NAME

pty − pseudo terminal driver

DESCRIPTION

The pty driver provides a device-pair termed a pseudo terminal. A pseudo terminal is a pair of character devices, a master device and a slave device.  The slave device provides processes an interface identical to that described in termio(7). However, whereas all other devices which provide the interface described in termio(7) have a hardware device of some sort behind them, the slave device has, instead, another process manipulating it through the master half of the pseudo terminal. That is, anything written on the master device is given to the slave device as input and anything written on the slave device is presented as input on the master device.

The following ioctl calls apply only to pseudo terminals:

TIOCPKT
Enable/disable packet mode.  Packet mode is enabled by specifying (by reference) a nonzero parameter and disabled by specifying (by reference) a zero parameter.  When applied to the master side of a pseudo terminal, each subsequent read from the terminal will return data written on the slave part of the pseudo terminal preceded by a zero byte (symbolically defined as TIOCPKT_DATA), or a single byte reflecting control status information.  In the latter case, the byte is an inclusive-or of zero or more of the bits:

TIOCPKT_FLUSHREAD
whenever the read queue for the terminal is flushed.

TIOCPKT_FLUSHWRITE
whenever the write queue for the terminal is flushed.

TIOCPKT_STOP
whenever output to the terminal is stopped a la ^S.

TIOCPKT_START
whenever output to the terminal is restarted.

TIOCPKT_DOSTOP
whenever t_stopc is ^S and t_startc is ^Q. 

TIOCPKT_NOSTOP
whenever the start and stop characters are not ^S/^Q.

This mode is used by rlogin(1C) and rlogind(1M) to implement a remote-echoed, locally ^S/^Q flow-controlled remote login with proper back-flushing of output; it can be used by other similar programs.

ALLOCATION

The code sequence shown below demonstrates how to allocate pseudo terminals.  Note that pseudo terminals, like all files, must have the correct file permissions to be accessible. 

#include <fcntl.h>
#include <sys/types.h>
#include <sys/sysmacros.h>
#include <sys/stat.h>
#include <stdio.h>
 externint open(/*char *, int ...*/);
externvoid close(/*int*/);
externint fstat(/*int, struct stat **/);
 /*
 * Find a pseudo tty to use.  Fill in "name" with the name of the tty.
 * Return the open descriptor for the controlling side.  Caller is
 * responsible for opening the slave side, if it wants to.
 */
int
findPseudoTTY(name, ptynum)
char *name;
int *ptynum;
{
int fd;
struct stat sb;
 fd = open("/dev/ptc", O_RDWR|O_NDELAY);
if (fd >= 0) {
if (fstat(fd, &sb) < 0) {
close(fd);
return -1;
}
*ptynum = minor(sb.st_rdev);
sprintf(name, "/dev/ttyq%d", *ptynum);
}
return fd;
}

FILES

/dev/ptc - master pseudo terminal
/dev/ttyq[0-99] - slave pseudo terminals

Version 3.6  —  December 20, 1987