\" $NetBSD: posix_openpt.3,v 1.11 2025/08/06 00:07:16 gutteridge Exp $
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\" by Christos Zoulas.
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Dd August 6, 2025
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 ,
Dv O_CLOEXEC ,
and
Dv O_CLOFORK
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 .
