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.

posix_openpt — open a pseudo-terminal device

#include <stdlib.h>
#include <fcntl.h>
int posix_openpt(int oflag);

The posix_openpt() 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 Section 2.14, 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 oflag.

Values for oflag are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>:

Open for reading and writing.
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 file descriptor for a master pseudo-terminal device and return a non-negative integer representing the file descriptor. Otherwise, -1 shall be returned and errno set to indicate the error.

The posix_openpt() 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.

The posix_openpt() function may fail if:

The value of oflag is not valid.
Out of pseudo-terminal resources.
Out of STREAMS resources.

The following sections are informative.

#include <fcntl.h>
#include <stdio.h>
int masterfd, slavefd;
char *slavedevice;
masterfd = posix_openpt(O_RDWR|O_NOCTTY);
if (masterfd == -1
    || grantpt (masterfd) == -1
    || unlockpt (masterfd) == -1
    || (slavedevice = ptsname (masterfd)) == NULL)
    return -1;
printf("slave device is: %s\n", slavedevice);
slavefd = open(slavedevice, O_RDWR|O_NOCTTY);
if (slavefd < 0)
    return -1;

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, respectively.

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 unlockpt() functions.

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;


Section 2.14, File Descriptor Allocation, grantpt(), open(), ptsname(), unlockpt()

The Base Definitions volume of POSIX.1‐2017, <fcntl.h>, <stdlib.h>

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 .

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2017 IEEE/The Open Group