CODEWITHSUNDEEP
cstdinfgetcfflush
pro neon
pro neon

Reputation: 253

undefined behaviour while using fgetc(stdin)

Before asking this question, I have read: fgetc(stdin) in a loop is producing strange behaviour. I am writing a program to ask the user if they want to exit. Here is my code: 

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

int ask_to_exit()
{
    int choice;
    while(choice!='y'||choice!='n')
    {   printf("Do you want to continue?(y/n):");
        while((fgetc(stdin)) != '\n');
        choice = fgetc(stdin);
        if(choice=='y') return 0;
        else if(choice=='n') return 1;
        else printf("Invalid choice!\n");
    }
}

int main()
{
    int exit = 0;
    while(!exit)
    {
        exit = ask_to_exit();
    }
    return 0;
}

Since fflush(stdin) is undefined behaviour, I did not use it. After following the solution in the other question, I still get the error. Here is a test run of the above program:

$./a.out
Do you want to continue?(y/n):y<pressed enter key>
<pressed enter key>
Invalid choice!
Do you want to continue?(y/n):y<pressed enter key>
<pressed enter key>
Invalid choice!
Do you want to continue?(y/n):n<pressed enter key>
<pressed enter key>
Invalid choice!
Do you want to continue?(y/n):n<pressed enter key>
n<pressed enter key>
<program exits>

Upvotes: 0

Views: 173

Answers (2)

Stephen Docy
Stephen Docy

Reputation: 4788

You need to get some input before checking the value of choice or it will be uninitialized. And also, you are draining the left-over input before grabbing the first character, you should do it after:

int ask_to_exit()
{
    int choice;

    do
    {
        printf("Do you want to continue?(y/n):");
        choice = fgetc(stdin);

        while (fgetc(stdin) != '\n');

        if (choice == 'y') {
            return 0;
        } else if (choice == 'n') {
            return 1;
        } else {
            printf("Invalid choice!\n");
        }
    } while (1);
}

ouput:

Do you want to continue?(y/n):g
Invalid choice!
Do you want to continue?(y/n):h
Invalid choice!
Do you want to continue?(y/n):j
Invalid choice!
Do you want to continue?(y/n):k
Invalid choice!
Do you want to continue?(y/n):m
Invalid choice!
Do you want to continue?(y/n):y
Do you want to continue?(y/n):y
Do you want to continue?(y/n):y
Do you want to continue?(y/n):n
Press any key to continue . . .

Upvotes: 1

R Sahu
R Sahu

Reputation: 206577

I see the following problems in your code.

  1. You are using choice before initializing it.
  2. You are skipping a line of input before reading anything into choice.

Error noticed by @WhozCraig:

choice!='y'||choice!='n' is always true.

I suggest:

int ask_to_exit()
{
   int choice;
   while ( 1 )
   {
      printf("Do you want to continue?(y/n):");
      choice = fgetc(stdin);
      if ( choice == 'y' )
      {
         return 0;
      }
      if ( choice == 'n' )
      {
         return 1;
      }

      printf("Invalid choice!\n");

      // Skip rest of the line.
      int c;`
      while( (c = fgetc(stdin)) != EOF && c != '\n');

      // If EOF is reached, return 0 also.
      if ( c == EOF )
      {
         return 0;
      }
   }
}

Upvotes: 0

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