從地址空間理解fork

Linux Process Address Space

  high address   +---------------+
                 |               |
                 |     Stack     |    int local_b
                 |               |
                 +---------------+
                 |       |       |
                 |       v       |
                 |               |
                 |               |
                 |       ^       |
                 |       |       |
                 +---------------+
                 |               |
                 |     Heap      |    int * heap_c = malloc()
                 |               |
                 +---------------+
                 |     Data      |    int global_a
                 +---------------+
                 |     Code      |
  low address    +---------------+

上圖是 Linux 的進程地址空間，從低位到高位地址分別爲：

• Code Segment: 程序的代碼，CPU 執行的指令部分，共享只讀
• Data Segment: 可細分爲初始化數據段和未初始化數據段，常用於存儲全局變量等
• Stack: 函數以及自動變量(未加 static 的自動變量又稱爲局部變量)
• Heap: 動態分配內存，如 malloc() 分配的內存

更爲詳細的介紹請見 Anatomy of a Program in Memory。

---

Fork

                  Parent Process             Child Process

  high address   +---------------+          +---------------+
                 |               |          |               |
                 |     Stack     |          |     Stack     |
                 |               |          |               |
                 +---------------+          +---------------+
                 |       |       |          |       |       |
                 |       v       |          |       v       |
                 |               |          |               |
                 |               |          |               |
                 |       ^       |          |       ^       |
                 |       |       |          |       |       |
                 +---------------+          +---------------+
                 |               |          |               |
                 |     Heap      |          |     Heap      |
                 |               |          |               |
                 +---------------+          +---------------+
                 |     Data      |          |     Data      |
                 +---------------+----------+---------------+
                 |                   Code                   |
  low address    +------------------------------------------+

fork 是 linux 中最重要的系統調用之一，用於創建一個新進程，它完全的複製父進程地址空間的 data segment、 heap 和 stack，但是和父進程共享一個 code segment，因爲 code segment 通常爲只讀，從邏輯的角度來看，子進程和父進程的內存地址空間互相獨立，子進程修改自己的 data segment，heap 和 stack 並不影響父進程內存空間。每次調用 fork，返回兩次結果，其中父進程的返回值爲子進程的 pid，子進程的返回值爲 0。

#include<stdio.h>

include<stdlib.h>

include <unistd.h>

int global_a = 0; // data segment

int main(void)
{
int local_b = 0, status; // stack
int * heap_c = malloc(sizeof(int)); // heap
pid_t pid;

<span class="k">if</span><span class="p">(</span><span class="o">!</span><span class="n">fork</span><span class="p">()){</span>
    <span class="n">global_a</span> <span class="o">++</span><span class="p">;</span>
    <span class="n">local_b</span> <span class="o">++</span><span class="p">;</span>
    <span class="o">*</span><span class="n">heap_c</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
    <span class="n">exit</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">else</span><span class="p">{</span>
    <span class="n">wait</span><span class="p">(</span><span class="o">&amp;</span><span class="n">status</span><span class="p">);</span>
<span class="p">}</span>

<span class="n">printf</span><span class="p">(</span><span class="s">"global_a = %d, local_b = %d, heap_c = %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">global_a</span><span class="p">,</span> <span class="n">local_b</span><span class="p">,</span> <span class="o">*</span><span class="n">heap_c</span><span class="p">);</span>

}

程序的輸出結果如下：

$ ./a.out
global_a = 0, local_b = 0, heap_c = 0

Note：爲了減輕 fork 調用開銷，實際採用 copy on write(COW) 技術。

---

Properties shared by both parent and child process

雖然父進程和子進程的地址空間是獨立的，但是二者依舊共享很多其它的資源，以下摘自 Advanced Programming in the UNIX Environment, 3rd Edition：

• File descriptor
• Real user ID, real group ID, effective user ID, effective group ID
• Supplementary group IDs
• Process group ID
• Session ID
• Controlling terminal
• The set-user-ID and set-group-ID flags
• Current working directory
• Root directory
• File mode creation mask
• Signal mask and dispositions
• The close-on-exec flag for any open file descriptors
• Environment
• Attached shared memory segments
• Memory mappings
• Resource limits

轉自：http://wsfdl.com/linux/2015/04/06/%E4%BB%8E%E5%9C%B0%E5%9D%80%E7%A9%BA%E9%97%B4%E7%90%86%E8%A7%A3fork.html