Advanced Programming in the UNIX® Environment

ptg10805159

Section 19.6 Using theptyProgram 737

In this example, we assume that theSHELL variable is the Korn shell (probably

/bin/ksh). As we mentioned earlier,scriptcopies only what is output by the new

shell (and any processes that it invokes), but since the line discipline module above the

PTY slave normally has echo enabled, most of what we type is also written to the

typescriptfile.

Running Coprocesses

In Figure15.18, the coprocess couldn’t use the standardI/O functions, because standard

input and standardoutput do not refer to a terminal, so the standardI/O functions treat

them as fully buffered. If we run the coprocess underptyby replacing the line

if (execl("./add2", "add2", (char *)0) < 0)

with

if (execl("./pty", "pty", "-e", "add2", (char *)0) < 0)

the program now works, even if the coprocess uses standardI/O.

Figure19.15 shows the arrangement of processes when we run the coprocess with a

pseudo terminal as its input and output. It is an expansion of Figure19.6, showing all

the process connections and data flow.The box labeled ‘‘driving program’’ is the

program from Figure15.18, with theexeclchanged as described previously.

driving program forkexec parentpty fork childpty (coprocess)add2

terminal

line discipline

terminal

device driver

terminal

line discipline

PTY master PTY slave

fork,exec

pipe2

pipe1

user at a

terminal

Figure 19.15 Running a coprocess with a pseudo terminal as its input and output

This example shows the need for the-e(no echo) option for theptyprogram. The

ptyprogram is not running interactively,because its standardinput is not connected to

aterminal. In Figure19.11, theinteractiveflag defaults to false, since the call to