c - 共享内存是如何通过不同的内存地址来共享数据的呢？

Question

这个问题已经有答案了:

When could 2 virtual addresses map to the same physical address? (4 个回答)

已关闭 6 年前。

写:

#include "apue.h"
#include "errorlog.h"
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int main ()
{
        int segment_id;
        char* shared_memory;
        struct shmid_ds *shmbuffer;
        int segment_size;
        const int shared_segment_size = 0x6400;
        shmbuffer=malloc(sizeof(struct shmid_ds));
        /* Allocate a shared memory segment.  */
        if((segment_id = shmget (12345, shared_segment_size,IPC_CREAT|S_IRUSR | S_IWUSR))==-1)
        perror("shmget");
        /* Attach the shared memory segment.  */
        shared_memory = (char*) shmat (segment_id, 0, 0);
        printf ("shared memory attached at address %p\n", shared_memory);
        /* Determine the segment's size. */
        shmctl (segment_id, IPC_STAT, shmbuffer);
        segment_size  =  shmbuffer->shm_segsz;
        printf ("segment size: %d\n", segment_size);
        /* Write a string to the shared memory segment.  */
        sprintf (shared_memory, "Hello, world.");
        /* Detach the shared memory segment.  */
        shmdt (shared_memory);
        return 0;
}

输出:

shared memory attached at address 0xb77ab000
segment size: 25600

阅读:

#include "apue.h"
#include "errorlog.h"
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int main ()
{
        int segment_id;
        char* shared_memory;
        struct shmid_ds *shmbuffer;
        int segment_size;
        const int shared_segment_size = 0x6400;
        shmbuffer=malloc(sizeof(struct shmid_ds));
        /* Allocate a shared memory segment.  */
        segment_id = shmget (12345, shared_segment_size,
                        IPC_CREAT  | S_IRUSR | S_IWUSR);

        /* Attach the shared memory segment, at a different address.  */
        shared_memory = (char*) shmat (segment_id, (void*)0, 0);
        printf ("shared memory reattached at address %p\n", shared_memory);
        /* Print out the string from shared memory.  */
        printf ("%s\n", shared_memory);
        /* Detach the shared memory segment.  */
        shmdt (shared_memory);
        printf("current proc id %d",getpid());
        return 0;
}

输出:

shared memory reattached at address 0xb7783000
Hello, world.
current proc id 6530

我已经执行了这个共享内存的程序。我有一个疑问，不同的内存地址如何得到相同的数据。这里写入地址是0xb77ab000，读取地址是0xb7783000，但是给出了正确的数据“Hello, world”。请任何人解释一下这一点..

Answer 1

内核处理这个。

您从用户空间看到的所有内存都称为“虚拟内存”。它们可以是未分配的(如果没有写入任何内容)、在实际 RAM 上分配、在交换上分配、或指向内存映射文件或共享内存。操作系统负责将它们映射到物理内存地址。

当内存被共享时，页面被映射到不同的虚拟内存地址，但在内部它们共享相同的物理页面。