pts.7 (2010 09)

p
pts(7) pts(7)
NAME
pts - STREAMS slave pty (pseudo-terminal) driver
SYNOPSIS
#include <sys/stropts.h>
#include <sys/termios.h>
#include <sys/strtio.h>
int open("/dev/pts/
N", O_RDWR);
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 connec-
tion 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:
----------------
| pty functions |
Application <--> |----------------| <--> Server
Processes | Slave | Master | Process
| (pts) | (ptm) |
----------------
The slave driver,
pts with ptem (STREAMS pty emulation module) and ldterm (STREAMS line discip-
line module) pushed on top (not shown for simplicity), provides a terminal interface as described in ter-
mio(7). Whereas devices that provide the terminal interface described in termio (7) have a hardware dev-
ice 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 mas-
ter 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 dev-
ice 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) sys-
tem 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.
int fd_master, fd_slave;
char *slave;
...
fd_master = open("/dev/ptmx", O_RDWR);
grantpt(fd_master);
unlockpt(fd_master);
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1