posix_openpt man page


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 is 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.

Return Value

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:

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.


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;

Application Usage

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;

See Also

grantpt(), open(), ptsname(), unlockpt()

The Base Definitions volume of POSIX.1‐2008, <fcntl.h>, <stdlib.h>