Size of struct in C not as expected

Asked Mar 02 '17 at 02:06

Active Mar 02 '17 at 02:06

Viewed 42 times

I'm doing a sample test question in C and can't explain the 'right answer'. The question is what is the output of:

#include<stdio.h>
#include<stdlib.h>
struct st{
    int a;
    char b;
};

int main()
{
    struct st *st_ptr;
    st_ptr = malloc(sizeof(struct st));
    printf("%d\n",sizeof(struct st));
    return 0;
}

And possible answers

a) 8
b) 5
c) 0
d) none of the mentioned

I figured the answer was B (4 bytes + 1 byte) but they say the right answer is A (8 bytes). Their explanation is: Maximum size of the data type is 4 byte(int) in the structure.

Can someone perhaps explain the right answer better? Or is their right answer actually wrong.

asked Mar 02 '17 at 02:06

TSG

4,242
9
61
121

4

Lookup padding. – R Sahu Mar 02 '17 at 02:08
Reply to this [question](http://stackoverflow.com/questions/6968468/padding-in-structures-in-c) will give you more clarity. – Milind Deore Mar 02 '17 at 02:12
Try adding `__attribute__((__packed__))` between `struct` and your struct name. Assuming you are using gcc of course. – Paul Rooney Mar 02 '17 at 02:13
@PaulRooney: That would limit between field padding, but it wouldn't prevent trailing padding to align the whole structure; that's necessary for the array scenario described by yellowantphil. – ShadowRanger Mar 02 '17 at 02:20
sure, but it would give him the 5 he had expected to see for a single element. – Paul Rooney Mar 02 '17 at 02:22
@yellowantphil this case it's because of padding at the end, not alignment – phuclv Mar 02 '17 at 02:41
@TSG `sizeof` returns `size_t` which [must be printed using `%zu`, not `%d`](http://stackoverflow.com/q/940087/995714) – phuclv Mar 02 '17 at 02:41

Size of struct in C not as expected

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