PostgreSQL使用程式架構,每個連線對應一個後臺程式,為了更好的理解這種架構,有必要深入理解程式的相關知識.本節主要介紹了Linux下的程式虛擬記憶體結構,並以使用C語言樣例程式進行說明.

### 一、虛擬記憶體
程式虛擬記憶體的詳細結構如下圖所示:

![](http://img.blog.itpub.net/blog/2019/09/17/c5cae0dce04f5fe1.png?x-oss-process=style/bb)

C樣例程式如下:
```
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

/**                                                                                                      
 * f - print locations of various elements                                                               
 *                                                                                                       
 * Returns: nothing                                                                                      
 */
void f(void)//子函式
{
     int a;
     int b;
     int c;

a = 98;
     b = 1024;
     c = a * b;
     printf("[f] a = %d, b = %d, c = a * b = %d\n", a, b, c);
     printf("[f] Adresses of a: %p, b = %p, c = %p\n", (void *)&a, (void *)&b, (void *)&c);
}

/**                                                                                                      
 * main - print locations of various elements                                                            
 *                                                                                                       
 * Return: EXIT_FAILURE if something failed. Otherwise EXIT_SUCCESS                                      
 */
int main(int ac, char **av, char **env)
{
     int a;//本地變數,位於棧中
     void *p;//指標,在堆中分配記憶體
     int i;
     int size;

printf("Address of a: %p\n", (void *)&a);
     p = malloc(98);//在堆中分配記憶體
     if (p == NULL)
     {
          fprintf(stderr, "Can't malloc\n");
          return (EXIT_FAILURE);
     }
     printf("Allocated space in the heap: %p\n", p);
     printf("Address of function main: %p\n", (void *)main);//main函式所在的記憶體地址
     printf("First bytes of the main function:\n\t");
     for (i = 0; i < 15; i++)
     {
          printf("%02x ", ((unsigned char *)main)[i]);//首16個位元組
     }
     printf("\n");
     printf("Address of the array of arguments: %p\n", (void *)av);//輸入引數陣列地址
     printf("Addresses of the arguments:\n\t");//
     for (i = 0; i < ac; i++)
     {
          printf("[%s]:%p ", av[i], av[i]);//列印輸入引數
     }
     printf("\n");
     printf("Address of the array of environment variables: %p\n", (void *)env);//環境變數地址
     printf("Address of the first environment variables:\n");//環境變數資訊
     for (i = 0; i < 3; i++)
     {
          printf("\t[%p]:\"%s\"\n", env[i], env[i]);
     }
     /* size of the env array */
     i = 0;
     while (env[i] != NULL)//以NULL作為終結
     {
          i++;
     }
     i++; /* the NULL pointer */
     size = i * sizeof(char *);
     printf("Size of the array env: %d elements -> %d bytes (0x%x)\n", i, size, size);//計算環境變數陣列大小
     f();//呼叫函式
     getchar();//接受輸入,程式不退出
     return (EXIT_SUCCESS);
}
```
編譯並執行
```
[root@localhost hacker]# gcc -Wall -Wextra -Werror main-7.c -o 7
[root@localhost hacker]# ./7 Hello Hacker Jack!
Address of a: 0x7fff5b327bb8 --> 位於棧中
Allocated space in the heap: 0x9b6010 --> 位於堆中
Address of function main: 0x400729 --> 可執行檔案
First bytes of the main function:
    55 48 89 e5 48 83 ec 40 89 7d dc 48 89 75 d0 
Address of the array of arguments: 0x7fff5b327cb8 -->引數陣列地址
Addresses of the arguments:
    [./7]:0x7fff5b329808 [Hello]:0x7fff5b32980c [Hacker]:0x7fff5b329812 [Jack!]:0x7fff5b329819 --> 輸入引數
Address of the array of environment variables: 0x7fff5b327ce0 --> 環境變數陣列
Address of the first environment variables:
    [0x7fff5b32981f]:"XDG_SESSION_ID=1"
    [0x7fff5b329830]:"HOSTNAME=localhost.localdomain"
    [0x7fff5b32984f]:"SELINUX_ROLE_REQUESTED="
Size of the array env: 27 elements -> 216 bytes (0xd8)
[f] a = 98, b = 1024, c = a * b = 100352 
[f] Adresses of a: 0x7fff5b327b7c, b = 0x7fff5b327b78, c = 0x7fff5b327b74 --> 函式f中的變數地址

```
透過udis86可反編譯,得到彙編程式碼
```
[root@localhost install]#  echo "55 48 89 e5 48 83 ec 10 48 8d 45 f0 48 89 c6" | udcli -64 -x -o 400729
0000000000400729 55               push rbp                
000000000040072a 4889e5           mov rbp, rsp            
000000000040072d 4883ec10         sub rsp, 0x10           
0000000000400731 488d45f0         lea rax, [rbp-0x10]     
0000000000400735 4889c6           mov rsi, rax            
[root@localhost install]# 
```

檢視該程式的記憶體對映資訊
```
[root@localhost install]# ps aux | grep "./7" | grep -v grep
root      6471  0.0  0.0   4348   348 pts/0    S+   15:24   0:00 ./7 Hello Hacker Jack!
[root@localhost install]# cat /proc/6471/maps
00400000-00401000 r-xp 00000000 fd:00 134287606                          /data/source/hacker/7
00600000-00601000 r--p 00000000 fd:00 134287606                          /data/source/hacker/7
00601000-00602000 rw-p 00001000 fd:00 134287606                          /data/source/hacker/7
009b6000-009d7000 rw-p 00000000 00:00 0                                  [heap]
7f95b5d16000-7f95b5ed8000 r-xp 00000000 fd:00 151914                     /usr/lib64/libc-2.17.so
7f95b5ed8000-7f95b60d8000 ---p 001c2000 fd:00 151914                     /usr/lib64/libc-2.17.so
7f95b60d8000-7f95b60dc000 r--p 001c2000 fd:00 151914                     /usr/lib64/libc-2.17.so
7f95b60dc000-7f95b60de000 rw-p 001c6000 fd:00 151914                     /usr/lib64/libc-2.17.so
7f95b60de000-7f95b60e3000 rw-p 00000000 00:00 0 
7f95b60e3000-7f95b6105000 r-xp 00000000 fd:00 151907                     /usr/lib64/ld-2.17.so
7f95b62f8000-7f95b62fb000 rw-p 00000000 00:00 0 
7f95b6301000-7f95b6304000 rw-p 00000000 00:00 0 
7f95b6304000-7f95b6305000 r--p 00021000 fd:00 151907                     /usr/lib64/ld-2.17.so
7f95b6305000-7f95b6306000 rw-p 00022000 fd:00 151907                     /usr/lib64/ld-2.17.so
7f95b6306000-7f95b6307000 rw-p 00000000 00:00 0 
7fff5b309000-7fff5b32a000 rw-p 00000000 00:00 0                          [stack]
7fff5b3f1000-7fff5b3f3000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
[root@localhost install]# 
```

### 二、參考資料
[Virtual memory](https://en.wikipedia.org/wiki/Virtual_memory)
[Hack the Virtual Memory: drawing the VM diagram](https://blog.holbertonschool.com/hack-the-virtual-memory-c-strings-proc/)
[Udis86](http://udis86.sourceforge.net/)</string.h></stdio.h></stdlib.h>

PostgreSQL 原始碼解讀（227）- Linux Kernel(程式虛擬記憶體#2)

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