union

1、union是为了让【不同的数据项】存放到【同一段内存】空间中。
2、union数据结构中的多个成员共用一段内存空间，空间大小由最大的成员决定。
3、union使用的范例如下：

  1、 union test    {        unsigned int a;        unsigned short b;        unsigned char c;    }union_data;   printf("size: %d\n",sizeof(union_data));   union_data.a = 0x11223344;        //内存中  44   33  22 11   union_data.b = 0x5566;            //内存中 (66) (55) 22 11   union_data.c = 0x88;              //内存中 (88)  55  22 11   printf("%x   %x   %x\n",union_data.a,union_data.b,union_data.c);

2、struct TypeData{    int type;  //用来区分union test1中使用哪个结构体   union test1   {       struct data1{           unsigned char  a;           unsigned char  b;           unsigned int   c;       }d1;       struct data2{           unsigned char  a;           unsigned char  b;       }d2;       struct data3{           unsigned int   c;       }d3;   }union_data1;}* p_struct_data1;void ShowValue(struct TypeData* data){    switch(data->type)    {        case 0x01:            printf("%x %x %x\n",data->union_data1.d1.a,data->union_data1.d1.b,data->union_data1.d1.c);            break;        case 0x02:            printf("%x %x\n",data->union_data1.d2.a,data->union_data1.d2.b);            break;        case 0x03:            printf("%x\n",data->union_data1.d3.c);            break;        default:            break;    }}   struct TypeData*  p_struct_data1;   printf("size: %d\n",sizeof(*p_struct_data1)); //12   unsigned  int data01[3] = {0x01,0x44332211,0x88776655};   unsigned  int data02[3] = {0x02,0x44332211,0x88776655};   unsigned  int data03[3] = {0x03,0x44332211,0x88776655};   p_struct_data1 = (struct TypeData *)&data01;   ShowValue(p_struct_data1);   printf("--------------------------------\n");   p_struct_data1 = (struct TypeData *)&data02;   ShowValue(p_struct_data1);   printf("--------------------------------\n");   p_struct_data1 = (struct TypeData *)&data03;   ShowValue(p_struct_data1);   printf("--------------------------------\n");