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

# unlockpt(3)

unlock a pseudoterminal master/slave pair

## NAME

unlockpt - unlock a pseudoterminal master/slave pair

## LIBRARY

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

## SYNOPSIS

```
#define _XOPEN_SOURCE
#include <stdlib.h>
```

**int unlockpt(int** *fd***);**

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

**unlockpt**():

```
    Since glibc 2.24:
        _XOPEN_SOURCE >= 500
    glibc 2.23 and earlier:
        _XOPEN_SOURCE
```

## DESCRIPTION

The **unlockpt**() function unlocks the slave pseudoterminal device corresponding to the master pseudoterminal referred to by the file descriptor *fd*.

**unlockpt**() should be called before opening the slave side of a pseudoterminal.

## RETURN VALUE

When successful, **unlockpt**() returns 0. Otherwise, it returns -1 and sets *errno* to indicate the error.

## ERRORS

- **EBADF**

  The *fd* argument is not a file descriptor open for writing.

- **EINVAL**

  The *fd* argument is not associated with a master pseudoterminal.

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **unlockpt**() | Thread safety | MT-Safe |

## STANDARDS

POSIX.1-2008.

## HISTORY

glibc 2.1. POSIX.1-2001.

## SEE ALSO

[**grantpt**(3)](/grantpt/3.md), [**posix\_openpt**(3)](/posix_openpt/3.md), [**ptsname**(3)](/ptsname/3.md), [**pts**(4)](/pts/4.md), [**pty**(7)](/pty/7.md)