结构填充编译标志 [英] structure padding compile flag

问题描述

typedef struct{
    char        t1;
    long long   t2;
    char        t3;
}struct_size_test;

printf("sizeof(long long)==[%ld]\n", sizeof(long long));
printf("sizeof(char)==[%ld]\n", sizeof(char));
printf("sizeof(struct_size_test)==[%ld]\n", sizeof(struct_size_test));

在iOS和OS X gcc中struct_size_test的结果不同。

在iOS中的结果是

The result of struct_size_test in iOS and in OS X gcc is different.
The result in iOS is

sizeof(long long)==[8]
sizeof(char)==[1]
sizeof(struct_size_test)==[24]

gcc is

sizeof(long long)==[8]
sizeof(char)==[1]
sizeof(struct_size_test)==[16]

我googled这个，添加-malign-double作为gcc编译标志。
但它显示

I googled this and found some advice that to add "-malign-double" as the gcc compile flag. But it shows

clang: error: unknown argument: '-malign-double' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future

任何想法如何同步iOS和OS X的结构填充？
非常感谢！

Any idea how to sync the structure padding for iOS and OS X? Thanks a lot!

推荐答案

我认为答案应该是你绝对不想这样做。

I think the answer should be that you most certainly don't want to do this.

您可以尝试nneonneo的解决方案或添加 __属性__（（packed））到您的结构，相同大小的OS X和iOS，但记住这是一个编译器特定的功能，将使你的代码更少便携

You could try nneonneo's solution or add __attribute__((packed)) to your structs to make the structs the same size on both OS X and iOS but keep in mind this is a compiler specific feature and will make your code less portable

此外，如果你改变了填充，你的程序可能会崩溃或运行效率较低的架构（特别是iOS / ARM）由于对齐要求不符合。有些ARM架构（不确定iDevices）不允许未对齐的读/写操作，如果不运行速度更慢，会直接崩溃（因为需要两次内存读取，而不是一次）。

Furthermore, if you did change the padding, your program would likely crash or run less efficiently on either architectures (especially iOS/ARM) due to alignment requirements not being met. Some ARM architectures (unsure about iDevices) won't allow unaligned reads/writes and will outright crash if not run slower (since two memory reads are required instead of one).

解决方法可能是使用最高的公共对齐方式（显示为8），这只需要更多的空间。

A workaround might be to use the highest common alignment (appears to be 8), which would simply take a bit more space. But this is still hacky and non-portable.

此外，为什么要这样做呢？

Why are you trying to do this anyway?

：nneonneo的答案应该看起来像这样：

nneonneo's answer probably should look like this:

typedef struct{
    char        t1 __attribute__ ((aligned (8)));
    long long   t2 __attribute__ ((aligned (8)));
    char        t3 __attribute__ ((aligned (8)));
} struct_size_test;

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