Data type consistency

This is a discussion on Data type consistency within the C Programming forums, part of the General Programming Boards category; Why when we allocating memory block using (ma|re|c)alloc we use sizeof([count *] primitive data type)? E.g. Code: int *a = ...

  1. #1
    Ugly C Lover audinue's Avatar
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    Question Data type consistency

    Why when we allocating memory block using (ma|re|c)alloc we use sizeof([count *] primitive data type)?

    E.g.
    Code:
    int *a = malloc(5 * sizeof(int)); //allocating 5 blocks of int
    Instead of
    Code:
    #define INT_SIZE 4
    int *a = malloc(5 * INT_SIZE);
    But, I'm not talking about
    Code:
    #define int char
    Is there possibility that sizeof(int) could be changed to (for example) 2 or 16?

  2. #2
    The larch
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    Yes, the size of int may be different on different platforms.

    Incidentally I believe this is even more better:
    Code:
    int* p = malloc(5 * sizeof(*p));
    This way, should you change the variable type you won't end up with:
    Code:
    short* p = malloc(5 * sizeof(int));
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
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  3. #3
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    The size of int depends on the processor (changes between different architectures - 16bit processors = 2 bytes, 32bit = 4 bytes, 64bit = 8 bytes, and this will be the size of the memory registers and others)
    Although some people told me it depends on the compiler (they showed me a proof - compiled sizeof(int) in Borland CPP and on Dev CPP and the results were different - 2 for Borland and 4 for Dev CPP)

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    Quote Originally Posted by eXeCuTeR View Post
    Although some people told me it depends on the compiler (they showed me a proof - compiled sizeof(int) in Borland CPP and on Dev CPP and the results were different - 2 for Borland and 4 for Dev CPP)
    I would imagine that the Borland compiler was of the fossil variety (compiled only 16-bit executables). Which is why sizeof(int) was 2.

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    Quote Originally Posted by swoopy View Post
    I would imagine that the Borland compiler was of the fossil variety (compiled only 16-bit executables). Which is why sizeof(int) was 2.
    Do you have any proof that it depends on the processor? I gotta figure what's right.

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    It depends more on the compiler than the processor. Sure, different processors have different optimal data sizes, but it's the compiler that makes the decisions. For example, 32-bit processors can use 64-bit data types, too, just a bit slower.

    On all current x86-64 compilers I know and/or have used, both on 64-bit Windows and 64-bit Linux, sizeof(int) is still 4, for backward compatibility.

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    It should depend on both, but int is intended to be the "native" integer size -- that is the size that the processor is built to handle most efficiently. For your typical Intel x86 processor that's 4 bytes. But it is up to the compiler to determine what that native size is, and principally it could be configured differently than expected.
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  8. #8
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    Quote Originally Posted by anon View Post
    Yes, the size of int may be different on different platforms.

    Incidentally I believe this is even more better:
    Code:
    int* p = malloc(5 * sizeof(*p));
    Unless p == NULL, in which case *p will blow up.

  9. #9
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    why so?

    Code:
    #include <stdio.h>
    
    int main() {
            int *p = NULL;
            printf("&#37;d", sizeof(*p)); // 4
    }
    works for me. (32-bit Linux, gcc 4.1.2)

  10. #10
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    Quote Originally Posted by cpjust View Post
    Unless p == NULL, in which case *p will blow up.
    What happens if p points to null? *p will still remain as an int, wouldn't it?

  11. #11
    Deathray Engineer MacGyver's Avatar
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    The sizeof operator is special in this case, I believe, since it's macro. No actual dereferencing is really taking place afaik.

  12. #12
    int x = *((int *) NULL); Cactus_Hugger's Avatar
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    Quote Originally Posted by cpjust View Post
    Unless p == NULL, in which case *p will blow up.
    sizeof is a language keyword, and not a function (it's an operator). Specifically,
    Quote Originally Posted by Standard, C89
    The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand, which is not itself evaluated. The result is an integer constant.
    sizeof(*p) will work as it should.
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  13. #13
    and the hat of int overfl Salem's Avatar
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    > #define INT_SIZE 4
    Well if you did
    #define INT_SIZE sizeof(int)
    It would be better.

    > Unless p == NULL, in which case *p will blow up.
    The only 'dereference' is through the compiler's expression tree to find the type of *p, and thus work out the size of the required object. Nothing happens at run-time, except to multiply by the constant which the compiler determined to be the size of a *p
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  14. #14
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    Quote Originally Posted by Salem View Post
    > Unless p == NULL, in which case *p will blow up.
    The only 'dereference' is through the compiler's expression tree to find the type of *p, and thus work out the size of the required object. Nothing happens at run-time, except to multiply by the constant which the compiler determined to be the size of a *p
    Thanks, I'll have to test that and see, but it's good to know.

  15. #15
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    Thanks, I'll have to test that and see, but it's good to know.
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