C-language structural body alignment

#include  <stdio.h>//gcc ubuntu 32-#pragma pack (8) typedef struct m{     char a ;    long b ;}  m;typedef struct n{    char a ;    char  b ;    long c ;}  N;typedef struct x{    char      a ;     struct m  b ;    long long c ;}  X;typedef struct y{    char      a ;     struct n  b ;    long long c ;}  Y; #pragma  pack () #define &NBSP;&NBSP;ADDR (ADDR)    ((Unsigned int) (ADDR)) Void main ( void) {    printf ("sizeof (char):%d  sizeof (short):%d  sizeof (int):%D sizeof (Long):%d sizeof (Long long):%d\n ",             sizeof (char), sizeof (short), sizeof (int), sizeof (long), sizeof (Long long));     printf ("sizeof (M):%d  sizeof (N):%d sizeof (X):%d  sizeof (Y):%d\n", sizeof (M), sizeof (N), sizeof (X), sizeof (Y))  ;        M m ;     n n ;    x x ;    y y  ;        printf ("m.a:%d m.b:%d \n", ADDR (&AMP;M.A)-ADDR (&m), ADDR (&AMP;M.B)-addr (&m));     printf ("n.a:%d n.b:%d  n.c:%d  \n ", ADDR (&AMP;N.A)-addr (&n), ADDR (&n.b)-addr (&n), ADDR (&AMP;N.C)-addr (&n));        printf ("x.a:%d x.b.a:%d  x.b.b:%d x.c:%d\n", ADDR (&AMP;X.A)- ADDR (&x), ADDR (&AMP;X.B.A)-ADDR (&x), ADDR (&x.b.b)-addr (&x), ADDR (&AMP;X.C)-addr (&x));     printf ("y.a:% D y.b.a:%d  y.b.b:%d y.b.c:%d y.c:%d\n ", ADDR (&AMP;Y.A)-addr (&y), ADDR (&AMP;Y.B.A)- ADDR (&y), ADDR (&y.b.b)-addr (&y), ADDR (&AMP;Y.B.C)-addr (&y), ADDR (&AMP;Y.C)-addr (&y));                  return ;} /* Program run Result: sizeof (char): 1  sizeof (short): 2  sizeof (int): 4 sizeof (Long): 4 sizeof ( Long long): 8sizeof (M): 8  sizeof (N): 8 sizeof (X): 20  sizeof (Y): 20m.a:0 m.b:4  n.a:0 n.b:1  n.c:4 x.a:0 x.b.a:4  x.b.b:8 x.c:12// The x.c here do not have an address that is divisible by 8                 Does the               //system press 4 bytes? 32-bit system 8-byte alignment doesn't make sense?                        &NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;//32-bit system with 8 bytes of data in 8-byte alignment, memory read                                //is also read multiple times, wasting space on it to 8 bytes, and does not speed reading speed                                //because 32 is a system, the data bus is 32 bits, at a time can only fetch 4                                //bytes of data, 8 bytes of data, aligned to 4 bytes and aligned to 8 bytes are the same                               // The hardware circuitry of the memory determines that the maximum is 4 byte aligned, and the alignment greater than 4 bytes isMeaningless y.a:0 y.b.a:4  y.b.b:5 y.b.c:8 y.c:12//here the y.c did not *//* to the address which divisible by 8 #pragma  Pack (8)    change to   #pragma  pack (1) program run Result: sizeof (char): 1  sizeof (short):2   sizeof (int): 4 sizeof (Long): 4 sizeof (Long long): 8sizeof (M): 5  sizeof (N): 6 sizeof ( X): 14  sizeof (Y): 15m.a:0 m.b:1 n.a:0 n.b:1  n.c:2 x.a:0 x.b.a:1   x.b.b:2 x.c:6y.a:0 y.b.a:1  y.b.b:2 y.b.c:3 y.c:7*//* put #pragma  Pack (8)    change to   #pragma  pack (4) program run Result: sizeof (char): 1  sizeof (short):2   sizeof (int): 4 sizeof (Long): 4 sizeof (Long long): 8sizeof (M): 8  sizeof (N): 8 sizeof ( X): 20  sizeof (Y): 20m.a:0 m.b:4 n.a:0 n.b:1  n.c:4 x.a:0 x.b.a:4   x.b.b:8 x.c:12y.a:0 y.b.a:4  y.b.b:5 y.b.c:8 y.c:12*/

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C-language structural body alignment