______________在多个变量的定义中 [英] __attribute__ in definitions of multiple variables

问题描述

我有一个最好的例子可以解释的问题.请考虑以下代码:

I have a question which is best explained by example. Please consider the following code:

unsigned char a,
              b;

这显然定义了两个类型为unsigned char的变量.

This obviously defines two variables of type unsigned char.

如果我想使变量与16字节边界对齐，那么我的第一个幼稚方法就是:

If I would like to make the variables aligned to 16-byte-boundaries, my first naive approach would be this:

 __attribute__((aligned(16))) unsigned char a,
                                            b;

我的问题是我不确定编译器是否始终将__attribute__((aligned(16)))应用于两个变量.

My problem is that I am not sure whether the compiler always applies __attribute__((aligned(16))) to both variables.

我特别担心，因为以下所有代码的编译均没有错误或警告:

I am particularly worried because all of the following code is compiled without errors or warnings:

unsigned char a __attribute__((aligned(16)));
unsigned char __attribute__((aligned(16))) b;
__attribute__((aligned(16))) unsigned char c;

根据我的研究，__attribute__((aligned(16)))对上面三行中的各个变量都做相同的操作.但是，如此弱的语法对于C语言来说并不常见，所以我有点不信任.

According to my research, __attribute__((aligned(16))) does the same to the respective variable in the three lines above. But such a weak syntax would be unusual for C, so I am somehow mistrustful.

回到我原来的问题，我知道我可以通过类似的方法轻松避免不确定性

Returning to my original problem, I am aware that I easily could avoid the uncertainty by something like

 __attribute__((aligned(16))) unsigned char a;
 __attribute__((aligned(16))) unsigned char b;

或者也许

 unsigned char a __attribute__((aligned(16))),
               b __attribute__((aligned(16)));

但是我真的很想知道在声明多个 all 应该具有属性的变量时，添加__attribute__装饰一次是否足够.

But I really would like to know whether it is sufficient to add the __attribute__ decoration once when declaring multiple variables which all should have the attribute.

当然，该问题涉及所有属性(不仅是aligned属性).

Of course, that question relates to all attributes (not only the aligned attribute).

作为一个额外的问题，将这样的属性不仅添加到变量定义中，而且还添加到变量声明中(例如，头文件中)，是否被认为是一种好风格?

As a bonus question, is it considered good style to add such attributes not only to the variable definitions, but also to the variable declarations (e.g. in header files)?

推荐答案

是；两者

__attribute__((aligned(16))) unsigned char   a, b;

和

unsigned char __attribute__((aligned(16)))    a, b;

将a和b对齐到16个字节的边界. gcc将__attribute__作为类型的一部分(例如const和volatile修饰符)进行处理，以便将诸如

align a and b to 16 byte boundary. gcc handles __attribute__ as part of the type (like const and volatile modifiers) so that mixed things like

char * __attribute__((__aligned__(16))) *  a;

也是可能的.

https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax 说:

属性说明符列表可能紧接在逗号，=或分号之前，终止了除函数定义以外的标识符声明.此类属性说明符适用于已声明的对象或函数

An attribute specifier list may appear immediately before the comma, = or semicolon terminating the declaration of an identifier other than a function definition. Such attribute specifiers apply to the declared object or function

这就是为什么

unsigned char   a __attribute__((aligned(16))), b;

仅适用于a，而不适用于b.

would apply to a only but not to b.

在另一种情况下

unsigned char   a, __attribute__((aligned(16))) b;

仅b对齐.在这里

在以逗号分隔的声明符列表中，属性说明符列表可能紧接在声明符(而不是第一个)之前出现...此类属性说明符仅适用于在它们出现在其声明符之前的标识符

An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators ... Such attribute specifiers apply only to the identifier before whose declarator they appear

来自 https://stackoverflow.com/a/31067623/5639126 .

为避免所有歧义，最好创建一个新类型并使用它.例如

To avoid all the ambiguities, it would be better to create a new type and use this. E.g.

typedef char __attribute__((__aligned__(16)))   char_aligned_t;
char_alignedt d, d1;

通过此示例和您的

unsigned char a __attribute__((aligned(16))), a1;
unsigned char __attribute__((aligned(16))) b, b1;
__attribute__((aligned(16))) unsigned char c, c1;

gcc创建(gcc -c)，并且readelf显示所描述的对齐方式

gcc creates (gcc -c) and readelf shows the described alignments

     8: 0000000000000010     1 OBJECT  GLOBAL DEFAULT  COM a
     9: 0000000000000001     1 OBJECT  GLOBAL DEFAULT  COM a1     <<< not aligned!
    10: 0000000000000010     1 OBJECT  GLOBAL DEFAULT  COM b
    11: 0000000000000010     1 OBJECT  GLOBAL DEFAULT  COM b1
    12: 0000000000000010     1 OBJECT  GLOBAL DEFAULT  COM c
    13: 0000000000000010     1 OBJECT  GLOBAL DEFAULT  COM c1
    14: 0000000000000010     1 OBJECT  GLOBAL DEFAULT  COM d
    15: 0000000000000010     1 OBJECT  GLOBAL DEFAULT  COM d1

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