl {
            unsigned char b[ sizeof( long )];
            long l;
    Are you sure your long is eight bytes?

    Hope is the first step on the road to disappointment.

  3. #3
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    New Zealand
    Whether it really is 8 bytes or not, all of those 8's should be sizeof(long)'s.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

    Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"

  4. #4
    Hurry Slowly vart's Avatar
    Join Date
    Oct 2006
    Rishon LeZion, Israel
    and mask should be 0xFF

    why do you need mask when converting back from array to long?
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

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