union、struct对齐

#include<stdio.h>

static union
{
    char c[4];
    unsigned long l;
}endian_test = { {'l','?','?','b'} };

#define ENDIANNESS ( (char)endian_test.l )


union U{
    char s[9]; //size: 9
    int n;     //size: 4 
    double d;  //size: 8
};             //size: 16

union 的最小的size是所包含的所有类型的基本长度的最小公倍数才行。


struct S
{
    char s[9]; //size: 9
    int n;     //size: 4
    double d;  //size: 8
};             //size: 24


int main()
{

    printf( "sizeof int = [%ld]
", sizeof( int  ) );
    printf( "sizeof unsigned long = [%ld]
", sizeof(unsigned long ) );
    printf( "sizeof double = [%ld]
", sizeof( double  ) );
    printf( "sizeof union U = [%ld]
", sizeof( union U ) );
    printf( "sizeof struct S = [%ld]
", sizeof( struct S ) );
    
    printf( "ENDIANESS = %c
", ENDIANNESS );

}

Detect endianness at run-time

 1 #include <stdint.h>
 2 
 3 enum {
 4   ENDIAN_UNKNOWN,
 5   ENDIAN_BIG,
 6   ENDIAN_LITTLE,
 7   ENDIAN_BIG_WORD,   /* Middle-endian, Honeywell 316 style */
 8   ENDIAN_LITTLE_WORD /* Middle-endian, PDP-11 style */
 9 };
10 
11 int endianness(void)
12 {
13   union
14   {
15     uint32_t value;
16     uint8_t data[sizeof(uint32_t)];
17   } number;
18 
19   number.data[0] = 0x00;
20   number.data[1] = 0x01;
21   number.data[2] = 0x02;
22   number.data[3] = 0x03;
23 
24   switch (number.value)
25   {
26   case UINT32_C(0x00010203): return ENDIAN_BIG;
27   case UINT32_C(0x03020100): return ENDIAN_LITTLE;
28   case UINT32_C(0x02030001): return ENDIAN_BIG_WORD;
29   case UINT32_C(0x01000302): return ENDIAN_LITTLE_WORD;
30   default:                   return ENDIAN_UNKNOWN;
31   }
32 }