POSIX_OPENPT(3POSIX)

💖 POSIX_OPENPT(3POSIX) 💖

okay bestie, so this one lives in the POSIX Programmer's Manual — that's the 3POSIX section, iykyk. 💅

⚠️ real talk first: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so go peek at the corresponding Linux manual page for the deets on Linux behavior), and honestly the interface may not even be implemented on Linux. not me giving you the tea AND the warning 🫶

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✨ NAME ✨

posix_openpt — open a pseudo-terminal device

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🌸 SYNOPSIS 🌸

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

int posix_openpt(int oflag);

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🦋 DESCRIPTION 🦋

so like, what does posix_openpt() even do? It establishes a connection between a master device for a pseudo-terminal and a file descriptor. That file descriptor gets allocated exactly like described in Section 2.14, File Descriptor Allocation, and then literally any other I/O function that refers to that pseudo-terminal can use it. slay. 👑

The file status flags and file access modes of the open file description get set according to whatever value you handed over in oflag. no cap.

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

• O_RDWR — open for reading and writing. both. we contain multitudes. 💜
• O_NOCTTY — if you set this, posix_openpt() shall not cause the terminal device to become the controlling terminal for the process. (protecting your peace fr fr 🙈)

💀 heads up: the behavior of any other values for the oflag argument is unspecified. so don't get cute with it.

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⭐ RETURN VALUE ⭐

Upon successful completion, posix_openpt() shall open a file descriptor for a master pseudo-terminal device and return a non-negative integer representing that file descriptor. iconic behavior.

Otherwise? -1 gets returned and errno is set to indicate the error. 😭

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💅 ERRORS 💅

The posix_openpt() function shall fail if:

• EMFILE — all file descriptors available to the process are currently open. every single one. full house, no vacancy.
• ENFILE — the maximum allowable number of files is currently open in the system. the whole system is booked, bestie.

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.

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🎀 EXAMPLES 🎀

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;

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🌈 APPLICATION USAGE 🌈

This function is the method for portably obtaining a file descriptor of a master terminal device for a pseudo-terminal. and then grantpt() and ptsname() are your little helpers — you use them to manipulate mode and ownership permissions, and to obtain the name of the slave device, respectively. team effort, honestly. 🫶

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🧚 RATIONALE 🧚

okay so the lore: the standard developers considered the matter of adding a special device for cloning master pseudo-terminals — the /dev/ptmx device. But consensus could not be reached 😭 and it was felt that adding a new function would permit other implementations. So posix_openpt() is designed to complement the grantpt(), ptsname(), and unlockpt() functions. the friend group of syscalls, periodt.

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;

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🍓 FUTURE DIRECTIONS 🍓

None.

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👑 SEE ALSO 👑

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

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

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💜 COPYRIGHT 💜

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 .

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 https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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okay that's the whole page, bestie 💖 courtesy of IEEE/The Open Group, dated 2017, and yes this gorgeous little document is POSIX_OPENPT(3POSIX) — now go open a pseudo-terminal, you absolute icon 🎀✨