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SETSID(2) Linux Programmer's Manual SETSID(2)

NAME

     setsid - creates a session and sets the process group ID

SYNOPSIS

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

     pid_t setsid(void);

DESCRIPTION

     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
     ID).

     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).

RETURN VALUE

     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.

ERRORS

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

CONFORMING TO

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

NOTES

     A child created via fork(2) inherits its parent's session ID.  The ses-
     sion 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 ses-
     sion 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 control-
     ling terminal.