进程组、会话、控制终端
process group - 进程组
session - 会话
controlling terminal - 控制终端
#include <unistd.h>#include <stdlib.h>#include <sys/stat.h>#include <signal.h>#include <syslog.h>#include <fcntl.h>#include <sys/resource.h>intmain(){inti, fd0, fd1, fd2;pid_tpid;struct rlimitrl;struct sigactionsa;/* * Clear file creation mask. */umask(0);/* * Get maximum number of file descriptors. */if (getrlimit(RLIMIT_NOFILE, &rl) < 0)exit(1);/* [1] */sleep(5);/* * Become a session leader to lose controlling TTY. */if ((pid = fork()) < 0)exit(1);else if (pid != 0) /* parent */exit(0);/* [2] */sleep(5);setsid();/* [3] */sleep(5);/* * Ensure future opens won't allocate controlling TTYs. */sa.sa_handler = SIG_IGN;sigemptyset(&sa.sa_mask);sa.sa_flags = 0;if (sigaction(SIGHUP, &sa, NULL) < 0)exit(1);if ((pid = fork()) < 0)exit(1);else if (pid != 0) /* parent */exit(0);/* [4] */sleep(5);/* * Change the current working directory to the root so * we won't prevent file systems from being unmounted. */if (chdir("/") < 0)exit(1);/* * Close all open file descriptors. */if (rl.rlim_max == RLIM_INFINITY)rl.rlim_max = 1024;for (i = 0; i < rl.rlim_max; i++)close(i);/* * Attach file descriptors 0, 1, and 2 to /dev/null. */fd0 = open("/dev/null", O_RDWR);fd1 = dup(0);fd2 = dup(0);/* * Initialize the log file. */openlog(NULL, LOG_CONS, LOG_DAEMON);if (fd0 != 0 || fd1 != 1 || fd2 != 2) {syslog(LOG_ERR, "unexpected file descriptors %d %d %d", fd0, fd1, fd2);exit(1);}return 0;}/*[1] 父进程：新建进程组（并且是进程组组长），与bash处于同一回话（bash为session leader）$ ps -u hwx -o pid,ppid,pgid,sid,comm PID PPID PGID SID COMMAND 1038 1037 1038 1038 bash 1425 1038 1425 1038 a.out[2] 第一个子进程，新建回话之前：与父进程处于同一进程组（但不是进程组组长），与bash处于同一回话$ ps -u hwx -o pid,ppid,pgid,sid,comm PID PPID PGID SID COMMAND 1038 1037 1038 1038 bash 1427 1 1425 1038 a.out [3] 第一个子进程，新建回话之后：新建回话（成为session leader），新建进程组（进程组组长）$ ps -u hwx -o pid,ppid,pgid,sid,comm PID PPID PGID SID COMMAND 1038 1037 1038 1038 bash 1427 1 1427 1427 a.out [4] 第二个子进程：既不是进程组组长，也不是session leader$ ps -u hwx -o pid,ppid,pgid,sid,comm PID PPID PGID SID COMMAND 1038 1037 1038 1038 bash 1432 1 1427 1427 a.out*/说明：
1、
pgid(process group ID) == pid of process group leader
sid(session ID) == pid of session leader
2、如果一个进程能新建 session，那么新建 session 的同时，必定新建 process group，并且是这个进程成为 session leader 和 process group leader
3、能获得控制终端(controlling terminal)的必定是 session leader，因此，如果一个进程不是 session leader，则这个进程不可能获得控制终端