POSIX_OPENPT(3) Library Functions Manual POSIX_OPENPT(3)

posix_openpt — open a pseudo-terminal device


#include <stdlib.h>

int posix_openpt(int oflag);

The posix_openpt () function shall establish a connection between a manager device for a pseudo-terminal and a file descriptor. The file descriptor shall be allocated as described in V2_chap02(3) 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:

Open for reading and writing.
Atomically set the FD_CLOEXEC flag on the file descriptor.
Atomically set the FD_CLOFORK flag on the file descriptor.
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 manager pseudo-terminal device and return a non-negative integer representing the file descriptor. Otherwise, -1 shall be returned and 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.

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>


int managerfd, subsidiaryfd;
char *subsidiarydevice;


managerfd = posix_openpt(O_RDWR|O_NOCTTY);


if (managerfd == -1
    || grantpt (managerfd) == -1
    || unlockpt (managerfd) == -1
    || (subsidiarydevice = ptsname (managerfd)) == NULL)
    return -1;


printf("subsidiary device is: %s\n", subsidiarydevice);


subsidiaryfd = open(subsidiarydevice, O_RDWR|O_NOCTTY);
if (subsidiaryfd < 0)
    return -1;

This function is a method for portably obtaining a file descriptor of a manager terminal device for a pseudo-terminal. The grantpt(3) function and the ptsname(3) and ptsname_r(3) functions can be used to manipulate mode and ownership permissions, and to obtain the name of the subsidiary device, respectively.

The standard developers considered the matter of adding a special device for cloning manager pseudo-terminal devices: 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(3), ptsname(3), ptsname_r(3), and unlockpt(3) functions.

On implementations supporting the /dev/ptmx clone device, opening the manager device of a pseudo-terminal is simply:

mfdp = open("/dev/ptmx", oflag );
if (mfdp < 0)
    return -1;

The O_CLOEXEC and O_CLOFORK flags are necessary to avoid a data race in multi-threaded applications. Without O_CLOFORK, a file descriptor is leaked into a child process created by one thread in the window between another thread creating a file descriptor with posix_openpt () and then using fcntl(3) to set the FD_CLOFORK flag. Without O_CLOEXEC, a file descriptor intentionally inherited by child processes is similarly leaked into an executed program if FD_CLOEXEC is not set atomically.

None.

V2_chap02(3), grantpt(3), open(3), ptsname(3), unlockpt(3)

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

First released in Issue 6.

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.

Austin Group Defects 411 and 1318 are applied, adding O_CLOEXEC and O_CLOFORK.

Austin Group Defect 508 is applied, changing the APPLICATION USAGE and RATIONALE sections to refer to ptsname_r(3) as well as ptsname(3).

Austin Group Defect 593 is applied, removing #include <fcntl.h> from the SYNOPSIS and a reference to <fcntl.h> from the DESCRIPTION section.

Austin Group Defect 1330 is applied, removing obsolescent interfaces.

Austin Group Defect 1466 is applied, changing the terminology used for pseudo-terminal devices.

June 14, 2024 posix.fail