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Without using sizeof how do i calculte structure size

Posted on 2008-06-11
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Last Modified: 2012-05-05
Hi,
I have one structure having two int and a char. how do i calculate size of the structure?
thanks,
santanu
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Question by:arijit_rebaca
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9 Comments
 
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Expert Comment

by:evilrix
ID: 21758843
>> I have one structure having two int and a char.
The size of these is platform dependent (their size isn't defined in the standard)

>> how do i calculate size of the structure?
At best you can approximate it, by adding together the (assumed) size of the types it aggregates but structs can also contain padding and unless you also know how this has been implemented you'll not necessarily get the right result. In other words the sum of the parts might be less than the whole.
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by:Infinity08
ID: 21758851
sizeof is the easiest way :

        typedef struct Test {
            int i;
            char c;
        } Test;

        sizeof(Test);

Other methods will not be reliable due to padding that might be added to the struct.
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by:Infinity08
ID: 21758858
Any reason you don't want to use sizeof ?
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Accepted Solution

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peetm earned 500 total points
ID: 21759291
#include <stdio.h>

int main(void)
{
    struct thing
    {
        int     a;
        char    b;
        float   c;
    };
   
    struct thing t;

    struct thing * ptr = &t;

    printf("%u\n", (char *)(ptr + 1) - (char *)ptr);

    return 0;
}
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Expert Comment

by:Infinity08
ID: 21759435
>> (char *)(ptr + 1)

Hehe ... that's cheating lol ... But true : sizeof is not used explicitly.
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Expert Comment

by:peetm
ID: 21759455
>>Hehe ... that's cheating lol ... But true : sizeof is not used explicitly.

:-)

And, one would not of course want to have a struct larger than ptrdiff_t, and, I'm not so sure about using 'u' either - still I reckon it's the kind of thing that someone's got in mind here.
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Expert Comment

by:Infinity08
ID: 21759491
>> still I reckon it's the kind of thing that someone's got in mind here.

There's a good chance of that :)
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