setsid - creates a session and sets the process group ID


   #include <unistd.h>

   pid_t setsid(void);


   setsid()  creates a new session if the calling process is not a process
   group leader.  The calling process is the leader  of  the  new  session
   (i.e., its session ID is made the same as its process ID).  The calling
   process also becomes the process group leader of a new process group in
   the session (i.e., its process group ID is made the same as its process

   The calling process will be the only process in the new  process  group
   and in the new session.

   Initially, the new session has no controlling terminal.  For details of
   how a session acquires a controlling terminal, see credentials(7).


   On success, the (new) session ID of the calling  process  is  returned.
   On  error,  (pid_t) -1  is  returned,  and errno is set to indicate the


   EPERM  The process group ID of  any  process  equals  the  PID  of  the
          calling  process.   Thus,  in  particular, setsid() fails if the
          calling process is already a process group leader.


   POSIX.1-2001, POSIX.1-2008, SVr4.


   A child created via fork(2) inherits  its  parent's  session  ID.   The
   session ID is preserved across an execve(2).

   A  process  group leader is a process whose process group ID equals its
   PID.  Disallowing a process group leader from calling setsid() prevents
   the  possibility  that  a  process  group leader places itself in a new
   session while other processes  in  the  process  group  remain  in  the
   original  session;  such  a  scenario  would break the strict two-level
   hierarchy of sessions and process groups.  In order  to  be  sure  that
   setsid() will succeed, call fork(2) and have the parent _exit(2), while
   the child (which by definition can't be a process group  leader)  calls

   If  a  session has a controlling terminal, and the CLOCAL flag for that
   terminal is not set, and a terminal hangup  occurs,  then  the  session
   leader is sent a SIGHUP signal.

   If  a process that is a session leader terminates, then a SIGHUP signal
   is sent to  each  process  in  the  foreground  process  group  of  the
   controlling terminal.


   setsid(1),     getsid(2),    setpgid(2),    setpgrp(2),    tcgetsid(3),
   credentials(7), sched(7)


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