linux知识复习1-dup dup2

 #include <sys/stat.h>
 #include <string.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <unistd.h>
 int main(void)
 {
     #define STDOUT 1
     int nul, oldstdout;
     char msg[] = "This is a test";
     /* create a file */
     nul = open("DUMMY.FIL", O_CREAT | O_RDWR, S_IREAD | S_IWRITE);
     /* create a duplicate handle for standard
     output */

     oldstdout = dup(STDOUT); //复制用于后续恢复
     /*
     redirect standard output to DUMMY.FIL
     by duplicating the file handle onto the
     file handle for standard output.
     */
     dup2(nul, STDOUT); //将标准输出重定向到nul
     /* close the handle for DUMMY.FIL */
     close(nul);
     /* will be redirected into DUMMY.FIL */
     write(STDOUT, msg, strlen(msg));
     /* restore original standard output
     handle */
     dup2(oldstdout, STDOUT); //将标准输出重定向到之前保存的标准输出
     /* close duplicate handle for STDOUT */
     close(oldstdout);
     return ;
 }

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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；

详细参考:

http://www.cnblogs.com/GODYCA/archive/2013/01/05/2846197.html