'\" et
.TH POSIX_OPENPT "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
posix_openpt
\(em open a pseudo-terminal device
.SH SYNOPSIS
.LP
.nf
#include <stdlib.h>
#include <fcntl.h>
.P
int posix_openpt(int \fIoflag\fP);
.fi
.SH DESCRIPTION
The
\fIposix_openpt\fR()
function shall establish a connection between a master device for a
pseudo-terminal and a file descriptor. The file descriptor shall be
allocated as described in
.IR "Section 2.14" ", " "File Descriptor Allocation"
and can be used by other I/O functions that refer to that pseudo-terminal.
.P
The file status flags and file access modes of the open file
description shall be set according to the value of
.IR oflag .
.P
Values for
.IR oflag
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
.IR <fcntl.h> :
.IP O_RDWR 12
Open for reading and writing.
.IP O_NOCTTY 12
If set
\fIposix_openpt\fR()
shall not cause the terminal device to become the controlling terminal
for the process.
.P
The behavior of other values for the
.IR oflag
argument is unspecified.
.SH "RETURN VALUE"
Upon successful completion, the
\fIposix_openpt\fR()
function shall open a file descriptor for a master pseudo-terminal
device and return a non-negative integer representing the file
descriptor. Otherwise, \-1 shall be returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIposix_openpt\fR()
function shall fail if:
.TP
.BR EMFILE
All file descriptors available to the process are currently open.
.TP
.BR ENFILE
The maximum allowable number of files is currently open in the system.
.P
The
\fIposix_openpt\fR()
function may fail if:
.TP
.BR EINVAL
The value of
.IR oflag
is not valid.
.TP
.BR EAGAIN
Out of pseudo-terminal resources.
.TP
.BR ENOSR
Out of STREAMS resources.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Opening a Pseudo-Terminal and Returning the Name of the Slave Device and a File Descriptor"
.sp
.RS 4
.nf

#include <fcntl.h>
#include <stdio.h>
.P
int masterfd, slavefd;
char *slavedevice;
.P
masterfd = posix_openpt(O_RDWR|O_NOCTTY);
.P
if (masterfd == -1
    || grantpt (masterfd) == -1
    || unlockpt (masterfd) == -1
    || (slavedevice = ptsname (masterfd)) == NULL)
    return -1;
.P
printf("slave device is: %s\en", slavedevice);
.P
slavefd = open(slavedevice, O_RDWR|O_NOCTTY);
if (slavefd < 0)
    return -1;
.fi
.P
.RE
.SH "APPLICATION USAGE"
This function is a method for portably obtaining a file descriptor of a
master terminal device for a pseudo-terminal. The
\fIgrantpt\fR()
and
\fIptsname\fR()
functions can be used to manipulate mode and ownership permissions, and
to obtain the name of the slave device, respectively.
.SH RATIONALE
The standard developers considered the matter of adding a special
device for cloning master pseudo-terminals: the
.BR /dev/ptmx
device. However, consensus could not be reached, and it was felt that
adding a new function would permit other implementations. The
\fIposix_openpt\fR()
function is designed to complement the
\fIgrantpt\fR(),
\fIptsname\fR(),
and
\fIunlockpt\fR()
functions.
.P
On implementations supporting the
.BR /dev/ptmx
clone device, opening the master device of a pseudo-terminal is simply:
.sp
.RS 4
.nf

mfdp = open("/dev/ptmx", oflag );
if (mfdp < 0)
    return -1;
.fi
.P
.RE
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.14" ", " "File Descriptor Allocation",
.IR "\fIgrantpt\fR\^(\|)",
.IR "\fIopen\fR\^(\|)",
.IR "\fIptsname\fR\^(\|)",
.IR "\fIunlockpt\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<fcntl.h>\fP",
.IR "\fB<stdlib.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .