---
name: "posix_openpt"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/posix_openpt/3"
---

# posix\_openpt(3)

open a pseudoterminal device

## NAME

posix\_openpt - open a pseudoterminal device

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

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

**int posix\_openpt(int** *flags***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**posix\_openpt**():

```
    _XOPEN_SOURCE >= 600
```

## DESCRIPTION

The **posix\_openpt**() function opens an unused pseudoterminal master device, returning a file descriptor that can be used to refer to that device.

The *flags* argument is a bit mask that ORs together zero or more of the following flags:

- **O\_RDWR**

  Open the device for both reading and writing. It is usual to specify this flag.

- **O\_NOCTTY**

  Do not make this device the controlling terminal for the process.

## RETURN VALUE

On success, **posix\_openpt**() returns a file descriptor (a nonnegative integer) which is the lowest numbered unused file descriptor. On failure, -1 is returned, and *errno* is set to indicate the error.

## ERRORS

See [**open**(2)](/open/2.md).

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **posix\_openpt**() | Thread safety | MT-Safe |

## STANDARDS

POSIX.1-2008.

## HISTORY

glibc 2.2.1. POSIX.1-2001.

It is part of the UNIX 98 pseudoterminal support (see [**pts**(4)](/pts/4.md)).

## NOTES

Some older UNIX implementations that support System V (aka UNIX 98) pseudoterminals don't have this function, but it can be easily implemented by opening the pseudoterminal multiplexer device:

```
int
posix_openpt(int flags)
{
    return open("/dev/ptmx", flags);
}
```

Calling **posix\_openpt**() creates a pathname for the corresponding pseudoterminal slave device. The pathname of the slave device can be obtained using [**ptsname**(3)](/ptsname/3.md). The slave device pathname exists only as long as the master device is open.

## SEE ALSO

[**open**(2)](/open/2.md), [**getpt**(3)](/getpt/3.md), [**grantpt**(3)](/grantpt/3.md), [**ptsname**(3)](/ptsname/3.md), [**unlockpt**(3)](/unlockpt/3.md), [**pts**(4)](/pts/4.md), [**pty**(7)](/pty/7.md)