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OPENPTY(3) Library Functions Manual OPENPTY(3)

openpty, login_tty, forkptytty utility functions

#include <termios.h>
#include <util.h>

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

login_tty(int fd);

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

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

The () 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 () function works in the following way: first it attempts to allocate the pseudo-tty through the /dev/ptm device (see pty(4) for details) and if that fails it searches for a free pseudo-tty by iterating through all existing pseudo-tty devices in /dev. When a free pseudo-tty is found, 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 first method can work for any user, the second method requires super-user privileges in most cases.

The () 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 () function combines openpty(), (), 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.

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.

master pseudo terminals
slave pseudo terminals
pseudo terminal management device

openpty() will fail if:

There are no available ttys.

login_tty() will fail if ioctl() fails to set fd to the controlling terminal of the current process. forkpty() will fail if either openpty() or fork() fails.

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

June 5, 2013 OpenBSD-5.8