Thread: Overwriting Console Output

Hello,
I got a problem trying to programm under Linux. I was trying to implement a fuction that will overwrite an input sentence. The function shall overwrite the read letters by hitting the Enter Button.

Code:

int main(void) {
    char Password[10];
    int i=0,j;
    do{
        Password[i]=getche();
        if(Password[i]!=8) i++;         
        else{                       // special case: Backspace
            printf(" \b");          
            if(i>0) i--;
        }
    }while(Password[i-1]!=13);
    for(j=0;j<i-1;j++)printf("*"); // overwrite it with '*'
    printf("\n%s",Password);
}

It works fine under Windows but there is no getche() function under Linux.

Do you know another possibility?

With best regards

I copied it to my Eclipse IDE, but it still waits for me to press Enter to continue... Do i have to link the terminos header to the linker library like math.h with a "m"?

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <termios.h>
char getch(){
    char Wahl = 0;
    struct termios old = { 0 };

    fflush(stdout);
    if (tcgetattr(0, &old) < 0){
        perror("tcsetattr()");
    }
    old.c_lflag &= ~ICANON; /* deactivate */
    old.c_lflag &= ~ECHO;
    old.c_cc[VMIN] = 1; /* send each byte */
    old.c_cc[VTIME] = 0;/* don't wait */
    if (tcsetattr(0, TCSANOW, &old) < 0){
        perror("tcsetattr ICANON");
    }
    if (read(0, &Wahl, 1) < 0){
        perror("read()");
    }
    old.c_lflag |= ICANON; /* activate */
    old.c_lflag |= ECHO;
    if (tcsetattr(0, TCSADRAIN, &old) < 0){
        perror("tcsetattr ~ICANON");
    }
    //printf("%c\n", Wahl);
    return Wahl;
}

int main(void) {
    char Passwort[10];
    int i=0,j;
    do{
        Passwort[i]=getch();
        printf("%c",Passwort[i]);
        if(Passwort[i]!=8) i++;     // normal input
        else{                       // Special case: Backspace
            printf("  ");          
            if(i>0) i--;
        }
    }while(Passwort[i-1]!=13);
    for(j=0;j<i-1;j++)printf("*"); // Pressing of Enter will overwrite the output
    return 0;
}

getch() is working now but the Problem is now. By pressing Backspace it wont delete a letter but creating a weird output on screen. Same goes with the function of Enter...

Okay i got it:

Code:

int main(void) {
    char Passwort[10];
    int i=0,j;
    printf("Gebe das Passwort ein:\n");
    do{
        Passwort[i]=getch();
        if(Passwort[i]==10) break;
        if(Passwort[i]!=127){
            printf("%c",Passwort[i]);
            i++;                         // Normale Eingabe
        }
        else{                       // Sonderfall: Backspace
            printf("\b \b");          // Überschreibe Eingabe in Konsole
            if(i>0) i--;
        }

    }while(Passwort[i]!=10);
    printf("\r");
    for(j=0;j<i;j++)printf("*"); // Bei Drücken von Enter wird der vorherige Text überschrieben
}

Figured out the ascii symbols for Enter and backspace

That's pretty close. You need to ignore the backspace character completely if there's no characters in the password. You need to ensure that you don't overflow the password string. You should probably print asterisks as the password is entered. And you need to zero-terminate the string.

Code:

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

void die(const char *msg) {
    perror(msg);
    exit(EXIT_FAILURE);
}

char getch() {
    char ch = 0;
    struct termios old = { 0 };
    fflush(stdout);
    if (tcgetattr(0, &old) < 0)              die("tcsetattr()");
    old.c_lflag &= ~ICANON; // deactivate
    old.c_lflag &= ~ECHO;
    old.c_cc[VMIN] = 1;     // send each byte
    old.c_cc[VTIME] = 0;    // don't wait
    if (tcsetattr(0, TCSANOW, &old) < 0)     die("tcsetattr ICANON");
    if (read(0, &ch, 1) < 0)                 die("read()");
    old.c_lflag |= ICANON;  // activate
    old.c_lflag |= ECHO;
    if (tcsetattr(0, TCSADRAIN, &old) < 0)   die("tcsetattr ~ICANON");
    return ch;
}

int main(void) {
    enum { ENTER = 10, BACKSPACE = 127 };
    char pass[20];
    int i = 0;

    printf("Enter your password:\n");

    while (i < (int)sizeof pass - 1) {
        char ch = getch();
        if (ch == ENTER) break;
        if (ch != BACKSPACE) {
            putchar('*');
            pass[i++] = ch;
        }
        else if (i > 0) {
            printf("\b \b");
            i--;
        }
    }
    pass[i] = '\0';

    printf("\npass: %s\n", pass);

    return 0;
}

Code:

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

void die(const char *msg) {
    perror(msg);
    exit(EXIT_FAILURE);
}

char getch() {
    char ch = 0;
    struct termios old = { 0 };
    fflush(stdout);
    if (tcgetattr(0, &old) < 0)              die("tcsetattr()");
    old.c_lflag &= ~ICANON; // deactivate
    old.c_lflag &= ~ECHO;
    old.c_cc[VMIN] = 1;     // send each byte
    old.c_cc[VTIME] = 0;    // don't wait
    if (tcsetattr(0, TCSANOW, &old) < 0)     die("tcsetattr ICANON");
    if (read(0, &ch, 1) < 0)                 die("read()");
    old.c_lflag |= ICANON;  // activate
    old.c_lflag |= ECHO;
    if (tcsetattr(0, TCSADRAIN, &old) < 0)   die("tcsetattr ~ICANON");
    return ch;
}

int main(void) {
    enum { ENTER = 10, BACKSPACE = 127 };
    char pass[20];
    int i = 0, j;

    printf("Enter your password:\n");

    while (i < (int)sizeof pass - 1) {
        char ch = getch();
        if (ch == ENTER) break;
        if (ch != BACKSPACE) {
            putchar(ch);
            pass[i++] = ch;
        }
        else if (i > 0) {
            printf("\b \b");
            i--;
        }
    }
    pass[i] = '\0';
    printf("\r");
    for(j=0;j<strlen(pass);j++)printf("*");
    printf("\n");

    return 0;
}

Thanks alot! But my plan was to show the input password on screen till you press Enter. Not the other way around
So the code above is now the right one.

Originally Posted by dani2304

But my plan was to show the input password on screen till you press Enter. Not the other way around

Obviously I only displayed the password for debugging purposes.
Only an idiot would display a password!

Yeah that would be weird

But maybe you can help me with another little problem.

Code:

#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
void Zeitmessung(){
    printf(" Wrong Login data for the third time in a row. 200 seconds time-out!");
    clock_t start,end;
    double duration=0;
    start=clock();
    for(;;){
        end=clock();
        duration= (double)((end - start) / CLOCKS_PER_SEC);
        if(duration>=200) break;
    }
    return;
}

int main(){
Zeitmessung();
}

Working with linux the Output of the printf() command is also delayed by the duration. But shouldnt it print out first and then go to the infinite loop?
At Windows it does excatally what i want it to do...

You need to flush the output. Either end your print statement with a newline, or follow it with fflush(stdout).