linux知识复习1-dup dup2

 1 #include 
 2 #include 
 3 #include 
 4 #include 
 5 #include 
 6 int main(void)
 7 {
 8     #define STDOUT 1 
 9     int nul, oldstdout;
10     char msg[] = "This is a test";
11     /* create a file */
12     nul = open("DUMMY.FIL", O_CREAT | O_RDWR, S_IREAD | S_IWRITE);
13     /* create a duplicate handle for standard
14     output */
15 
16     oldstdout = dup(STDOUT); //复制用于后续恢复
17     /*
18     redirect standard output to DUMMY.FIL
19     by duplicating the file handle onto the
20     file handle for standard output.
21     */
22     dup2(nul, STDOUT); //将标准输出重定向到nul
23     /* close the handle for DUMMY.FIL */
24     close(nul);
25     /* will be redirected into DUMMY.FIL */
26     write(STDOUT, msg, strlen(msg));
27     /* restore original standard output
28     handle */
29     dup2(oldstdout, STDOUT); //将标准输出重定向到之前保存的标准输出
30     /* close duplicate handle for STDOUT */
31     close(oldstdout);
32     return 0;
33 }

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进程fd表 ---- 文件表----v节点表

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指向关系改变:

dup2(nul, STDOUT)

STDOUT指向了nul的文件表；

dup2(oldstdout, STDOUT)

STDOUT指向了oldstdout保存的文件表；

打开文件的每个进程都有独立的文件表项，可以使每个进程都有自己对于该文件的当前偏移；