为什么子进程和父进程的变量地址相同

发布于 2024-12-02 00:21:46 字数 497 浏览 0 评论 0原文

这是我的代码

int main()
{
  pid_t pid;
  int y = 3;  
  if ( (pid = fork()) <0 )
   return -1;;

  if( pid == 0 )  /* child */
  {
    printf(" before: %d %p\n", y, &y );
    y *= 10;
    printf("after: %d %p\n", y, &y );
  }
  else /* father */
  {
   sleep(1);
   printf("father: %d %p\n" , y , &y );

  }
  return 0;
}

程序的输出如下：

before: 3 ffbff440
after: 30 ffbff440
father: 3 ffbff440

我的问题是为什么子变量和父变量的地址相同但值不同？

原文

Here is my code

int main()
{
  pid_t pid;
  int y = 3;  
  if ( (pid = fork()) <0 )
   return -1;;

  if( pid == 0 )  /* child */
  {
    printf(" before: %d %p\n", y, &y );
    y *= 10;
    printf("after: %d %p\n", y, &y );
  }
  else /* father */
  {
   sleep(1);
   printf("father: %d %p\n" , y , &y );

  }
  return 0;
}

The output of the program is like following:

before: 3 ffbff440
after: 30 ffbff440
father: 3 ffbff440

My question is why is address of variable of child and parent same but the value different?

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离鸿 2024-12-09 00:21:46

因为它是一个虚拟地址，而不是物理地址。

每个进程都有自己的地址空间（例如，32 位系统可能允许每个进程拥有自己的完整 4G 范围的地址空间）。

它是内存管理单元，它将虚拟地址映射到物理地址（如果需要从辅助存储买回换出的页面，则处理页面错误等问题）。

下图可能会有所帮助，每个部分代表一个 4K 内存块：

   Process A           Physical Memory      Process B
   +-------+           +-------------+      +-------+
0K |       |---->   0K |  (shared)   | <----|       | 0K
   +-------+           +-------------+      +-------+
4K |       |--+     4K |             | <----|       | 4K
   +-------+  |        +-------------+      +-------+
8K |       |  +->   8K |             |      |       | 8K
   +-------+           +-------------+      +-------+
       |                : : : : : : :           |
       |               +-------------+          |
       |          128K |             | <--------+
       |               +-------------+
       +--------> 132K |             |
                       +-------------+

在该图中，您可以看到虚拟内存地址和物理内存地址之间的脱节（以及进程共享内存块的可能性）。左右两侧的地址是进程看到的虚拟地址。

中央块中的地址是数据“真正”所在的实际物理地址，并且由 MMU 处理映射。

要更深入地解释 fork （和 exec），您可能还需要查看这个答案。

Because it's a virtual address, not a physical one.

Each process gets its own address space (for example, a 32-bit system may allow each process to have its own address space with the full 4G range).

It's the memory management unit that will map virtual addresses to physical ones (and handle things like page faults if swapped out pages need to be bought back in from secondary storage).

The following diagram may help, each section representing a 4K block of memory:

   Process A           Physical Memory      Process B
   +-------+           +-------------+      +-------+
0K |       |---->   0K |  (shared)   | <----|       | 0K
   +-------+           +-------------+      +-------+
4K |       |--+     4K |             | <----|       | 4K
   +-------+  |        +-------------+      +-------+
8K |       |  +->   8K |             |      |       | 8K
   +-------+           +-------------+      +-------+
       |                : : : : : : :           |
       |               +-------------+          |
       |          128K |             | <--------+
       |               +-------------+
       +--------> 132K |             |
                       +-------------+

You can see, in that diagram, the disconnect between virtual memory addresses and physical memory addresses (and the possibility for processes to share memory blocks as well). The addresses down the left and right sides are virtual addresses which the processes see.

The addresses in the central block are actual physical addresses where the data "really" is, and the MMU handles the mapping.

For a deeper explanation of fork (and exec), you may also want to look at this answer.

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