Migrating Character-Based User Interfaces | Unix Application Migration Guide (Patterns & Practices)

Not all UNIX-based interfaces are graphical. Character-based interfaces were the original mainstay of UNIX computing long before the graphical workstation was conceived. There are two options for character-based interfaces. Either a migration to the Microsoft Interix environment can take place with minimal change, or a graphical interface (Windows-based or HTML) can replace the character-based interface. A preliminary port to POSIX smoothes the migration to Interix.

POSIX Terminal I/O

The POSIX termios structure and a new set of access calls replace the two traditional terminal hardware interfaces, namely termio structures in System V and stty structures in BSD.

The POSIX input/output (I/O) model is very similar to the System V model. Two modes exist: canonical and noncanonical. Canonical input is line-based , like BSD cooked mode. Noncanonical mode is character-based, like BSD raw or cbreak mode. The Interix subsystem includes a true, noncanonical mode, with support for cc_c[VMIN ] and cc_c[VTIME] .

The termios structure is defined in Termios.h, as shown in the following listing:

 struct termios {    tcflag_t c_iflag;  /* input mode         */    tcflag_t c_oflag;  /* output mode        */    tcflag_t c_cflag;  /* control mode       */    tcflag_t c_lflag;  /* local mode         */    speed_t c_ispeed;  /* input speed        */    speed_t c_ospeed;  /* output speed       */    cc_t c_cc[NCCS];   /* control characters */ };

The Interix SDK extends the POSIX.1 set of flags for c_iflag to include IMAXBEL and VBELTIME. For c_cc , VMIN and VTIME do not have the same values as VEOF and VEOL. For a portable application, however, a developer should take into consideration that VMIN and VTIME can be identical to VEOF and VEOL on a POSIX.1 system.

Table11.8 shows the 12 new functions that replace the terminal I/O ioctl() calls, which include ioctl(fd, TIOCSETP, buf) and ioctl(fd, TIOCGETP, buf) or stty() and gtty() . They were changed because the data type of the final argument for terminal I/O ioctl() calls depends on an action that makes type checking impossible .

Table 11.8: Functions that Replace Terminal I/O ioctl()
Function	Description
tcgetattr()	Fetches attributes (termios structure).
tcsetattr()	Sets attributes (termios structure).
cfgetispeed()	Gets input speed.
cfgetospeed()	Gets output speed.
cfsetispeed()	Sets input speed.
cfsetospeed()	Sets output speed.
tcdrain()	Waits for all output to be transmitted.
tcflow()	Suspends transmit or receive.
tcflush()	Flushes pending I/O.
tcsendbreak()	Sends BREAK character.
tcgetpgrp ()	Gets foreground process group identifier (ID).
tcsetpgrp()	Sets foreground process group ID.

To get the window size , use the TIOCGWINSZ command for ioctl() or the winsize structure, which are both supported.

Porting Curses and Terminal Routines to Interix

The Interix SDK libraries include Libcurses.a, a port of the ncurses package, and Libtermcap.a, termcap routines. The Interix SDK also supports pseudoterminals. Porting such applications to Interix should be straightforward, but note the following:

The curses routines make use of the terminfo database, stored in /usr/share/terminfo. This location is different from the location used in traditional systems. To link with the terminfo routines, link with the curses library.
Interix supports both the BSD /dev/ptynn and the System V /dev/ptmx methods for opening the master side of a pseudoterminal. The System V method is slightly faster because the search for an available master device is handled in the subsystem. Currently, the Interix subsystem supports 265 ptys named /dev/pty[p “zA “E][0 “9a “f] on the master side. The corresponding subordinate side names are /dev/tty[p “zA “E][0 “9a “f].
When using /dev/ptmx, the subordinate (slave) tty name can be obtained with ptsname() . BSD-based ioctl() calls can be used with the pty master side.
Provided that it is a session leader, a process without a controlling tty acquires a controlling terminal on the first open () call to a tty, unless NOCTTY is specified in the open() call.

Older character-based terminal applications placed the cursor on the physical display screen based on the capabilities of the terminal. These capabilities were typically stored in the /etc/termcap file with around 15,000 lines of terminal capabilities.

The Console APIs can be used to create character-based applications with an addressable cursor. SetConsoleCursorPosition() , WriteConsole() , and ReadConsole() are three functions among the many available for use in the Console API.