gets() function gets skipped from the second time onwards

Question

I was playing with C and wrote this code:

   1 #include<stdio.h>
   2 #define ASK_PROMPT printf("\nDo you want to continue(Y/N):");
   3 main()
   4 {
   5    char main[20], i;
   6    start:
   7    printf("Please enter your string:\n");
   8    gets(main);
   9    printf("\nstring entered was:\n \n%s\n", main);
   10   ASK_PROMPT;
   11   scanf("%c",&i);
   12       
   13   if(i=='Y'||i=='y')
   14       goto start;
   15   getch();
   16   return;
   17 }

when I execute this code, the goto loop is not working correctly. On providing y or Y response to question asked from line 10, loop does work and line 7 is again executed/printed but line 8 was skipped(doesn't wait for the input to be provided).

• Can anyone explain why it is happening?
• How do I fix it?

Answer 1

I would suggest not to use gets.While using gets what if string entered is larger than the size of main[20] .gets won't prevent this and it will cause UB.

Use fgets-

fgets(main,20,stdin);

And reason for your problem is that '\n' after the scanf("%c",&i); which remains in stdin causes this as gets return as it encounter '\n'.

To avoid '\n' you can do this after scanf statement -

int c;
while ( (c = getchar()) != EOF && c != '\n' );

Answer 2

The problem:

After typing yEnter in response to your prompt, the input stream for your program contains the characters 'y' and '\n' (the newline character). The scanf call removes the 'y' character from the input stream, leaving the newline character in place.

gets reads from the input stream until it sees a newline character, which it discards; since the first thing it sees is the newline character from the previous entry, it returns without reading any further input.

The solution:

First, don't use gets. Ever. It was deprecated in the 1999 revision and has been removed completely from the 2011 revision. Using it will introduce a point of failure/security hole in your program. Replace that with a call to fgets instead, just be aware that fgets will try to store the newline in the target buffer if there's room.

Second, you need to consume that trailing newline from the input stream. The answer given by vish4071 shows one approach - use scanf to read the character following your input (which should be the newline) and discard it:

scanf( " %c%*c", &i ); 

The leading blank in the format string tells scanf to skip any leading whitespace, the %c conversion specifier tells scanf to read the next character from the input stream and store its value into i, and the %*c conversion specifier tells scanf to read the next character from the input stream and discard it.

Another option is to consume input until you see a newline:

scanf( " %c", &i );
while ( getchar() != '\n' )  // repeatedly read characters from input stream
  ;                          // until we see a newline

Yet another option is to read your input as another string:

char answer[3]; // space for y/n response plus newline plus 0 terminator
...
if ( fgets( answer, sizeof answer, stdin ) )
{
  if ( tolower( answer[0] ) == 'y' )
  {
    // do stuff
  }
}

You will probably want to check for the presence of the newline character in the answer buffer; if it isn't there, then the user typed in more characters than your buffer was sized to hold, which will likely foul up a future read, and you need to clear that spurious input before your next read operation:

if ( !strchr( answer, '\n' ) )
  while ( getchar() != '\n' )
    ;

Answer 3

Fix

Change scanf("%c",&i); to scanf("%c%*c",&i);.

Problem

The \n after inputting i is what is causing gets() not to wait for input.

Good practices

• Stop using gets(), goto etc. Instead, you can use fgets(), scanf() etc. Never use deprecated functions.
• Change char array name from main to something else, say m.

Thanks @Coolguy for the comment on use of scanf.

Answer 4

Minimal fix to existing code, just use gets(main) a second time. If the code is going to use ASK_PROMPT;, then the define for ASK_PROMPT doesn't need a trailing ; . As already suggested, you can use fgets() instead of gets(), and use a proper loop instead of start: and goto.

#include<stdio.h>
#define ASK_PROMPT printf("\nDo you want to continue(Y/N):")
int main()
{
    char main[20];
    start:
    printf("Please enter your string:\n");
    gets(main);
    printf("\nstring entered was:\n \n%s\n", main);
    ASK_PROMPT;
    gets(main);
    if(main[0]=='Y'||main[0]=='y')
        goto start;
    getch();
    return(0);
}