小談 mmap() 與 VMA

ゝ一世哀愁。 2022-08-24 04:16 137阅读 0赞

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           <p style="margin-top: 10px; margin-bottom: 10px; ">最近在 program loader 專欄裡，整理並分享了 kernel 的 ELF loader 主題；接著下來就是 dynamic linking 的議題了。在正式介紹 dynamic linking 前，先來簡單提一下「memory mapping」的觀念；近期在進行 Linux programming 的教育訓練，正好可以跟同學做個小討論。</p> 
           <p style="margin-top: 10px; margin-bottom: 10px; ">在 IPC 的課程中提到 mapped memory 的行程間通訊機制，是透過「shared file」來做訊息的傳遞；將 shared file mapping 到 process address space 的 system call 為 'mmap()'。以下是其中一個簡單的程式範例 （<i>mmap_write.c</i>）：</p> 
           <blockquote style="margin-top: 10px; margin-bottom: 10px; "> 
            <pre style="margin-top: 10px; margin-bottom: 10px; ">#include &lt;stdio.h&gt; 
#include &lt;fcntl.h&gt;
#include &lt;string.h&gt;
#include &lt;sys/mman.h&gt;
#include &lt;sys/stat.h&gt;
#define FILE_LENGTH 0x400

int main(int argc, char *argv[])
{
   int fd;
   void *map_memory;

/* Open a file to be mapped. */
   fd = open("/tmp/shared_file", O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
   lseek(fd, FILE_LENGTH+1, SEEK_SET);
   write(fd, "", 1);
   lseek(fd, 0, SEEK_SET);

/* Create map memory. */
   <b>map_memory = mmap(0, FILE_LENGTH, PROT_WRITE, MAP_SHARED, fd, 0);
   close(fd);</b>

/* Write to mapped memory. */
   if (strlen(argv[1]) &lt; FILE_LENGTH)
      sprintf((char *)map_memory, "%s", argv[1]);

<b><font>sleep(10);</font></b>

exit(0);
}</pre> 
           </blockquote> 
           <p style="margin-top: 10px; margin-bottom: 10px; ">當程式將檔案 mapping 到自己的記憶體空間（process address space）時，mmap system call 便會建立相對應的 VMA 區段；觀念上來說，只要透過 mmap system call 將檔案 mapping 到記憶體，便會產生對應的 VMA。</p> 
           <p style="margin-top: 10px; margin-bottom: 10px; ">修改一下範例程式，讓程式在結束前小睡 10 秒鐘，並做觀察：</p> 
           <blockquote style="margin-top: 10px; margin-bottom: 10px; "> 
            <pre style="margin-top: 10px; margin-bottom: 10px; "># ./mmap_write hello&amp;
[1] 14667
# cat /proc/14667/maps
08048000-08049000 r-xp 00000000 16:42 213757     /tmp/mmap_write
08049000-0804a000 rw-p 00000000 16:42 213757     /tmp/mmap_write
40000000-40015000 r-xp 00000000 16:42 12828674   /lib/ld-2.3.2.so
40015000-40016000 rw-p 00014000 16:42 12828674   /lib/ld-2.3.2.so
40016000-40017000 rw-p 00000000 00:00 0
<font>40017000-40018000 -w-s 00000000 16:42 213753     /tmp/shared_file</font>
42000000-4212e000 r-xp 00000000 16:42 13991938   /lib/tls/libc-2.3.2.so
4212e000-42131000 rw-p 0012e000 16:42 13991938   /lib/tls/libc-2.3.2.so
42131000-42133000 rw-p 00000000 00:00 0
bfffe000-c0000000 rwxp fffff000 00:00 0</pre> 
           </blockquote> 
           <p style="margin-top: 10px; margin-bottom: 10px; ">除了 mmap system call 外，IPC 技術中的 'shared memory' 同樣也能觀察到對應的 VMA 區段。</p> 
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