integer overflow

This is a discussion on integer overflow within the C Programming forums, part of the General Programming Boards category; Hi I need help to know why am I getting "integer overflow" when trying to assign 10^10 to a unsigned ...

  1. #1
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    Question integer overflow

    Hi

    I need help to know why am I getting "integer overflow" when trying to assign 10^10 to a
    unsigned long int. My compiler is (gcc 4.2).

    Doesn't unsigned long int vary from 0 to 4,294,967,295 in a ordinary 32bit IMB/PC?

    Thanks.

  2. #2
    Code Goddess Prelude's Avatar
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    >why am I getting "integer overflow" when trying to assign 10^10 to a unsigned long int
    >Doesn't unsigned long int vary from 0 to 4,294,967,295 in a ordinary 32bit IMB/PC?
    You're aware that 10^10 has one more digit than a 32-bit unsigned integer can hold, right?

    10000000000 - 10^10
    4294967296 - 2^32
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  3. #3
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    Quote Originally Posted by Prelude View Post
    >why am I getting "integer overflow" when trying to assign 10^10 to a unsigned long int
    >Doesn't unsigned long int vary from 0 to 4,294,967,295 in a ordinary 32bit IMB/PC?
    You're aware that 10^10 has one more digit than a 32-bit unsigned integer can hold, right?

    10000000000 - 10^10
    4294967296 - 2^32
    Sorry. My fault.

    But I still receive integer overflow when switching to unsigned long long...

  4. #4
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    You need to make sure your arithmetic is being done using unsigned long long. In particular, there's a suffix, either LLU or ULL, for constants to make them unsigned long long.

  5. #5
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    What does your code look like?

    The following works for MS VC.Net:
    Code:
    #include <stdio.h>
    #include <math.h>
    
    int main()
    {
    	_int64 a;
    	
    	a = 1000I64 * 1000 * 1000 * 1000;
    	printf("%I64d\n", a);
    	return 0;
    }
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  6. #6
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    Quote Originally Posted by matsp View Post
    What does your code look like?

    The following works for MS VC.Net:
    Code:
    #include <stdio.h>
    #include <math.h>
    
    int main()
    {
    	_int64 a;
    	
    	a = 1000I64 * 1000 * 1000 * 1000;
    	printf("&#37;I64d\n", a);
    	return 0;
    }
    Actually, I have:
    Code:
    # define h (1e-2)
    # define t_min -500.0
    # define t_max 500.0
    # define y_min -500.0
    # define y_max 500.0
    # define lin (unsigned long long)((y_max-y_min)/h+2)
    # define col (unsigned long long)((t_max-t_min)/h+1)
    And in the main()
    Code:
       double *grid;
       grid=malloc(lin*col*sizeof(double));    //line 33
    When I try to compile it, i have:
    blah.c:32: warning: large integer implicitly truncated to unsigned type

    But when I used the lines
    Code:
    # define lin (unsigned long)((y_max-y_min)/h+2)
    # define col (unsigned long)((t_max-t_min)/h+1)
    I got integer overflow on line 33.

    I'm using these macros because y_min/max etc are parameters that i'm constantly changing.

    Thanks

  7. #7
    CSharpener vart's Avatar
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    why to divide on 1e-2 and not multiply on 100? - It can be done using unsigned int arithmetics
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    the last 10% takes the other 90% of the time.

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    Quote Originally Posted by vart View Post
    why to divide on 1e-2 and not multiply on 100? - It can be done using unsigned int arithmetics
    It's because I need h, t_max/min, y_max/min to be double and they're not always cute like 500 and 1e-2...

    Thanks again...

    PS I've just read the following: http://gcc.gnu.org/ml/gcc-bugs/2000-10/msg00479.html

    Can it possibly be my case?

  9. #9
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    Right, you do not want to use long long for malloc - if you ACTUALLY need long long size numbers, you won't be able to malloc that memory anyways!

    Sorry for the slow reply, but I had to double-check the math: You are creating an array that is 100001 * 100002 * 8 bytes long - that is just a few times larger than the absolute amount of memory you can use in a 32-bit OS. So no matter what you do, you can't achieve that unless you get y'self a 64-bit OS. [And even then, you need a fair chunk of RAM in it, like 80GB of RAM - better get one o' them tha' 4 CPU Opteron motherboards with 4 sticks of 8GB per CPU...

    Get the idea that this isn't going to fly?

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    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  10. #10
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    Quote Originally Posted by matsp View Post
    Right, you do not want to use long long for malloc - if you ACTUALLY need long long size numbers, you won't be able to malloc that memory anyways!

    Sorry for the slow reply, but I had to double-check the math: You are creating an array that is 100001 * 100002 * 8 bytes long - that is just a few times larger than the absolute amount of memory you can use in a 32-bit OS. So no matter what you do, you can't achieve that unless you get y'self a 64-bit OS. [And even then, you need a fair chunk of RAM in it, like 80GB of RAM - better get one o' them tha' 4 CPU Opteron motherboards with 4 sticks of 8GB per CPU...

    Get the idea that this isn't going to fly?

    --
    Mats
    LOL, my adviser wouldn't let me use the super... :P
    Can you point me a way to read such a big matrix as a (100e3)x(100e3) of doubles in a ordinary PC?

    Thanks

  11. #11
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    Hi

    I just realized that this matrix is unnecessarily large...
    I can do my stuff with much smaller matrices...

    Thanks anyway!!!

  12. #12
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    Yes, using smaller matrices is the only solution I can think of immediately.

    If you only need some of the values (as in, there is a large amount of the matrix that holds some constant value that you don't actually need to store), you could use a technique called "sparse arrays" or "sparse matrices", where you only store some of the data in the matrix, and other parts (not filled in) are considered a constant value (such as zero, 1.0, -1.0 or whatever is a common value in the matrix). That way, you only need a fraction of the memory, because you are "recycling" the same constant value for all those cells that don't hold a specific value.

    --
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    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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