fork（）和使用它创建的内存共享b/w进程-yiteyi-C++库

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那么，当我们做fork（）时，这两个进程实际上共享哪些部分呢？每个进程的堆内存是多少？全局变量是否共享？ malloc会将相同的地址返回给双方吗？

让我们运行下面的程序，看看它的输出，以澄清上面的问题。

                     // C program to demonstrate working of fork()                   
                     #include <unistd.h>                   
                     #include <sys/types.h>                   
                     #include <errno.h>                   
                     #include <stdio.h>                   
                     #include <sys/wait.h>                   
                     #include <stdlib.h>                   
                             
                     int                               globalVar;                               /*  A global variable*/                   
                             
                     int                               main(                               void                               )                   
                     {                   
                                         int                               localVar = 0;                   
                                         int                               * p = (                               int                               *)                               malloc                               (2);                   
                                         pid_t childPID = fork();                   
                             
                                         // Putting value at allocated address                   
                                         *p = 0;                   
                             
                                         if                               (childPID >= 0)                               // fork was successful                   
                                         {                   
                                         if                               (childPID == 0)                               // child process                   
                                         {                   
                                         printf                               (                               " Child Process Initial Value :: localVar"                   
                                         " = %d, globalVar = %d"                               , localVar,                   
                                         globalVar);                   
                                         localVar++;                   
                                         globalVar++;                   
                             
                                         int                               c = 500;                   
                                         printf                               (                               " Child Process :: localVar = %d, "                   
                                         "globalVar = %d"                               , localVar, globalVar);                   
                                         printf                               (                               " Address of malloced mem child = %p "                   
                                         "and value is %d"                               , p, *p);                   
                                         printf                               (                               " lets change the value pointed my malloc"                               );                   
                             
                                         *p = 50;                   
                                         printf                               (                               " Address of malloced mem child = %p "                   
                                         "and value is %d"                               , p, *p);                   
                                         printf                               (                               " lets change the value pointed my "                   
                                         "malloc in child"                               );                   
                             
                                         *p = 200;                   
                                         printf                               (                               " Address of malloced mem child = %p "                   
                                         "and value is %d"                               , p, *p);                   
                                         }                   
                                         else                               // Parent process                   
                                         {                   
                                         printf                               (                               " Parent process Initial Value :: "                   
                                         "localVar = %d, globalVar = %d"                               ,                   
                                         localVar, globalVar);                   
                             
                                         localVar = 10;                   
                                         globalVar = 20;                   
                                         printf                               (                               " Parent process :: localVar = %d,"                   
                                         " globalVar = %d"                               , localVar, globalVar);                   
                                         printf                               (                               " Address of malloced mem parent= %p "                   
                                         "and value is %d"                               , p, *p);                   
                             
                                         *p = 100;                   
                                         printf                               (                               " Address of malloced mem parent= %p "                   
                                         "and value is %d"                               , p, *p);                   
                                         printf                               (                               " lets change the value pointed my"                   
                                         " malloc in child"                               );                   
                                         *p = 400;                   
                                         printf                               (                               " Address of malloced mem child = %p"                   
                                         " and value is %d "                               , p, *p);                   
                                         }                   
                                         }                   
                                         else                               // fork failed                   
                                         {                   
                                         printf                               (                               " Fork failed, quitting!!!!!!"                               );                   
                                         return                               1;                   
                                         }                   
                             
                                         return                               0;                   
                     }                   

 Parent process Initial Value :: localVar = 0, globalVar = 0
 Parent process :: localVar = 10, globalVar = 20
 Address of malloced mem parent= 0x1bb5010 and value is 0
 Address of malloced mem parent= 0x1bb5010 and value is 100
 lets change the value pointed my malloc in child
 Address of malloced mem child = 0x1bb5010 and value is 400 

 Child Process Initial Value :: localVar = 0, globalVar = 0
 Child Process :: localVar = 1, globalVar = 1
 Address of malloced mem child = 0x1bb5010 and value is 0
 lets change the value pointed my malloc
 Address of malloced mem child = 0x1bb5010 and value is 50
 lets change the value pointed my malloc in child
 Address of malloced mem child = 0x1bb5010 and value is 200

说明：：

因此，每个进程的子进程和父进程都有自己的globalVariable和localvar副本，否则，如果他们共享了它，我们会在子进程中得到“子进程初始值：：localvar=[10]，globalVariable[20]”，这是在首先执行的父进程中分配的，但我们没有。
虽然malloc返回的内存地址相同，但实际上它们指向或映射到不同的物理地址，否则当父级在内存地址0x1535010处分配值100时，我们应该在子级中得到相同的100，但我们得到了0。

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