The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


posix_openpt - open a pseudo-terminal device


[XSI] [Option Start] #include <stdlib.h>
#include <fcntl.h>

int posix_openpt(int
oflag); [Option End]


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 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.
[OB XSR] [Option Start] Out of STREAMS resources. [Option End]

The following sections are informative.


Opening a Pseudo-Terminal and Returning the Name of the Slave Device and a File Descriptor
#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;




File Descriptor Allocation, grantpt, open, ptsname, unlockpt

XBD <fcntl.h>, <stdlib.h>


First released in Issue 6.

Issue 7

SD5-XBD-ERN-4 is applied, changing the definition of the [EMFILE] error.

SD5-XSH-ERN-51 is applied, correcting an error in the EXAMPLES section.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0253 [835] and XSH/TC2-2008/0254 [835] are applied.

End of informative text.


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