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.Dd $Mdocdate: June 5 2013 $
.Dt OPENPTY 3
.Os
.Sh NAME
.Nm openpty ,
.Nm login_tty ,
.Nm forkpty
.Nd tty utility functions
.Sh SYNOPSIS
.In termios.h
.In util.h
.Ft int
.Fn openpty "int *amaster" "int *aslave" "char *name" "struct termios *termp" "struct winsize *winp"
.Ft int
.Fn login_tty "int fd"
.Ft pid_t
.Fn forkpty "int *amaster" "char *name" "struct termios *termp" "struct winsize *winp"
.Sh DESCRIPTION
The
.Fn openpty ,
.Fn login_tty ,
and
.Fn forkpty
functions perform manipulations on ttys and pseudo-ttys.
.Pp
The
.Fn openpty
function finds an available pseudo-tty and returns file descriptors
for the master and slave in
.Fa amaster
and
.Fa aslave .
If
.Fa name
is non-null, the filename of the slave is returned in
.Fa name
(a string of at least 16 characters).
If
.Fa termp
is non-null, the terminal parameters of the slave will be set to the
values in
.Fa termp .
If
.Fa winp
is non-null, the window size of the slave will be set to the values in
.Fa winp .
.Pp
The
.Fn openpty
function works in the following way:
first it attempts to allocate the pseudo-tty through the
.Pa /dev/ptm
device (see
.Xr pty 4
for details) and if that fails it searches for a free pseudo-tty by iterating
through all existing pseudo-tty devices in
.Pa /dev .
When a free pseudo-tty is found, its ownership is changed to
the UID of the caller, permissions are set to correct values,
and all earlier uses of that device are revoked (see
.Xr revoke 2
for details).
The first method can work for any user, the second method requires
super-user privileges in most cases.
.Pp
The
.Fn login_tty
function prepares for a login on the tty
.Fa fd
(which may be a real tty device, or the slave of a pseudo-tty as
returned by
.Fn openpty )
by creating a new session, making
.Fa fd
the controlling terminal for the current process, setting
.Fa fd
to be the standard input, output, and error streams of the current
process, and closing
.Fa fd .
.Pp
The
.Fn forkpty
function combines
.Fn openpty ,
.Fn fork ,
and
.Fn login_tty
to create a new process operating in a pseudo-tty.
The file
descriptor of the master side of the pseudo-tty is returned in
.Fa amaster ,
and the filename of the slave in
.Fa name
if it is non-null.
The
.Fa termp
and
.Fa winp
parameters, if non-null, will determine the terminal attributes and
window size of the slave side of the pseudo-tty.
.Sh RETURN VALUES
If a call to
.Fn openpty ,
.Fn login_tty ,
or
.Fn forkpty
is not successful, \-1 is returned and
.Va errno
is set to indicate the error.
Otherwise,
.Fn openpty ,
.Fn login_tty ,
and the child process of
.Fn forkpty
return 0, and the parent process of
.Fn forkpty
returns the process ID of the child process.
.Sh FILES
.Bl -tag -width /dev/tty[p-zP-T][0-9a-zA-Z]x -compact
.It Pa /dev/pty[p-zP-T][0-9a-zA-Z]
master pseudo terminals
.It Pa /dev/tty[p-zP-T][0-9a-zA-Z]
slave pseudo terminals
.It Pa /dev/ptm
pseudo terminal management device
.El
.Sh ERRORS
.Fn openpty
will fail if:
.Bl -tag -width Er
.It Bq Er ENOENT
There are no available ttys.
.El
.Pp
.Fn login_tty
will fail if
.Fn ioctl
fails to set
.Fa fd
to the controlling terminal of the current process.
.Fn forkpty
will fail if either
.Fn openpty
or
.Fn fork
fails.
.Sh SEE ALSO
.Xr fork 2 ,
.Xr revoke 2 ,
.Xr pty 4