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HP-UX 11i Version 3: February 2007

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ptm — STREAMS master pty (pseudo-terminal) driver


#include <sys/stropts.h> #include <sys/ptyio.h> #include <sys/strtio.h> int open("/dev/ptmx", O_RDWR);


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:

---------------- | pty functions | Application <--> |----------------| <--> Server Processes | Slave | Master | Process | (pts) | (ptm) | ----------------

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 pts(7) for details on slave pty). There are no nodes in the file system for each individual master device. Rather, the master driver is set up as a STREAMS clone driver (see clone(7)) with its major device number set to the major for the clone driver and its minor device number set to the major for the ptm driver. The master driver is opened using the open() system call with /dev/ptmx 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. Only one open is allowed on a master device whereas multiple open are allowed on the slave device. When the master device is opened, the corresponding slave device is automatically locked out (see pts(7) on how to unlock the slave and obtain the slave device name). 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. These two streams are automatically connected by the master and slave devices when they are opened. The user may then push the necessary modules on the master and slave streams (e.g., 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. Hence, 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 on the slave device but it will be able to read any data remaining on the slave stream. Finally, when all the data have been read, the read() system call will return 0 (zero) 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() or getmsg() system calls and a 0 is returned. The user of the master device decides whether to close the master device file which will dismantle the streams on the master side. If the master device remains opened, the corresponding slave device can be opened and used again by another user.

Unlike the slave device, the master device does not act like a terminal. If O_NDELAY or O_NONBLOCK is set, a read on the master device returns -1 with errno set to EAGAIN if no data is available, and a write returns -1 with errno set to EAGAIN if there is internal flow control on the stream.

The master ptm driver supports the following ioctl() requests:


Determines whether the file descriptor is that of an open master device. On success, it returns the major and minor number (type dev_t) of the master device which can be used to determine the name of the corresponding slave device. On failure, it returns -1 with errno set to EINVAL. ISPTM on HP-UX can return valid device number with negative value. For example, with major number of the STREAMS pty master being 0x9c, ICPTM will return 0x9C000000 which is a negative number. Therefore, it is imperative that applications check for an explicit -1 instead of "< 0" (less than 0) on the return value.

ISPTM is used by functions grantpt(), unlockpt(), and ptsname(). User applications normally do not need to invoke this ioctl. The format of this ioctl is:

int ioctl(master_fd, ISPTM, 0)


Unlocks the master and the corresponding slave devices. On success, it returns 0. On failure, it returns -1 with errno set to EINVAL. UNLKPT is used by function unlockpt(). User applications normally do not need to invoke this ioctl. The format of this ioctl is:

int ioctl(master_fd, UNLKPT, 0)


This ioctl puts the STREAMS pty in and out of Remote Mode. When Remote Mode is on, input data will be flow-controlled and passed through ldterm without any input processing regardless of the terminal mode. When the pty master driver receives this ioctl, it will send an M_CTL message downstream to ldterm via ptm, pts, and ptem. The command in the M_CTL message is set to MC_NO_CANON or MC_DO_CANON depending whether to turn on or off the Remote Mode. The format of this ioctl is:

int ioctl(master_fd, TIOCREMOTE, argument)

where the argument is set to 1 to turn on Remote Mode and 0 to turn it off. Remote Mode is normally used when doing remote line editing in a window manager, or whenever flow-controlled input is required. Each write to the master device produces a record boundary for the process reading the slave devices. In normal usage, a write of data is like the data typed as a line on the terminal; a write of 0 (zero) bytes is like typing an EOF (End-of-File) character.


This ioctl allows the master process to send a signal to the slave process. The format of this ioctl is:

int ioctl(master_fd, TIOCSIGNAL, argument)

where the argument is the signal number as defined in the header file <sys/signal.h>. For example the master process can send an SIGINT signal to the slave process by doing:

ioctl(master_fd, TIOCSIGNAL, SIGINT)


ptm was developed by HP and OSF.



Streams pty master clone device


Streams pty slave devices (0 <= N < NSTRPTY), where NSTRPTY is a kernel tunable parameter which can be changed via SAM.

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