.\"	$OpenBSD: openpty.3,v 1.3 1996/11/24 23:53:12 millert Exp $
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.Dd June 17, 2025
.Dt OPENPTY 3
.Os
.Sh NAME
.Nm openpty ,
.Nm login_tty ,
.Nm forkpty
.Nd tty utility functions
.Sh SYNOPSIS
.Fd #include <util.h>
.Ft int
.Fn openpty "int *aprimary" "int *areplica" "char *name" "struct termios *termp" "struct winsize *winp"
.Ft int
.Fn login_tty "int fd"
.Ft pid_t
.Fn forkpty "int *aprimary" "char *name" "struct termios *termp" "struct winsize *winp"
.Sh DESCRIPTION
The
.Fn openpty ,
.Fn login_tty ,
and
.Fn forkpty
functions manipulate ttys and pseudo-ttys.
.Pp
The
.Fn openpty
function allocates a pseudo-tty and returns file descriptors
for the primary and replica in
.Fa aprimary
and
.Fa areplica .
If
.Fa name
is non-null, the pathname of the replica is stored in the buffer
pointed to by
.Fa name ,
which must be at least 128 bytes.
If
.Fa termp
is non-null, the terminal parameters of the replica will be set to the
values in
.Fa termp .
If
.Fa winp
is non-null, the window size of the replica will be set to the values in
.Fa winp .
.Pp
The
.Fn login_tty
function prepares for a login on the tty
.Fa fd
(which may be a real tty device, or the replica 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 creates a new process operating in a pseudo-tty.
The file
descriptor of the primary side of the pseudo-tty is returned in
.Fa aprimary ,
and the pathname of the replica is stored in the buffer pointed to by
.Fa name
if it is non-null.
The buffer must be at least 128 bytes long.
The
.Fa termp
and
.Fa winp
parameters, if non-null, will determine the terminal attributes and
window size of the replica 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 ERRORS
.Fn openpty
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
There are no available pseudo-ttys.
.It Bq Er ENXIO
The
.Xr sysctl 3
limit
.Dq kern.tty.ptmx_max
number of pseudo-terminal devices was reached.
.El
.Pp
The
.Fn openpty
function may also fail and set
.Va errno
for any of the errors specified for the
.Xr open 2
system call.
.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 FILES
.Bl -tag -width /dev/ttys[0-9][0-9][0-9] -compact
.It Pa /dev/ptmx
cloning pseudo-tty device
.It Pa /dev/ttys[0-9][0-9][0-9]
replica pseudo-tty devices
.El
.Sh SEE ALSO
.Xr fork 2 ,
.Xr ptsname 3 ,
.Xr sysctl 3
.Sh CAVEATS
Due to the risk of accidentally passing a too-small buffer as the
.Fa name
argument to the
.Fn forkpty
and
.Fn openpty
functions, it is advisable to use
.Xr ptsname_r 3
instead.