Sizeof union larger than expected. how does type alignment work here?

Question

#include <stdio.h> union u1 { struct { int *i; } s1; struct { int i, j; } s2; }; union u2 { struct { int *i, j; } s1; struct { int i, j; } s2; }; int main(void) { printf(" size of int: %zu\n", sizeof(int)); printf("size of int pointer: %zu\n", sizeof(int *)); printf(" size of union u1: %zu\n", sizeof(union u1)); printf(" size of union u2: %zu\n", sizeof(union u2)); return 0; } 

Results in:

 $ gcc -O -Wall -Wextra -pedantic -std=c99 -o test test.c $ ./test size of int: 4 size of int pointer: 8 size of union u1: 8 size of union u2: 16 

Why does adding an integer of 4 bytes to the nested structure s1 of the union u2 increase the size of the whole by 8 bytes?

Answer 1

The structure of u2.s2 is 16 bytes due to alignment restrictions. The compiler ensures that if you create an array of such structures, each pointer will be aligned on an 8-byte boundary. The *i field takes 8 bytes, then j takes 4 bytes, and the compiler inserts 4 bytes of padding. Because the structure has 16 bytes, the union containing it is also 16 bytes.

Answer 2

This is because the compiler must support the entire structure (as well as the union) aligned to 8 bytes, because there is a pointer inside. (which in your case is 8 bytes)

Thus, even if you add only 4 bytes with an extra int , aligning the structure causes everything to be aligned to 8 bytes - hence +8, to bring the total size to 16 bytes.

The result is the following:

 struct { int *i, j; } s1; 

has a size of 16 bytes. Since the union must be no less than the largest element, it is also forced to 16.

http://en.wikipedia.org/wiki/Data_structure_alignment

Answer 3

Since the pointer on your platform takes 8 bytes, it is likely that alignment of 8 bytes is also required. Therefore, when adding 4 more bytes, the structure cannot be 12 bytes in size, and in this case, individual elements, for example, the u2 array, will not be correctly aligned with the boundaries of 8 bytes, which is necessary for the pointer element. Therefore, you need to increase its size to the next multiple of 8, which is 16. The additional 4 bytes are simply not used / undefined.