共享記憶體是程式間通訊的一種方式,PostgreSQL使用共享記憶體快取資料以及各種資料結構.
下面是演示程式碼,邏輯很簡單,自行參考程式碼註釋.

```C

/*
申請一段共享記憶體，父程式寫入一串字元，子程式讀出。
*/

#include <stdio.h>
#include <sys>
#include <sys>
#include <sys>
#include <unistd.h>
#include <stdlib.h>
//1k共享記憶體
#define SHM_SIZE 1024
#define SHM_ID 10086

int main()
{
    //共享記憶體id,子程式id
    int shmid, pid;
    //共享記憶體指標
    char *ptr = NULL;

//申請共享記憶體
    shmid = shmget((key_t)SHM_ID, SHM_SIZE, IPC_CREAT | 0600);

//對映共享記憶體到程式地址空間
    ptr = (char *)shmat(shmid, 0, 0);

printf("Attach pointer addr is %p \n", ptr);

ptr = "This is shared memory!";

printf("The String of Parent Process is : %s \n", ptr);

if((pid = fork()) == -1)
    {
        perror("fork process error!");
        exit(0);
    }
    else if(!pid)
    {
        printf("Child Process PID is : %d,String is %s \n", pid,ptr);
        exit(0);
    }else{
        sleep(1);
        //解除對映
        shmdt(ptr);
        //刪除共享記憶體
        shmctl(shmid, IPC_RMID, 0);
    }

return 0;
}

```
執行輸出
```
[pg12@localhost ipc]$ gcc -std=c11 -o fork fork.c 
In file included from fork.c:7:0:
/usr/include/sys/ipc.h:24:3: warning: #warning "Files using this header must be compiled with _SVID_SOURCE or _XOPEN_SOURCE" [-Wcpp]
 # warning "Files using this header must be compiled with _SVID_SOURCE or _XOPEN_SOURCE"
   ^
[pg12@localhost ipc]$ ./fork 
Attach pointer addr is 0x7f61ffb6b000 
The String of Parent Process is : This is shared memory! 
Child Process PID is : 0,String is This is shared memory! 
[pg12@localhost ipc]$

```</stdlib.h></unistd.h></sys></sys></sys></stdio.h>

資料庫實現原理#6（共享記憶體）

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