PTSNAME(3) | Library Functions Manual | PTSNAME(3)

# NAME

`grantpt`, `ptsname`, `unlockpt` — pseudo-terminal access functions

# SYNOPSIS

`#include <stdlib.h>`

int
`grantpt`(int fildes);

char *
`ptsname`(int fildes);

int
`unlockpt`(int fildes);

# DESCRIPTION

The `grantpt`(), `ptsname`(), and `unlockpt`() functions allow access to pseudo-terminal devices. These three functions accept a file descriptor that references the master half of a pseudo-terminal pair. This file descriptor is created with posix_openpt(3).

The `grantpt`() function is used to establish ownership and permissions of the slave device counterpart to the master device specified with fildes. The slave device's ownership is set to the real user ID of the calling process, and the permissions are set to user readable-writable and group writable. The group owner of the slave device is also set to the group “`tty`”.

The `ptsname`() function returns the full path name of the slave device counterpart to the master device specified with fildes. This value can be used to subsequently open the appropriate slave after posix_openpt(3) and `grantpt`() have been called.

The `unlockpt`() function clears the lock held on the pseudo-terminal pair for the master device specified with fildes.

# RETURN VALUES

The `grantpt`() and `unlockpt`() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The `ptsname`() function returns a pointer to the name of the slave device on success; otherwise a `NULL` pointer is returned.

# ERRORS

The `grantpt`(), `ptsname`() and `unlockpt`() functions may fail and set errno to:

[`EBADF`]
  fildes is not a valid open file descriptor.
[`EINVAL`]
  fildes is not a master pseudo-terminal device.

In addition, the `grantpt`() function may set errno to:

[`EACCES`]
  The slave pseudo-terminal device could not be accessed.

# SEE ALSO

posix_openpt(3), pty(4), tty(4)

# STANDARDS

The `ptsname`() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

This implementation of `grantpt`() and `unlockpt`() does not conform to IEEE Std 1003.1-2008 (“POSIX.1”), because it depends on posix_openpt(3) to create the pseudo-terminal device with proper permissions in place. It only validates whether fildes is a valid pseudo-terminal master device. Future revisions of the specification will likely allow this behaviour, as stated by the Austin Group.

# HISTORY

The `grantpt`(), `ptsname`() and `unlockpt`() functions appeared in OpenBSD 5.3.

June 13, 2025 | Debian