OPENPTY(3) | Library Functions Manual | OPENPTY(3)

# NAME

`getptmfd`, `openpty`, `fdopenpty`, `login_tty`, `forkpty`, `fdforkpty` — tty utility functions

# SYNOPSIS

System Utilities Library (libutil, -lutil)
`#include <termios.h>`
`#include <util.h>`

int
`getptmfd`(void);

int
`openpty`(int *amaster, int *aslave, char *name, const struct termios *termp, const struct winsize *winp);

int
`fdopenpty`(int ptmfd, int *amaster, int *aslave, char *name, const struct termios *termp, const struct winsize *winp);

int
`login_tty`(int fd);

pid_t
`forkpty`(int *amaster, char *name, const struct termios *termp, const struct winsize *winp);

pid_t
`fdforkpty`(int ptmfd, int *amaster, char *name, const struct termios *termp, const struct winsize *winp);

# DESCRIPTION

The `openpty`(), `login_tty`(), and `forkpty`() functions perform manipulations on ttys and pseudo-ttys.

The `openpty`() function finds an available pseudo-tty and returns file descriptors for the master and slave in amaster and aslave. If name is non-null, the filename of the slave is returned in name (a string of at least 16 characters). If termp is non-null, the terminal parameters of the slave will be set to the values in termp. If winp is non-null, the window size of the slave will be set to the values in winp.

The `openpty`() function allocates the pseudo-tty through the /dev/ptm device (see pty(4) for details) which means that its ownership is changed to the UID of the caller, permissions are set to correct values, and all earlier uses of that device are revoked (see revoke(2) for details).

The `fdopenpty`() and `fdforkpty`() functions work like `openpty`() and `forkpty`() but expect a /dev/ptm file descriptor ptmfd obtained from the `getptmfd`() function.

The `login_tty`() function prepares for a login on the tty fd (which may be a real tty device, or the slave of a pseudo-tty as returned by `openpty`()) by creating a new session, making fd the controlling terminal for the current process, setting fd to be the standard input, output, and error streams of the current process, and closing fd.

The `forkpty`() function combines `openpty`(), `fork`(), and `login_tty`() to create a new process operating in a pseudo-tty. The file descriptor of the master side of the pseudo-tty is returned in amaster, and the filename of the slave in name if it is non-null. The termp and winp parameters, if non-null, will determine the terminal attributes and window size of the slave side of the pseudo-tty.

# RETURN VALUES

If a call to `openpty`(), `login_tty`(), or `forkpty`() is not successful, -1 is returned and errno is set to indicate the error. Otherwise, `openpty`(), `login_tty`(), and the child process of `forkpty`() return 0, and the parent process of `forkpty`() returns the process ID of the child process.

# FILES

/dev/pty[p-zP-T][0-9a-zA-Z]
  master pseudo terminals
/dev/tty[p-zP-T][0-9a-zA-Z]
  slave pseudo terminals
/dev/ptm
  pseudo terminal management device

# ERRORS

`getptmfd`() may fail and set errno for any of the errors specified for the routine open(2).

`openpty`() and `fdopenpty`() will fail if:

[`ENOENT`]
  There are no available ttys.

`fdopenpty`() and `fdforkpty`() will fail if `getptmfd`() fails. `forkpty`() and `fdforkpty`() will fail if either `openpty`() or `fork`() fails.

`login_tty`() will fail if `ioctl`() fails to set fd to the controlling terminal of the current process.

# SEE ALSO

fork(2), revoke(2), pty(4)

June 6, 2025 | Debian