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HP-UX 11i Version 3: February 2007

Technical documentation

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tels, telm — STREAMS Telnet slave (pseudo-terminal) driver, STREAMS Telnet master driver (used by telnetd only), respectively


#include <sys/termios.h> #include <sys/strtio.h>

int open("/dev/pts/ tN ", O_RDWR);


A Telnet pseudo-terminal consists of a tightly-coupled pair of character devices, called the master device and slave device. The master and slave device drivers work together to provide a Telnet connection on the server side where the master provides a connection to telnetd and the slave provides a terminal device special file access for the Telnet application processes, as depicted below:

-------------------------- | Pseudo terminal functions| Application <--> |--------------------------| <--> telnetd Processes | Slave | Master | | (tels) | (telm) | --------------------------

The slave driver, tels 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 telnetd manipulating it through the master side of the Telnet pseudo terminal.

There are no nodes in the file system for each individual master device. Rather, the master driver is set up as a STREAMS clone(7) driver with its major device number set to the major for the clone driver and its minor device number set to the major for the telm driver. The master driver is opened by telnetd using the open(2) system call with /dev/telnetm as the device file parameter. The clone open finds the next available minor number for the master device. The master device is available only if it and its corresponding slave device are not already opened.

In order to use the STREAMS Telnet subsystem, a node for the master driver /dev/telnetm and N number of Telnet slave devices must be installed.

The number of slave devices is set by a kernel tunable parameter called nstrtel. This can be modified using SAM; its default and minimum value is 60. The value of nstrtel is the upper limit of the number of telnet sessions that can be opened.

Multiple opens are allowed on the Telnet slave device.

The master and slave drivers pass all STREAMS messages to their adjacent drivers. When the connection is closed from the Telnet client side, 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(2) 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(2) system call will return 0, indicating that the slave can no longer be used.


tels() and telm() were developed by HP.



Streams Telnet master clone device


Streams slave devices where N is the minor number of the slave device and 0 < N < nstrtel.

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