sizeof an array of structure without using 'sizeof' operator


 
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# 1  
Old 03-30-2009
sizeof an array of structure without using 'sizeof' operator

Hi All,

is it possible to find out the size of an array of structures ( without using 'sizeof' operator). The condition is we have the array of structure instant but we are not aware of the elements inside the structure.

Can someone help me out?


Thanks in advance.
# 2  
Old 03-30-2009
Yes, I guess you can do; something like

Code:
#pragma pack(1)
#include <header.h> // Header file which contain opaque structure definition
int main()
{
     char begin;
     struct unknown_structure array[100];
     char end;

     printf("Size of structure = %u ", (&begin > &end) ? (&begin - &end - 1)/100 : (&end - &begin - 1)/100);

     return 0;
}

Let me know the output.
This may not be accurate, refer to alignment and structure padding
# 3  
Old 03-30-2009
I was hoping it should produce a correct number, but that would happen in absence of processors not needing any padding at the end of structures
# 4  
Old 03-30-2009
Not aware of the size of the structure, as in, it contains elements that the compiler has not seen defined?

sizeof doesn't work on those because it really, genuinely doesn't know the size. You can't trick it into giving you information it does not possess; you can't make anything but pointers to those kind of types, and it certainly won't know the size of anything but the pointer.

Otherwise, why not use sizeof? That's what its there for.
# 5  
Old 03-30-2009
Quote:
Originally Posted by rvan
Hi All,

is it possible to find out the size of an array of structures ( without using 'sizeof' operator). The condition is we have the array of structure instant but we are not aware of the elements inside the structure.

Can someone help me out?


Thanks in advance.
Try this -

Code:
int main()
{
     struct whatever w;
     char s;

      if( (char*)&w > & z)
        printf("Size of whatever %d\n",(char*)&w-&z)
      else
        printf("Size of whatever %d\n",(char*)&w-&z)
}

# 6  
Old 03-30-2009
Thank you pshaikh. it exactly gives the sizeof the structure. While executing your first sample piece of code... it is mostly &begin is greater than &end. Here all the local variables begin, end and array[100] gets stored in the stack. When you subtract the begin address and end address and divide it by the sizeof the array it gives the sizeof structure.

But if i try to subtract address of say &array[1] - &array[0] doesn't give the sizeof the array instead it gives 1 always. I just tried printing &array[1] and &array[0] which gives the address. When we subtract the address of begin and end variable it gives the number of bytes occupied between those two and why not in &array[1] - &array[0]. Kindly clarify.


Thanks
# 7  
Old 03-30-2009
This is because the compiler has incorrect knowledge about the structure.

Are you casting the array from void * to the right structure type?
Is the structure datatype declared somewhere that you can reference it?

Please post the first few lines of the function that is trying to find the size of the struct.
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