[英]struct alignment on a 64-bit machine
I have the following struct on a 64-bit Linux machine. 我在64位Linux机器上有以下结构。
struct __wait_queue_head {
spinlock_t lock;
struct list_head task_list;
};
where
typedef struct {
raw_spinlock_t raw_lock;
} spinlock_t;
and
struct list_head {
struct list_head *next, *prev;
};
raw_spinlock_t is defined as:
typedef struct {
volatile unsigned int slock;
} raw_spinlock_t;
Now I want to understand the alignment of the struct __wait_queue_head on a 64-bit Linux machine following the LP64 standard. 现在我想了解遵循LP64标准的64位Linux机器上struct __wait_queue_head的对齐方式。 From what I know, since the first field of this struct ie.
据我所知,自从这个结构的第一个领域即。
spinlock_t lock
is an unsigned int, which occupies 4 bytes on a 64-bit machine, this struct should begin at a 4-byte aligned address. 是一个无符号整数,在64位机器上占用4个字节,这个结构应该从一个4字节对齐的地址开始。 However, I have seen that is not the case on a real system.
但是,我已经看到在真实系统上并非如此。 Instead, the struct begins at an 8 byte aligned address, although the alignment requirement of the first field would have been met by a 4 byte aligned address.
相反,结构开始于8字节对齐的地址,尽管第一个字段的对齐要求将由4字节对齐的地址满足。 Basically, what governs alignment of a struct?
基本上,什么控制结构的对齐? Please note that I am clear about the padding concept of fields within a struct.
请注意,我清楚结构中字段的填充概念。 The alignment requirement of a struct itself is what I am finding confusing.
结构本身的对齐要求是我发现令人困惑的。
The alignment requirement of a struct is the biggest alignment requirement of any of its members. 结构的对齐要求是其任何成员的最大对齐要求。 In this case, since
struct list_head
contains pointers, the alignment of struct list_head
is 8 bytes. 在这种情况下,由于
struct list_head
包含指针,因此struct list_head
的对齐方式为8个字节。 And since struct __wait_queue_head
contains a struct list_head
, its alignment is 8 bytes as well. 由于
struct __wait_queue_head
包含struct list_head
,因此它的对齐也是8个字节。 This is required because if the struct had a looser alignment requirement, then the struct padding wouldn't be enough to guarantee that the members would be properly aligned. 这是必需的,因为如果struct具有更宽松的对齐要求,那么struct padding将不足以保证成员正确对齐。
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