Thread: How to save a 64 bit number with 2 32 bit ints?

  1. 04-05-2010 #1
    homer_3
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    How to save a 64 bit number with 2 32 bit ints?

    This isn't really a C question, but I'm hoping some people here can help anyway. I have 2 32 bit unsigned ints representing a 64 bit unsigned int. I want to save the value to a string so I can print it out. My first thought was

    Code:
    sprintf(val, "%u", ((top << 32) | bottom));
    But that doesn't work because I shift the top half right out of the memory space for the int. Anyone have any thoughts on how this can be accomplished, or links to point me to that discuss dealing with 64 bit values on a 32 bit system?
  2. 04-05-2010 #2
    MK27
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    A long int on a 32-bit system is 8 bytes.
    Code:
    	printf("%d", sizeof(long));
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  3. 04-05-2010 #3
    Subsonics
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    Quote Originally Posted by homer_3 View Post
    Anyone have any thoughts on how this can be accomplished, or links to point me to that discuss dealing with 64 bit values on a 32 bit system?
    A 32 bit system doesn't limit the size of available datatypes to 4 bytes. As MK suggested you have 64 bit integers readily available, you can even go crazy with it and use a long double for 128 bits (no int tho).
  4. 04-05-2010 #4
    itCbitC
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    Quote Originally Posted by homer_3 View Post
    This isn't really a C question, but I'm hoping some people here can help anyway. I have 2 32 bit unsigned ints representing a 64 bit unsigned int. I want to save the value to a string so I can print it out. My first thought was

    Code:
    sprintf(val, "%u", ((top << 32) | bottom));
    But that doesn't work because I shift the top half right out of the memory space for the int. Anyone have any thoughts on how this can be accomplished, or links to point me to that discuss dealing with 64 bit values on a 32 bit system?
    Save the two 32-bit ints into a 64-bit variable (type long long), then stringify and save to a char array using sprintf().
  5. 04-05-2010 #5
    vart
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    Quote Originally Posted by MK27 View Post
    A long int on a 32-bit system is 8 bytes.
    Code:
    	printf("%d", sizeof(long));
    depends on compiler... on most 64 bit long type is

    long 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
  6. 04-05-2010 #6
    Elysia
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    Quote Originally Posted by MK27 View Post
    A long int on a 32-bit system is 8 bytes.
    Code:
    	printf("%d", sizeof(long));
    What system would that be?
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
  7. 04-05-2010 #7
    quzah
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    Quote Originally Posted by Elysia View Post
    What system would that be?
    I think it's FictitiOS


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  8. 04-05-2010 #8
    Subsonics
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    Quote Originally Posted by quzah View Post
    I think it's FictitiOS


    Quzah.
    I have it here actually, mac os X 10.6.3, gcc 4.2.1
  9. 04-05-2010 #9
    quzah
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    That's not a 32 bit OS. Actually ... it's a confused OS. It doesn't look like it knows for sure what it is. It looks like it either may or may not be booted with 64 bit extensions. If you've got them running -- and more than likely you do if size of long is 64 bits -- then you're in 64 bit mode.

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    Last edited by quzah; 04-05-2010 at 05:30 PM.
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  10. 04-05-2010 #10
    Subsonics
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    No, I'm booting the 32 bit kernel. The thing is, from what I understand the reason for this is that 3rd party peripherals will be allowed some time to port their drivers.

    Any way, the size of long (or rather if it's name is long, or long long) should have nothing to do with pointer sizes, 64 bits then for the addressing.
  11. 04-06-2010 #11
    siavoshkc
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    Are you talking about Mac OS X? You mean it checks drivers to see if all are 64 bit or not and if not it loads 32bit kernel?

    BTW, are you all agreed that long long is always 64bits?
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  12. 04-06-2010 #12
    Elysia
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    Quote Originally Posted by siavoshkc View Post
    BTW, are you all agreed that long long is always 64bits?
    Yes. It's standardized in C99 and C++0x.
    In previous standards, all compilers that support long long makes them 64-bit AFAIK.
    Last edited by Elysia; 04-06-2010 at 03:21 PM.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
  13. 04-06-2010 #13
    Subsonics
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    Quote Originally Posted by siavoshkc View Post
    Are you talking about Mac OS X? You mean it checks drivers to see if all are 64 bit or not and if not it loads 32bit kernel?
    Yes mac os x, as far as I'm aware it boots with a 32 bit kernel by default on all macs but xserve at this point. My guess is that it will stay that way until a majority of 3d party device drivers have been ported to 64 bit. You can boot it into a 64bit kernel by pressing 6 4 keys on a restart though, or change the boot args.
    Last edited by Subsonics; 04-06-2010 at 09:11 AM.
  14. 04-06-2010 #14
    siavoshkc
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    Quote Originally Posted by Elysia
    Yes. It's standardized in C99 and C++0x.
    On previous standards, all compilers that support long long makes them 64-bit AFAIK.
    Is a 128 bit integer defined in the standard?
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  15. 04-06-2010 #15
    laserlight
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    Quote Originally Posted by siavoshkc
    Is a 128 bit integer defined in the standard?
    No, though there is nothing in the C and C++ standards that prevent an implementation from providing a 128 bit integer type.
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