Implementing ‘noecho’ Programming details regarding the Linux implementation for ‘struct termios’ objects

Basic issues to consider Normal tty operation is ‘canonical’ mode Input gets processed one line at a time Line-editing is allowed (e.g., backspace) User’s keystrokes echoed to the screen In C/C++ demo we could turn off echoing We used two standard interface functions: int tcgetattr( int fileno, struct termios *tty ); Int tcsetattr( int fileno, int flag, struct termios *tty );

Basic Algorithm Save a copy of the initial terminal settings Modify certain bits in the ‘c_lflag’ field: –Turn off the ECHO bit, and –Turn on the ECHONL bit Install the modified terminal settings Perform desired echo-free keyboard input Then reinstall the original terminal settings

How much storage space? We must reserve adequate space for storing the initial terminal settings But we don’t know how big this object is This issue was transparently handled for us in our ‘noecho.cpp” demo by including: #include struct termios save_termios; How can we accomplish this in assembly?

The ‘c_lflag’ field We needed to modify bits in ‘c_lflag’ field Those bits were represented by constants: ECHO and ECHONL Their values were defined in We did not need to know the actual values But header-file isn’t available in assembly So now we do need to know these values Another constant needed is TCSANOW

Memory addressing Where’s ‘c_lflag’ field within termios object We can’t modify its bits until we know that One idea is to study the Linux header-file But where is it? There seem to be several Nested type-declarations add to confusion Another approach: let’s write a program to report the specific information we’ll need Our ‘ttyinfo.cpp’ demo program shows us that sizeof( struct termios ) equals 60 bytes

.section.data # we need storage for two ‘termios’ objects origtty:.space60# original settings worktty:.space60# a ‘working copy’ # Insufficient space would cause big trouble! origttyworktty system would overwite part of the next data-area tcgetattr() does not know the size that we have allocated

Constants in assembly We can use the.equ directive to create our own manifest constants:.equECHO, 0x equECHONL, 0x equTCSANOW, 0x Question: will our program also work on other versions of UNIX besides Linux?

Copying a structure object We can create a program loop to copy the contents of a data-structure Here again we need to know the number of bytes in the structure we want to copy We can use a ‘counted loop’ to do copying Three initialization steps: –put source-address in register %esi –put dest’n address in register %edi –put the byte-count into register %ecx Advance %esi and %edi as each byte is copied

The actual loop code movl$origtty, %esi movl$worktty, %edi movl$60, %ecx again:movb(%esi), %al movb%al, (%edi) incl%esi incl%edi loopagain

Modifying some flag bits Determine offset of the ‘c_lflag’ field (=12) Setup this offset in a CPU register (%edi) movl $12, %edi Use AND-instruction to turn a bit off: andl $~ECHO, worktty(%edi) Use OR-instruction to turn a bit on: orl $ECHONL, worktty(%edi) (Here other approaches also are possible)

Non-canonical terminal modes Certain kinds of applications do not lend themselves to ‘canonical’ terminal input We may want an instant response to each individual keystroke (not to an entire line) Example: computer game using keyboard We can ‘turn off’ canonical line-processing Very similar to ‘turning off’ the input echo i.e., worktty.c_lflag &= ~ICANON;

Two further adjustments Need two changes in the c_cc[] array: worktty.c_cc[ VMIN ] = 1; worktty.c_cc[ VTIME ] = 0; First change causes ‘read()’ function to return as soon as at least one character has been typed Second change causes ‘read() function to return without any time-delay

An application You could use this ‘non-canonical’ terminal mode to implement visual “user feedback” in your ‘password’ program Whenever the user presses a new key, the program immediately responds by printing a neutral character (e.g. ‘*’) to confirm that it has indeed received the user’s input Special handling for ‘backspace’ is needed Suggestion: Try it first in a C/C++ program