Section: POSIX Programmer's Manual (3P)
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.
--- open a pseudo-terminal device
int posix_openpt(int oflag);
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
, File Descriptor Allocation
and can be used by other I/O functions that refer to that pseudo-terminal.
The file status flags and file access modes of the open file
description shall be set according to the value of
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
Open for reading and writing.
shall not cause the terminal device to become the controlling terminal
for the process.
The behavior of other values for the
argument is unspecified.
Upon successful completion, the
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
set to indicate the error.
function shall fail if:
All file descriptors available to the process are currently open.
The maximum allowable number of files is currently open in the system.
function may fail if:
The value of
is not valid.
Out of pseudo-terminal resources.
Out of STREAMS resources.
The following sections are informative.
Opening a Pseudo-Terminal and Returning the Name of the Slave Device and a File Descriptor
int masterfd, slavefd;
masterfd = posix_openpt(O_RDWR|O_NOCTTY);
if (masterfd == -1
|| grantpt (masterfd) == -1
|| unlockpt (masterfd) == -1
|| (slavedevice = ptsname (masterfd)) == NULL)
printf("slave device is: %s\n", slavedevice);
slavefd = open(slavedevice, O_RDWR|O_NOCTTY);
if (slavefd < 0)
This function is a method for portably obtaining a file descriptor of a
master terminal device for a pseudo-terminal. The
functions can be used to manipulate mode and ownership permissions, and
to obtain the name of the slave device, respectively.
The standard developers considered the matter of adding a special
device for cloning master pseudo-terminals: the
device. However, consensus could not be reached, and it was felt that
adding a new function would permit other implementations. The
function is designed to complement the
On implementations supporting the
clone device, opening the master device of a pseudo-terminal is simply:
mfdp = open("/dev/ptmx", oflag );
if (mfdp < 0)
, File Descriptor Allocation,
The Base Definitions volume of POSIX.1-2017,
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
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