为什么打包不能在同级联合或结构中起作用 [英] Why does packing not work across sibling unions or structs

问题描述

在下面的示例中，我希望 complex_t 的大小与 uint16_t 相同：2个字节，但是这是3个字节。

In the following example I expect the size of complex_t to be the same as uint16_t: 2 bytes, however it's 3 bytes.

删除第二个联合（ proximity_unsafe）可将大小减小到2个字节，但我无法弄清楚打包规则的模型。

Removing the second union ("proximity_unsafe") reduces the size to 2 bytes, but I can't figure out the model of the packing rules.

#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)); 
}

推荐答案

因为合法服用不是位域的任何命名结构成员的地址，此类非位域成员必须从字节边界开始。尽管不可能获取匿名成员的地址，因此从理论上讲，编译器可以允许此类对象从任意位边界开始，但这将暗示结构的布局将根据其成员是否不同而有所不同。被命名。

Because it is legal to take the address of any named struct member that is not a bitfield, such non-bitfield members are required to start at byte boundaries. While it is not possible to take the address of anonymous members, and it would thus theoretically be possible for a compiler to allow such objects to start at arbitrary bit boundaries, that would imply that the layout of a structure would vary based upon whether its members were named.

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