CODEWITHSUNDEEP
clinuxinputscreenncurses
BowPark
BowPark

Reputation: 1460

ncurses and getch - Handling erase and arrow characters

In a C program under Linux and ncurses, I need to acquire characters from the keyboard (so, from the user) and to store them into a string, which is considered complete only when the user presses Enter. At the same time, however, I need to display a screen echo for the user, so that he can see what he is writing and he can correct some typos, if necessary.

My bunch of code is working, but it can't handle the erase and arrow characters.

In this question a solution is provided to correctly store the string, when Backspace or Del are pressed. But what solution could be choosen in order to display a correct screen echo, even considering the arrow keys?

My code is essentially:

while(1)
{
if (getch() =! ERR)
// store the character into an array
if (getch() == 10)
// terminate the string and print it on screen
}

The characters are acquired through a getch() one by one. Like in the linked question, if I don't use noecho() and I press Backspace, the sequence ^? is displayed instead of removing the previous character from the screen. If I use noecho(), the program should show real-time to the user what's happening. Should I re-print the string at every while cycle? It would be cumbersome.

So, how can I correctly display to the user what's happening?

Upvotes: 1

Views: 3356

Answers (3)

dnt994
dnt994

Reputation: 31

This is my solution, it's pretty easy and don't need noecho();

getyx( stdscr, y, x ); //get current cursor position
x-= 3; //go three position back, one for the char to erase, the other two to erase the backspace char ^? 
mvprintw( y, x, "   " ); //erase chars
move( y, x ); //get in right position for new input

Upvotes: -1

Thomas Dickey
Thomas Dickey

Reputation: 54505

If the OP's program used the keypad() function, then left-cursor (arrow) and the erase key would have the same effect. getnstr does not support inline editing (moving the cursor within the line). (Incidentally, other implementations of curses do nothing with cursor-keys).

As an example of a program which does support inline-editing, dialog is useful (it works with UTF-8). On the other hand, because it stores the responses as a plain character string, it is more complicated, say, than something explicitly written to use wget_wch.

cdk does not handle UTF-8.

ncurses does not provide a more interesting function because:

  • that functionality is already in the form library
  • it does not qualify as an extension because one can readily develop various functions of this sort without knowing the library's internals.

Recently there was a related question (for blocking I/O) in ncurses, print and contemporary acquire strings (again, dialog does that).

Upvotes: 2

rici
rici

Reputation: 241721

If you want to read characters until the user presses Enter, you probably want to use the getnstr function instead of reading a character at a time. getnstr will interpret erase and kill characters, although it is far from a complete line-editing system.

Otherwise, you are going to end up having to handle all the cursor movement characters yourself. That's obviously more flexible but it's also a lot more work. If you go down that route, I suggest turning echo off and manually echoing (non-control) characters because that will give you much better control over the cursor position.

Upvotes: 0

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