setsid - create session and set process group ID


     #include <sys/types.h>
     #include <unistd.h>

     pid_t setsid(void);


     The setsid() function creates a new session, if the  calling
     process  is not a process group leader. Upon return the cal-
     ling process will be the session leader of this new session,
     will be the process group leader of a new process group, and
     will have no controlling terminal. The process group  ID  of
     the calling process will be set equal to the process
      ID of the calling process. The calling process will be  the
     only  process  in the new process group and the only process
     in the new session.


     Upon successful completion, setsid() returns  the  value  of
     the  process  group  ID of the calling process. Otherwise it
     returns (pid_t)-1 and sets errno to indicate the error.


     The setsid() function will fail if:

     EPERM The calling process is already a process group leader,
           or  the  process group  ID of a process other than the
           calling process matches the process  ID of the calling


     See attributes(5) for descriptions of the  following  attri-

    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    | MT-Level                    | Async-Signal-Safe           |


     getsid(2), setpgid(2), setpgrp(2), attributes(5)


     A call to setsid() by a process  that  is  a  process  group
     leader  will  fail.  A  process  can  become a process group
     leader by being the last member of a pipeline started  by  a
     job  control  shell. Thus, a process that expects to be part
     of a pipeline, and that calls setsid(), should always  first
     fork;  the parent should exit and the child should call set-
     sid().  This will ensure that the calling process will  work
     reliably when started by both job control shells and non-job
     control shells.

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