The tty Interface An introduction to “systems programming” in the Linux environment

The “login” program An important “interactive” application Illustrates a human-computer dialogue: –Computer says: What’s your name? –User replies: (typing in his/her username) –Computer says: What’s your password? –User replies: (typing in his/her password) –Computer performs a “lookup” operation –Computer replies: ok, access is allowed (or alternatively: access is denied)

Login program-structure main process_dataprint_outputobtain_input We saw this same basic program-structure once before (i.e., in our ‘manydots.s’ demo)

But ‘login’ needs a ‘tweak’ Login requires user to type a password The password is supposed to be secret But computer prints it onscreen (‘echo’) Anyone passing by will know the secret! How can we stop the ‘automatic echo’? It is a “systems programming” problem

How does the ‘tty’ work? TeleType displaY device HARDWARE SOFTWARE application tty_driver c_lflag input handling c_iflag c_cc output handling c_oflag terminal_driver c_cflag User space Kernel space struct tty { c_iflag; c_oflag; c_cflag; c_lflag; c_line; c_cc[ ]; };

The ‘c_lflag’ field It’s an array of flag-bits Individual bits have symbolic names Names conform to a POSIX standard Linux names match other UNIX’s names Though actual symbol-values may differ Your C/C++ program should use: #include for portability to other UNIX environments

The ‘c_lflag’ field (continued) Symbolic names defined in a header-file Header-files are in ‘/usr/include’ directory (and in its sub-directories) Important flag-bit names: ECHO, ECHONL You can search for them (by using ‘grep’): – $ grep ECHO /usr/include/*.h – $ grep ECHO /usr/include/*/*.h ECHO is in ‘/usr/include/asm/termbits.h’

How to turn off tty echo Two programming interface-functions: –int tcgetattr( int fd, struct termios *tios ); –Int tcsetattr( int fd, int fl, struct termios *tios ); Algorithm: –Use ‘tcgetattr()’ to get current settings –Clear the ECHO flag in c_lflag –Set the ECHONL flag in c_lflag –Use ‘tcsetattr()’ to install new settings

Demo ‘noecho.cpp’ High-level language: briefer and clearer So we use C++ to demonstrate this idea We worry later about assembly language A minor side-issue: converting ascii codes for letters from lowercase to uppercase Helps to know the C++ logic operators

AND, OR, XOR, NOT Important C++ operators: &, |, ^, and ~ Needed often in systems programming Examples (used in ‘noecho.cpp’): c_lflag &= ~ECHO; c_lflag |= ECHONL; Useful also for converting letters from lowercase to uppercase (or vice-versa)