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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)

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.

Re: How to calculate the size of struct?

Thank you xpi0t0s again!,I get it!~~haha