.\" $NetBSD: posix_openpt.3,v 1.10 2022/11/15 22:17:53 gutteridge Exp $
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.\" by Christos Zoulas.
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.Dd November 15, 2022
.Dt POSIX_OPENPT 3
.Os
.Sh NAME
.Nm posix_openpt
.Nd open a pseudo-terminal device
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.In fcntl.h
.Ft int
.Fn posix_openpt "int oflag"
.Sh DESCRIPTION
The
.Fn posix_openpt
function searches for an unused master pseudo-terminal device,
opens it, and returns a file descriptor associated with the now
used pseudo-terminal device.
The
.Fa oflag
argument has the same meaning as in the
.Xr open 2
call.
.Sh RETURN VALUES
If successful,
.Fn posix_openpt
returns a non-negative integer, which corresponds to a file descriptor
pointing to the master pseudo-terminal device.
Otherwise, a value of \-1 is returned and
.Va errno
is set to indicate the error.
.Pp
Note that unlike implementations on some other operating systems,
.Fn posix_openpt
does not return
.Er EINVAL
if a flag supplied in
.Fa oflag
would be deemed invalid, instead it is simply ignored.
This means it is not possible to dynamically test which
.Xr open 2
flags are possible to set, and apply a fallback if
.Er EINVAL
is received.
However, this is unlikely to be a concern in practice, as flags such as
.Dv O_NONBLOCK
and
.Dv O_CLOEXEC
are supported.
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr open 2 ,
.Xr grantpt 3 ,
.Xr ptsname 3 ,
.Xr unlockpt 3
.Sh RATIONALE
The standards committee did not want to directly expose the cloning device,
thus decided to wrap the functionality in this function.
The equivalent code would be:
.Bd -literal
	int
	posix_openpt(int oflag) {
		return open("/dev/ptmx", oflag);
	}
.Ed
.Sh STANDARDS
The
.Fn posix_openpt
function conforms to
.St -p1003.1-2001 .
.Sh HISTORY
This function first appeared in
.Nx 3 .
Support for non-standard flags appeared in
.Nx 10 .