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How to calculate the size of struct?

Discussion in 'C' started by wdliming, Mar 15, 2012.

  1. wdliming

    wdliming New Member

    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:
     #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
    then ...
    I think the
    = (sizeof(float)) + 2*(sizeof(int)) + (sizeof(char))
    =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)
    Last edited: Mar 15, 2012
  2. xpi0t0s

    xpi0t0s Mentor

    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
    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.
  3. wdliming

    wdliming New Member

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

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