为什么结构对齐方式在iOS中有所不同 [英] Why struct alignment different in iOS

问题描述

我已经将Xcode中的结构对齐方式设置为8字节压缩.例如:

I have set the struct alignment in Xcode to 8-byte packing. For example :

struct Info
{
    unsigned short  kindID;
    char            kindName[32];
    int64_t         gameMonery;
}__attribute__ ((aligned(8)));

在iPhone5和iPhone 5S中，此结构大小均为48字节， kindName 地址偏移量为2字节，但在iPhone5中， gameMoney 偏移量为36字节，在iPhone 5S的 gameMoney 偏移量为40字节.

This struct size is 48 byte, both in iPhone5 and iPhone 5S, the kindName address offset is 2 byte, but in iPhone5 the gameMoney offset is 36 byte, in iPhone 5S the gameMoney offset is 40 byte.

为什么有这个不同?

推荐答案

64位类型( int64_t )的对齐方式在32位平台上为4，在64位平台上为8.

The alignment of 64 bit types (int64_t) is 4 on 32 bit platforms and 8 on 64 bit platforms.

通过在声明中添加 __ attribute __((packed))，可以使编译器禁止单个元素的对齐.未结盟成员的访问速度稍慢.

You can make the compiler suppress alignment of individual elements by adding __attribute__((packed)) to the declaration. Access of unaligned members is somewhat slower.

两个平台上结构的大小之所以没有不同的原因是，其大小取决于结构的对齐方式(如8所示):

The reason why the size of the struct does not differ on both platforms is that the size is subject to the struct's alignment (as specified to 8):

示例:

struct { char i; } __attribute__((aligned(8))) foo;
sizeof(foo) == 8;

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