A pseudoterminal pair is similar to a bidirectional pipe. Anything that is written on the master appears as input on the slave, and anything that is written on the slave appears as input on the master.
The point that distinguishes a pseudoterminal pair from a bidirectional pipe is that the slave side operates like a terminal device. The slave interprets input in the same way as a normal controlling terminal would interpret keyboard input. For example, if we write a Control-C character (the usual terminal interrupt character) to the pseudoterminal master, the slave will generate a SIGINT signal for its foreground process group. Just as with a conventional terminal, when a pseudoterminal slave operates in canonical mode (the default), input is buffered line by line. In other words, the program reading from the pseudoterminal slave will see (a line of) input only when we write a newline character to the pseudoterminal master.
Like pipes, pseudoterminals have a limited capacity. If we exhaust this capacity, then further writes are blocked until the process on the other side of the pseudoterminal has consumed some bytes.
On Linux, the pseudoterminal capacity is about 4 kB in each direction.
If we close all file descriptors referring to the pseudoterminal master, then:
If the slave device has a controlling process, a SIGHUP signal is sent to that process (see Section 34.6).
A read() from the slave device returns end-of-file (0).
A write() to the slave device fails with the error EIO. (On some other UNIX implementations, write() fails with the error ENXIO in this case.)
If we close all file descriptors referring to the pseudoterminal slave, then:
A read() from the master device fails with the error EIO. (On some other UNIX implementations, a read() returns end-of-file in this case.)
A write() to the master device succeeds, unless the input queue of the slave device is full, in which case the write() blocks. If the slave device is subsequently reopened, these bytes can be read.
UNIX implementations vary widely in their behavior for the last case. On some UNIX implementations, write() fails with the error EIO. On other implementations, write() succeeds, but the output bytes are discarded (i.e., they can’t be read if the slave is reopened). In general, these variations don’t present a problem. Normally, the process on the master side detects that the slave has been closed because a read() from the master returns end-of-file or fails. At this point, the process performs no further writes to the master.
Packet mode is a mechanism that allows the process running above a pseudoterminal master to be informed when the following events related to software flow control occur on the pseudoterminal slave:
the input or output queue is flushed;
terminal output is stopped or started (Control-S/Control-Q); or
flow control was enabled or disabled.
Packet mode helps with handling software flow control in certain pseudoterminal applications that provide network login services (e.g., telnet and rlogin).
Packet mode is enabled by applying the ioctl() TIOCPKT operation to the file descriptor referring to the pseudoterminal master:
int arg;
arg = 1; /* 1 == enable; 0 == disable */
if (ioctl(mfd, TIOCPKT, &arg) == -1)
errExit("ioctl");When packet mode is in operation, reads from the pseudoterminal master return either a single nonzero control byte, which is a bit mask indicating the state change(s) that occurred on the slave device, or a 0 byte followed by one or more bytes of data that were written on the pseudoterminal slave.
When a state change occurs on a pseudoterminal that is operating in packet mode, select() indicates that an exceptional condition (the exceptfds argument) has occurred on the master, and poll() returns POLLPRI in the revents field. (Refer to Chapter 63 for descriptions of select() and poll().)
Packet mode is not standardized in SUSv3, and some details vary on other UNIX implementations. Further details of packet mode on Linux, including the bit-mask values used to indicate state changes, can be found in the tty_ioctl(4) manual page.