pts(7)

DESCRIPTION

A pseudo-terminal (pty) consists of a tightly-coupled pair of character devices, called the master device and slave device. The pty master and slave device drivers work together to simulate a terminal connection where the master provides a connection to the pseudo terminal server process and the slave provides a terminal device special file access for the terminal application processes, as depicted below:

The slave driver, pts with ptem (STREAMS pty emulation module) and ldterm (STREAMS line discipline module) pushed on top (not shown for simplicity), provides a terminal interface as described in termio(7). Whereas devices that provide the terminal interface described in termio(7) have a hardware device behind them; in contrast, the slave device has another process manipulating it through the master side of the pty. Data written on the master device is given to the slave device as input and data written on the slave device is presented as input on the master device.

In order to use the STREAMS pty subsystem, a node for the master pty driver /dev/ptmx and N number of slave pty devices must be installed (see ptm(7) for more details on master pty). When the master device is opened, the corresponding slave device is automatically locked out. No user can open that slave device until its permissions are changed (via the grantpt() function) and the device is unlocked (via the unlockpt() function). The user then call the ptsname() function to obtain the name of the slave device and invoke the open() system call to open the slave device. Although only one open is allowed on a master device, multiple opens are allowed on the slave device. After both the master and slave have been opened, the user has two file descriptors which represent the end points of a full duplex connection composed of two streams that are automatically connected by the master and slave devices when they are opened. The user may then push the desired modules (for example, ptem and ldterm, on pts for terminal semantics and pckt on ptm for Packet Mode feature).

The master and slave drivers pass all STREAMS messages to their adjacent drivers. Only the M_FLUSH message needs some special processing because the read queue of the master is connected to the write queue of the slave and vice versa. For example, the FLUSHR flag is changed to FLUSHW flag and vice versa whenever a M_FLUSH message travels across the master-slave link. When the master device is closed, an M_HANGUP message is sent to the corresponding slave device which will render that slave device unusable. The process on the slave side gets the errno ENXIO when attempting a write() system call to the slave device file but it will be able to read any data remaining in the slave stream. Finally, when all the data has been read, the read() system call will return 0, indicating that the slave can no longer be used. On the last close of the slave device, a zero-length M_DATA message is sent to the corresponding master device. When the application on the master side issues a read(2) or getmsg(2) system calls, a 0 (zero) is returned. The user of the master device may decide to close the master device file, which dismantles the stream on the master side. If the master device remains opened, the corresponding slave device can be opened and used again by another user.

EXAMPLES

The following example shows how a STREAMS pty master and slave devices are typically opened.

AUTHOR

pts was developed by HP and OSF.

FILES

SEE ALSO

insf(1M), sam(1M), getmsg(2), ioctl(2), open(2), read(2), write(2), grantpt(3C), ptsname(3C), unlockpt(3C), ldterm(7), ptem(7), ptm(7), streamio(7), termio(7).