关于thread_union的两个疑问

dsfhs

在内核2.6中堆栈这么定义：
union thread_union {
      struct thread_info thread_info;
      unsigned long stack[THREAD_SIZE/sizeof(long)];
};
根据内核的配置，THREAD_SIZE既可以是4K字节(1个页面)也可以是8K字节(2个页面)。thread_info是52个字节长。
在深入理解linux内核中可以看到这副图：


从union角度来看，某一时刻不是只有一个变量存在于这块内存中吗，这个图这么画对不？union声明时，变量长度不是最大变量长度的整数倍吗，这个里面应该是thread_info是最大变量，但8k不是52字节的整数倍，这是怎么回事？

bensenq

stack[0]的地址与thread_info地址相同，也就是说每个thread的thread_info保存在进程内核栈的最低端。

firkraag

Although the size of 'struct thread_info' is 52 bytes long, but it is not 52 bytes aligned. It 's 4 bytes aligned. 4096 or 8192 can be divided by 4.

dsfhs

firkraag 发表于 2013-08-28 08:15
Although the size of 'struct thread_info' is 52 bytes long, but it is not 52 bytes aligned. It 's 4  ...

Why it is 4 bytes aligned? And what's your opinion on the first question?Thank you!

firkraag

回复 4# dsfhs

Because the largest field of 'struct thread_info' is 4 bytes long in 32bits machine. You can baidu the articles about 'bytes aligned'.


For 1st question:

When you reference the field of 'thread_info' in the union, you get the 'thread_info',  when you reference the 'stack' field you get the 'stack' array as well. They share the same memory area. What is it is determined by how you reference it. BTW, the graph is right.
   

瀚海书香

回复 1# dsfhs

The graph is right! You misunderstand union struct.

   

junnyg

联合体的地址对齐是以最大成员长度对齐