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       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.  The new session has no controlling terminal.


       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, fork(2)  and
       _exit(2), and have the child do setsid().


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


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