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       setsid - creates a session and sets the process group ID


       #include <sys/types.h>
       #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

       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 error.


       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


       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 setsid().

       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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