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The tty Interface

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)

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The tty Interface

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

  2. 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)

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

  4. 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

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

  6. 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 <termios.h> for portability to other UNIX environments

  7. 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’

  8. 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

  9. 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

  10. 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)

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