Thread: How to shadow user input (as 'su' does for example)

Hi,

I am writing a small program ( still quite new to this), and i need to hide the input that the user enters (as su does for example, you cant see what you are typing). How can i do this?

You might want to have a look at this thread, it basically explains everything you want to do.

Hi,

Thanks for the reply, but its not quite working yet.

I now have the following code:

Code:

#include <stdlib.h>
#include <curses.h>
#include <stdio.h>

void passgets(char pword[], int maxlength)
{
        int counter = 0;
        char ch;
        while( (ch = getch() ) != '\r' && counter < maxlength-1 )
        {
                if(ch == '\b' && counter > 0)
                {
                        printf("\b \b");
                        counter--;
                }
                else
                {
                        pword[counter++] = ch;
                        putchar('*');
                }
        }
        pword[counter] = '\0';
}

int main()
{
        char password[20];
        passgets(password, 19);
        return 1;

}

I then compile it with

gcc prog.c -o prog -lcurses

When I run the program all it does is print 19 *'s and exits, doesnt take any input, etc. What could it be?

No what it is supposed to do is the following:
every time a key is pressed, it is added to the character buffer and a * is printed. Once the user pressed enter it quits
What it currently does is, when run, it prints 19 *'s and quits, as if my whole program consisted of printf("*******************");

I was looking at the man pages of getch and i read the following:

The getch, wgetch, mvgetch and mvwgetch, routines read a character from
the window. In no-delay mode, if no input is waiting, the value ERR is
returned. In delay mode, the program waits until the system passes
text through to the program.

What is this no-delay mode and does it have to do with that?

The quote you gave explains what no-delay mode is.

To expand ....

The behaviour of getch() and related functions is affected by settings that your program can control. One of those settings is no-delay mode which is changed using the nodelay() function -- Enter "man nodelay" (I seem to recall it's "man 3 nodelay" for the library function, but may be wrong on that) for information on the function call.

In no-delay mode, if there is no input pending, getch will return an error status. This is potentially useful if you want to poll for user input, but do something (eg count down for some period, so the program effectively only waits for a fixed time for input, but tells the user how long is remaining).

If not in no-delay mode, if there is no input pending, getch() will sit and wait until input is available.

Thanks for the reply, but I finally found something that works. It is slightly above my head but i am trying to fully understand what it does. BTW backspaces dont work here, any ideas on that?

Code:

#include <stdio.h>
#include <termios.h>
#include <unistd.h>
#include <ctype.h>

int mygetch ( void ) 
{
  int ch;
  struct termios oldt, newt;
  
  tcgetattr ( STDIN_FILENO, &oldt );
  newt = oldt;
  newt.c_lflag &= ~( ICANON | ECHO );
  tcsetattr ( STDIN_FILENO, TCSANOW, &newt );
  ch = getchar();
  tcsetattr ( STDIN_FILENO, TCSANOW, &oldt );
  
  return ch;
} 

int main()
{
  
  int ch;
  char pword[BUFSIZ];
  int i = 0;
  
  puts ("Enter your password");
  fflush(stdout);
  
  while ((ch = mygetch()) != EOF 
          && ch != '\n' 
          && ch != '\r' 
          && i < sizeof(pword) - 1)
  {
    if (ch == '\b' && i > 0) 
    {
      printf("\b \b");
      fflush(stdout);
      i--;
      pword[i] = '\0';
    }
    else if (isalnum(ch))
    {
      putchar('*');
      pword[i++] = (char)ch;
    }
  }

  pword[i] = '\0';
  
  printf ("\nYou entered >%s<", pword);
  
  return 0;
}

Only thing is backspace doesnt work, any ideas on that?

What do you get as output, for example?

With the new code (i.e. the code I posted above), I see everything I typed. What this means is that backspaces are effectively ignored, so if i type "hi <backspace>a" (intending to type ha), it will print hia.

Originally Posted by KONI

You might want to have a look at this thread, it basically explains everything you want to do.

The final version of the code has a backspace feature. If you study it, you're bound to see how it could work in your program, as well. Nevermind our silly squabbles.

Adak