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OPENPTY(3) Library Functions Manual OPENPTY(3)

openpty, login_tty, forkpty
tty utility functions

#include <termios.h>
#include <util.h>
int
openpty(int *amaster, int *aslave, char *name, struct termios *termp, struct winsize *winp);
int
login_tty(int fd);
pid_t
forkpty(int *amaster, char *name, struct termios *termp, struct winsize *winp);

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 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.

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.

/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

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)
August 28, 2015 OpenBSD-6.1