c - fork() 之后地址、值和指针会发生什么

Question

我正在研究一个问题，我要在 C 中检查 fork() 调用前后的值和地址。我的方法是显示变量值和地址，假设看到一个fork() 之后的地址不同。令我惊讶的是，上述变量的地址保持不变。

我的问题是为什么它们相同？如果我更改子变量中的变量会怎样？它会改变 parent 和 child 吗？如果不是，我如何才能更改该地址中的值，同时父子地址相同。

代码(供引用):

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>

int main()
{
  int status;
  pid_t pid;

  int a = 123456;
  float b = 123.456;
  char c = 'Z';
  int *e;
  e=&a;

  //Retriving address's
  void *ap=&a, *bp=&b, *cp=&c, *ep=&e;

    printf("Parent Before Fork:\n");
    printf("Integer a: \tvalue = %d, \taddress = %p\n", a, ap);
    printf("Float b: \tvalue = %f, \taddress = %p\n", b, bp);
    printf("Char c: \tvalue = %c, \t\taddress = %p\n", c, cp);
    printf("Pointer e: \tvalue = %p, address = %p\n", e, ep);  

    pid = fork();

    if(pid > 0)
    {
      pid = wait(&status);
      printf("\nParent After Fork:\n");
      printf("Integer a: \tvalue = %d, \taddress = %p\n", a, ap);
      printf("Float b: \tvalue = %f, \taddress = %p\n", b, bp);
      printf("Char c: \tvalue = %c, \t\taddress = %p\n", c, cp);
      printf("Pointer e: \tvalue = %p, address = %p\n", e, ep);

      sleep(1);
    }
    else if(pid == 0)
    {
      printf("\nChild After Fork:\n");
      printf("Integer a: \tvalue = %d, \taddress = %p\n", a, ap);
      printf("Float b: \tvalue = %f, \taddress = %p\n", b, bp);
      printf("Char c: \tvalue = %c, \t\taddress = %p\n", c, cp);
      printf("Pointer e: \tvalue = %p, address = %p\n", e, ep);
   }
   else
     printf("fork() did not work");

return 0; 
}

输出(供引用):

Parent Before Fork:
Integer a:  value = 123456,         address = 0x7fff8b8e378c
Float b:    value = 123.456001,     address = 0x7fff8b8e3790
Char c:     value = Z,              address = 0x7fff8b8e3787
Pointer e:  value = 0x7fff8b8e378c, address = 0x7fff8b8e3798

Child After Fork:
Integer a:  value = 123456,         address = 0x7fff8b8e378c
Float b:    value = 123.456001,     address = 0x7fff8b8e3790
Char c:     value = Z,              address = 0x7fff8b8e3787
Pointer e:  value = 0x7fff8b8e378c, address = 0x7fff8b8e3798

Parent After Fork:
Integer a:  value = 123456,         address = 0x7fff8b8e378c
Float b:    value = 123.456001,     address = 0x7fff8b8e3790
Char c:     value = Z,              address = 0x7fff8b8e3787
Pointer e:  value = 0x7fff8b8e378c, address = 0x7fff8b8e3798

Answer 1

子进程拥有父地址空间的副本。在现代系统中，地址是虚拟化的，因此任何特定的指针地址都可以在一个进程中映射到与在另一个进程中不同的物理地址。

What happens if I change a variable in the child? Will it change in both parent and child?

子项有自己的变量副本，因此更改子项中的变量不会影响父项中变量的值。

If not, how am I able to change the value in that address while the addres is the same for both parent and child.

这是由于两个进程中的相同地址映射到不同的物理地址。