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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

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    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)
     
    Last edited: Mar 15, 2012
  2. xpi0t0s

    xpi0t0s Mentor

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    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.
     
  3. wdliming

    wdliming New Member

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

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