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.
The posix_openpt() function shall establish a connection between a
master device for a pseudo-terminal and a file descriptor. The file
descriptor is used by other I/O functions that refer to that pseudo-
The file status flags and file access modes of the open file
description shall be set according to the value of oflag.
Values for oflag are constructed by a bitwise-inclusive OR of flags
from the following list, defined in <fcntl.h>:
O_RDWR Open for reading and writing.
O_NOCTTY If set posix_openpt() shall not cause the terminal device
to become the controlling terminal for the process.
The behavior of other values for the oflag argument is unspecified.
Upon successful completion, the posix_openpt() function shall open a
master pseudo-terminal device and return a non-negative integer
representing the lowest numbered unused file descriptor. Otherwise,
−1 shall be returned and errno set to indicate the error.
The posix_openpt() function shall fail if:
EMFILE All file descriptors available to the process are currently
ENFILE The maximum allowable number of files is currently open in the
The posix_openpt() function may fail if:
EINVAL The value of oflag is not valid.
EAGAIN Out of pseudo-terminal resources.
ENOSR Out of STREAMS resources.
The following sections are informative.
This function is a method for portably obtaining a file descriptor of
a master terminal device for a pseudo-terminal. The grantpt() and
ptsname() functions can be used to manipulate mode and ownership
permissions, and to obtain the name of the slave device,
The standard developers considered the matter of adding a special
device for cloning master pseudo-terminals: the /dev/ptmx device.
However, consensus could not be reached, and it was felt that adding
a new function would permit other implementations. The posix_openpt()
function is designed to complement the grantpt(), ptsname(), and
On implementations supporting the /dev/ptmx clone device, opening the
master device of a pseudo-terminal is simply:
mfdp = open("/dev/ptmx", oflag );if (mfdp < 0)return -1;
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
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
IEEE/The Open Group 2013 POSIX_OPENPT(3P)