how to limit digit amount when getting user input and how to automatically press enter-CodePudding

I want to write a line of code that when you enter 4 numbers the computer automatically presses enter after a set numbers of digit was entered, how do i do it?

CodePudding user response：

A more or less portable solution using GNU readline:

#include <readline/readline.h>
#include <stdio.h>
#include <stdlib.h>

static int limit_rl(FILE *f)
{
    if (rl_end >= 4)
    {
        return '\n';
    }
    return rl_getc(f);
}

int main(void)
{
    char *str;

    rl_getc_function = limit_rl;
    str = readline("> ");
    printf("%s\n", str);
    free(str);
    return 0;
}

CodePudding user response：

it depends on your OS:

as an example, on a linux system, with system calls:

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

int main(void){
  int c;
  int passwd[4]={0};
  system ("/bin/stty raw"); //use system call to make terminal send all keystrokes directly to stdin
  for(unsigned short nb_char = 0;  nb_char < 4; nb_char  ) 
  {
    do
    {
        c=getchar();
    }while((c < 48) || (c > 57));
    passwd[nb_char] = c;
  }
  printf("\npassword is:");
  for(unsigned short nb_char = 0;  nb_char < 4; nb_char  ) 
  {
      putchar(passwd[nb_char]);
  }
  system ("/bin/stty cooked");//use system call to set terminal behaviour to more normal behaviour
  return 0;
}

output is:

1234
password is:1234

for windows, see getch():

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
int main(void){
  int c;
  int passwd[4]={0};

  for(unsigned short nb_char = 0;  nb_char < 4; nb_char  ) 
  {
    do
    {
        c=getch();
    }while((c < 48) || (c > 57));
    passwd[nb_char] = c;
  }
  printf("\npassword is:");
  for(unsigned short nb_char = 0;  nb_char < 4; nb_char  ) 
  {
      putchar(passwd[nb_char]);
  }
  return 0;
}

See this answer for a complete answer