为什么这两个地址不一样?

问题描述：

shmget.c:

#include<sys/types.h>
#include<string.h>
#include<sys/ipc.h>
#include<sys/shm.h>
#include<stdio.h>
main()
{
    key_t key;
    int shmid;
    char* addr1;
    key = ftok("/home/tamil/myc/pws.c",'T');
    shmid = shmget(key,128*1024,IPC_CREAT|SHM_R|SHM_W);

    addr1 = shmat(shmid,0,0);

    printf("\nIPC SHARED MEMORY");
    printf("\n SENDER ADDRESS");
    printf("\nTHE ADDRESS IS %p",addr1);
    printf("\nENTER THE MESSAGE:");
    scanf("%s",addr1);
    printf("\nMESSAGE STORED IN %p IS %s",addr1,addr1);  
}

shmget2.c:

#include<sys/types.h>
#include<string.h>
#include<sys/ipc.h>
#include<sys/shm.h>
#include<stdio.h>

main()
{
    int shmid;
    char* addr1;
    key_t key;


    key = ftok("/home/tamil/myc/pws.c",'T');
    shmid = shmget(key,128*1024,SHM_R|SHM_W);

    addr1 = shmat(shmid,0,0);


    printf("\nIPC SHARED MEMORY");
    printf("\n SENDER ADDRESS");
    printf("\nTHE ADDRESSS IS %p",addr1);
    printf("\nMESSAGE STORED IN %p IS %s",addr1,addr1);

}

输出:

tamil@ubuntu:~/myc$ cc shmget.c
tamil@ubuntu:~/myc$ ./a.out

IPC SHARED MEMORY
 SENDER ADDRESS
**THE ADDRESS IS **0xb786c000****
ENTER THE MESSAGE:helloworld

MESSAGE STORED IN **0xb786c000** IS helloworldtamil@ubuntu:~/myc$ cc shmget2.c
tamil@ubuntu:~/myc$ ./a.out

IPC SHARED MEMORY
 SENDER ADDRESS
**THE ADDRESSS IS **0xb7706000****
MESSAGE STORED IN **0xb7706000** IS helloworldtamil@ubuntu:~/myc$

这一切都很好.但是地址不一样.这是为什么?

This all works well. But the address is not the same. Why is this?

答

共享内存可以映射到不同进程地址空间中的不同区域.这是完全正常的.但是如果你在共享内存中存储指向共享内存其他部分的指针，那么你就有麻烦了.您需要使用类似偏移指针在那种情况下.

Shared memory can be mapped to different regions in the address space of different processes. This is perfectly normal. But if you're storing pointers inside the shared memory to other parts of the shared memory, then you're in trouble. You'll need to use something like an offset pointer in that case.

为什么这两个地址不一样?

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