C 为什么打包不能跨同级联合或结构工作
在下面的示例中,我希望C 为什么打包不能跨同级联合或结构工作,c,c99,unions,bit-fields,bit-packing,C,C99,Unions,Bit Fields,Bit Packing,在下面的示例中,我希望complex\u t的大小与uint16\u t的大小相同:2个字节,但它是3个字节 删除第二个并集(“接近度_unsafe”)会将大小减少到2个字节,但我无法理解打包规则的模型 #include <stdint.h> #include <stdio.h> typedef union { uint16_t unsafe; struct { uint16_t backwardmotion_unsafe : 1;
complex\u t
的大小与uint16\u t
的大小相同:2个字节,但它是3个字节
删除第二个并集(“接近度_unsafe”)会将大小减少到2个字节,但我无法理解打包规则的模型
#include <stdint.h>
#include <stdio.h>
typedef union {
uint16_t unsafe;
struct {
uint16_t backwardmotion_unsafe : 1;
uint16_t batteryvoltage_unsafe : 1;
union {
uint16_t dropoff_unsafe : 4;
struct {
uint16_t dropofffrontleft_unsafe : 1;
uint16_t dropofffrontright_unsafe : 1;
uint16_t dropoffsideleft_unsafe : 1;
uint16_t dropoffsideright_unsafe : 1;
}__attribute__((__packed__));
}__attribute__((__packed__));
union {
uint16_t proximity_unsafe : 3;
struct {
uint16_t proximityfront_unsafe : 1;
uint16_t proximityleft_unsafe : 1;
uint16_t proximityright_unsafe : 1;
}__attribute__((__packed__));
}__attribute__((__packed__));
} __attribute__((__packed__));
} __attribute__((__packed__)) complex_t;
int main()
{
printf("sizeof(complex_t): %i", sizeof(complex_t));
printf("sizeof(uint16_t): %i", sizeof(uint16_t));
}
#包括
#包括
typedef联合{
uint16不安全;
结构{
uint16\u t后移不安全:1;
uint16电池电压不安全:1;
联合{
uint16_t下车不安全:4;
结构{
uint16_t dropofffront left_不安全:1;
uint16_t dropofffrontright_不安全:1;
uint16\u t dropoffsideleft\u不安全:1;
uint16\u t dropoffsideright\u不安全:1;
}__属性_uuu((uuu-packed_uu));
}__属性_uuu((uuu-packed_uu));
联合{
uint16不安全:3;
结构{
uint16_t近处前方不安全:1;
uint16_t proximityleft_不安全:1;
uint16_t proximityright_不安全:1;
}__属性_uuu((uuu-packed_uu));
}__属性_uuu((uuu-packed_uu));
}_uuu属性_uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu;
}_uuu属性uuuuuuuuuuuu(uuuu压缩uuuuuuuuu))复合物ut;
int main()
{
printf(“sizeof(complex_t):%i”,sizeof(complex_t));
printf(“sizeof(uint16_t):%i”,sizeof(uint16_t));
}
因为获取任何非位域的命名结构成员的地址是合法的,所以此类非位域成员必须从字节边界开始。虽然不可能获取匿名成员的地址,因此理论上编译器可以允许此类对象从任意位边界开始,但这意味着结构的布局将根据其成员是否命名而有所不同。这是一个很好的解释!