Empty string ends for loop

Question

What's wrong in the below program (What's happening here)? It should break the for loop after the user inserts empty string (presses only ENTER), but in my case it ends in endless for loop. I tried what is in the comments with no success.

#include <stdio.h>
#include <string.h>                                      

struct S {
  char str [10];
};

int main(void)
{
  int n;
  struct S strings [10];
  for (n = 0; n < 10; n++) {
    # fflush(stdout);
    scanf("%s", strings[n].str);
    if (strlen(strings[n].str) == 0)
      break;
    # getchar();
  }
  printf("done");
  return 0;
}

When I replace scanf with gets(strings[n].str); done is never printed. How would you fix it?

This sample solution works. Is there a difference in comparison to my code?

It's not the cause of your problem, but please just avoid using scanf; it's notoriously hard to use, and the way you're using it is susceptible to a buffer overflow. (And never use gets). c-faq.com/stdio/scanfprobs.html — jamesdlin
– jamesdlin, Commented May 29, 2012 at 10:48
scanf("%s", strings[n].str); expects at least one non-whitespace character. scanf will not return until it finds one. — Daniel Fischer
– Daniel Fischer, Commented May 29, 2012 at 15:43
@DanielFischer OK, note that I tried gets too which is used in sample solution by Chuck Allison too. — xralf
– xralf, Commented May 29, 2012 at 16:59

Sachin Kainth · Accepted Answer · 2012-09-28 14:04:43Z

5

The enter key is not empty string, it is an ascii character or rather two characters a CR and LF (on Windows).

edited Sep 28, 2012 at 14:04

answered May 29, 2012 at 10:20

Sachin Kainth

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2 Comments

phoxis Over a year ago

end of line character depends, in *nix it is LF, Win: CRLF Mac CR , check : en.wikipedia.org/wiki/Newline

Sachin Kainth Over a year ago

Yes I know that - but I was just talking about Windows - not that the others don't matter :-)

Martin Thompson · Accepted Answer · 2012-05-29 10:34:30Z

2

You shouldn't use strlen to find out if the input is empty. As others have said, when you press ENTER you get one or two characters sent to you.

You could instead check the first character in the string and see if it is '\n' or '\r'

answered May 29, 2012 at 10:34

Martin Thompson

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Comments

phoxis · Accepted Answer · 2012-05-29 10:23:07Z

1

scanf returns exactly what you've input... i.e. a crlf pair I'd imagine!

edited May 29, 2012 at 10:23

phoxis

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answered May 29, 2012 at 10:20

Jeff Watkins

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1 Comment

phoxis Over a year ago

The word "returns" here is ambiguous.

Some programmer dude · Accepted Answer · 2012-05-29 10:26:27Z

1

The problem with using scanf is that it expects something, not an empty string. You solve this by using e.g. fgets instead of scanf:

if (fgets(strings[n].str, sizeof(strings[n].str), stdin))
{
    /* You got a string, it will contain the newline! */
}

answered May 29, 2012 at 10:26

Some programmer dude

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