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-   -   How to calculate the size of struct? (http://www.go4expert.com/forums/calculate-size-struct-t28025/)

wdliming 15Mar2012 17:19

How to calculate the size of struct?
 
hello!guys,recent days ,i come across a problem when i write a C program,that is "How to calculate the size of struct ",i have a part of C source code below:
Code:

#include <stdio.h>
 struct b
{
    float f;   
    int a[2];
    char p;
};

  int main(void)
{
    printf("sizeof(struct(b))=%d\n",sizeof(struct b));
    return 0;
}

After compiling using GCC 4.6.1,the result is
sizeof(struct(b))=16
then ...
I think the
= (sizeof(float)) + 2*(sizeof(int)) + (sizeof(char))
=4+2*4+1
=4+8+1
=13 not equal 16.

oh my god !!
Could some one tell me why?thank you very much! ^@^
(compile environment Windows XP sp3,TDM-GCC 4.6.1,X86 architecture)

xpi0t0s 16Mar2012 00:15

Re: How to calculate the size of struct?
 
Padding. In short, each item of a different type is on a 4-byte boundary. So char p effectively takes up 4 bytes (well, it doesn't because it's a char, but the other 3 bytes are not used.)

If you had
Code:

float f;
int a[2];
char p,q,r;

then this would probably also be 16. chars don't have to be on 4-byte boundaries so p,q,r use 3 of those 4 bytes.

This isn't something you should worry about; padding is very rarely an issue, and by the time it is, you'll know enough about programming to know why and how to solve it.

wdliming 16Mar2012 13:52

Re: How to calculate the size of struct?
 
Thank you xpi0t0s again!,I get it!~~haha


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