JOBCONTROL(2)                    System Calls                    JOBCONTROL(2)




NAME

       settpgrp, tcnewpgrp, tctpgrp - interface for the new job control model


SYNOPSIS

       #include <gno/gno.h>

       int tcnewpgrp(int fdtty);
       int settpgrp(int fdtty);
       int tctpgrp(int fdtty, int pid);


DESCRIPTION

       The job control interface is used to control what processes are 'in the
       foreground' on a particular terminal.  Every tty has a  process  group.
       Each process is a member of a process group.  A process is a foreground
       process on a tty if and only if that process and the terminal belong to
       the same process group.  Certain characters (such as ^C) typed on a tty
       with a non-zero process group produce signals  sent  to  every  process
       which is a member of the group.

       A process is suspended (stopped) if it performs a sufficiently invasive
       operation on a tty with a different process group.  This includes these
       job  control  calls, reads from a terminal, and writes to a terminal if
       it is configured to do so with ioctl(2).  When  a  tty  file  is  first
       opened,  it  is  assigned process group zero (init(8) has process group
       zero).  As init launches login processes on various  ttys,  it  assigns
       process groups to those ttys and processes.

       tcnewpgrp  allocates a new process group and assigns it to the terminal
       referred to by fdtty.  If the calling process is not in the foreground,
       it is sent SIGTTOU.

       settpgrp sets the current process to have the process group as fdtty.

       tctpgrp  sets the tty referred to by fdtty to the same process group as
       the process pid, where pid is the current process or  a  descendant  of
       it.


RETURN VALUE

       These  calls  will  return zero on success, otherwise they'll return -1
       and set errno accordingly.


ERRORS

       EBADF  fdtty is not a valid file descriptor.

       ENOTTY fdtty does not refer to a terminal file.

       ESRCH  pid is not a valid process identifier.


EXAMPLES

       The following are some example uses of the job control interface.

              Forking a pipeline in a job-control  shell:   The  shell  starts
              with  tcnewpgrp  so  that  the  tty  is in the new process group
              before there are even any children. It then forks  each  process
              in  the  pipeline.  Each process does settpgrp, thus joining the
              new process group.

              Handling a stopped child process:  When the shell  sees  that  a
              pipeline  has  stopped or exited, it does tctpgrp to set the tty
              to its own process group.  To resume the pipeline it does  tctp-
              grp where pid is one of the child processes, then sends SIGCONT.

              Starting a process under a new tty:  When, for instance, telnetd
              (8)  wants  to  grap  a pseudo-tty, it opens the pty and forks a
              child process.  The child does tcnewpgrp to give the tty a  real
              process group, then settpgrp to place itself into the foregroup.
       Security under this scheme is trivial.  there is no way a  process  can
       join a process group except by settpgrp, and that requires a descriptor
       open to a tty with that pgrp.  To make a tty have that requires  either
       tcnewpgrp  (in  which  case  nobody else is using the pgrp), or tctpgrp
       (which reduces to the first problem of having a process in the  process
       group).

       Note that 'using' must be defined as use both by ttys and by processes;
       the kernel keeps a table of pgrps, each with a total  tty  and  process
       reference  count.   When  the reference count reaches zero, the pgrp is
       automatically deallocated.


SEE ALSO

       ioctl(2), kill(2), signal(2), tty(4), GNO Shell Reference Manual.


CREDITS

       This job control interface was designed by Dan Bernstein <brnstnd@kram-
       den.acf.nyu.edu>.   He  was  inspired by Chris Torek, and dedicated the
       system to Mark Teitelbaum.  The text of this manpage  is  derived  from
       his original specifications.

       The GNO implementation was written strictly from specs.



GNO                             19 January 1997                  JOBCONTROL(2)

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