forkpty(3) terminal utility functions

Other Alias

openpty, login_tty


#include <pty.h>

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

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

#include <utmp.h>

int login_tty(int fd);

Link with -lutil.


The openpty() function finds an available pseudoterminal and returns file descriptors for the master and slave in amaster and aslave. If name is not NULL, the filename of the slave is returned in name. If termp is not NULL, the terminal parameters of the slave will be set to the values in termp. If winp is not NULL, the window size of the slave will be set to the values in winp.

The login_tty() function prepares for a login on the terminal fd (which may be a real terminal device, or the slave of a pseudoterminal as returned by openpty()) by creating a new session, making fd the controlling terminal for the calling 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(2), and login_tty() to create a new process operating in a pseudoterminal. The file descriptor of the master side of the pseudoterminal is returned in amaster, and the filename of the slave in name if it is not NULL. The termp and winp arguments, if not NULL, will determine the terminal attributes and window size of the slave side of the pseudoterminal.


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.


openpty() will fail if:
There are no available terminals.

login_tty() will fail if ioctl(2) fails to set fd to the controlling terminal of the calling process.

forkpty() will fail if either openpty() or fork(2) fails.


For an explanation of the terms used in this section, see attributes(7).
forkpty(), openpty() Thread safetyMT-Safe locale
login_tty() Thread safetyMT-Unsafe race:ttyname


These are BSD functions, present in glibc. They are not standardized in POSIX.


The const modifiers were added to the structure pointer arguments of openpty() and forkpty() in glibc 2.8.

In versions of glibc before 2.0.92, openpty() returns file descriptors for a BSD pseudoterminal pair; since glibc 2.0.92, it first attempts to open a UNIX 98 pseudoterminal pair, and falls back to opening a BSD pseudoterminal pair if that fails.


Nobody knows how much space should be reserved for name. So, calling openpty() or forkpty() with non-NULL name may not be secure.


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