[root@mylinux ap]# vi 9.4setsid.c 

      1 #include    <unistd.h>
      2 #include    <stdio.h>
      3 #include    <sys/types.h>
      4 #include    <fcntl.h>
      5
      6 int
      7 main(void)
      8 {      

      9     pid_t   pid;
     10

                                          /* 打印父进程的进程ID,及进程组ID  */
     11     printf("father's pid is %d,pgid is %d\n", getpid(), getpgid(0));
     12     if ((pid = fork()) < 0) {                  /* fork子进程  */
     13         printf("fork error\n");
     14         exit(1);
     15     }
     16     else if (pid == 0) {                       /* 子进程  */            
     17         printf("before set,child's pid is %d,pgid is %d\n", getpid(), getpgid(0));         //打印子进程的进程ID与进程组ID
     18         if (setsid() < -1) {                   //创建新会话
     19             printf("setsid error\n");
     20             exit(1);
     21         }
     22         printf("sid is %d\n", getsid(0));    //查看会话ID
     23         if (open("/dev/tty", O_RDWR) < 0) {  //验证子进程是否具有控制终端
     24             printf("open error\n");
     25             exit(1);
     26         }
     27         exit(0);
     28         }
     29     if (open("/dev/tty", O_RDWR) < 0) {  //难父进程是否具有控制终端
     30             printf("open error\n");
     31             exit(1);
     32     }
     33     else
     34         printf("father open ok\n");
     35     exit(0);
     36 }

[root@mylinux ap]# ./a.out
father's pid is 26252,pgid is 26252
before set,child's pid is 26253,pgid is 26252
sid is 26253         //创建新会话成功,会话的首进程的进程组ID为子进ID
open error           //子进程不具有控制终端
father open ok          //父进程具有控制终端