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 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 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/[pt]ty[pqrstuvwxyzPQRST][0123456789abcdef]
/dev/ptm
openpty() will fail if:
[ENOENT] 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)
OpenBSD 3.6 November 4, 1996
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