CodeGuru Home VC++ / MFC / C++ .NET / C# Visual Basic VB Forums Developer.com
Results 1 to 5 of 5
  1. #1
    Join Date
    Sep 2013
    Posts
    18

    convert 8 bytes into double and back?

    I have a question. I receive a byte stream. The first 8 bytes contain an Identification number. I receive at first the lowest byte and at the end the highest byte of the number.
    How can I transform this into a double value and later back into the bytestream? In the past I hard only 2 Byte values and there I could use things like MAKEWORD and HIBYTE and LOWBYTE to convert the values

  2. #2
    Join Date
    Apr 2000
    Location
    Belgium (Europe)
    Posts
    4,626

    Re: convert 8 bytes into double and back?

    if the 8 bytes represent an actualdouble in the double format...

    1) use a Union of a double and an array of 8 bytes.
    2) store the bytes in an 8 byte array, the copy the contents into a double with memcpy()
    3) store in an array, then typecast the array pointer to a double pointer and dereference.
    ...

    now. if the 8 bytes aren't in double format. you'll need to convert, can't help you there without more info.

  3. #3
    2kaud's Avatar
    2kaud is offline Super Moderator Power Poster
    Join Date
    Dec 2012
    Location
    England
    Posts
    7,822

    Re: convert 8 bytes into double and back?

    The format of a double is described in
    http://en.wikipedia.org/wiki/Double-...g-point_format

    Basically there are 52 bits for the fraction part, 11 bits for the exponent and 1 bit for the sign. From the OP post #1, the 8 bytes represent an identification number which is probably an integer and not a floating point (double) number so a double number cannot be used correctly to store the id.

    The type you probably need is long long or __int64 (both 8 byte integer).
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++23 Compiler: Microsoft VS2022 (17.6.5)

  4. #4
    2kaud's Avatar
    2kaud is offline Super Moderator Power Poster
    Join Date
    Dec 2012
    Location
    England
    Posts
    7,822

    Re: convert 8 bytes into double and back?

    One possible way of obtaining the id as a long long is

    Code:
    #include <stdio.h>
    
    int main()
    {
    unsigned char id[] = {1,2,3,4,5,6,7,8};
    
    long long res = 0;
    
    	for (int b = 0; b < 8; ++b) 
    		res |= ((long long)id[b]) << (b * 8);
    
    unsigned char *ares = (unsigned char*)&res;
    
    	for (int i = 0; i < 8; ++i)
    		printf("%02x ", ares[i]);
    
    	return 0;
    }
    All advice is offered in good faith only. All my code is tested (unless stated explicitly otherwise) with the latest version of Microsoft Visual Studio (using the supported features of the latest standard) and is offered as examples only - not as production quality. I cannot offer advice regarding any other c/c++ compiler/IDE or incompatibilities with VS. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/ and can be used without reference or acknowledgement. Also note that I only provide advice and guidance via the forums - and not via private messages!

    C++23 Compiler: Microsoft VS2022 (17.6.5)

  5. #5
    Join Date
    Sep 2013
    Posts
    18

    Re: convert 8 bytes into double and back?

    Thank you the last post is helping me. so far I created me this type for my 8 byte value:
    typedef unsigned __int64 QWORD;

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  





Click Here to Expand Forum to Full Width

Featured