c Exit if string is empty

Question

I have the below code which is supposed to exit if the provided user input is empty i.e they press [ENTER] when asked for the input. However it doesnt do anything if [ENTER] is pressed.

printf("Enter the website URL:\n");
scanf("%s", str);
if(strlen(str) == 0) {
    printf("Empty URL");
    exit(2);
}

Ismail Badawi · Accepted Answer · 2012-03-12 17:42:47Z

4

If the user just presses enter, the input will still contain a newline ('\n'). Your condition should be

if (!strcmp(str, "\n"))

answered Mar 12, 2012 at 17:42

Ismail Badawi

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2 Comments

user229044 Over a year ago

This fails if ctrl-d is used, or if input is being redirected from a file. The correct solution would be to trim whitespace from the input and then check for an empty string.

Adam Mihalcin Over a year ago

In a sense this is a correct answer, but it misses a big issue for beginners in C: using scanf instead of a safe replacement like fgets. I'll upvote if you edit to include a discussion of why scanf is bad.

Richard J. Ross III · Accepted Answer · 2012-03-12 17:47:28Z

2

I use a isempty function:

int isempty(const char *str)
{
    for (; *str != '\0'; str++)
    {
        if (!isspace(*str))
        {
           return 0;
        }
    }

    return 1;
}

Also, I would recommend using fgets over scanf, as scanf is unsafe and can lead to buffer overflows.

fgets(str, /* allocated size of str */, stdin);

answered Mar 12, 2012 at 17:47

Richard J. Ross III

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Comments

FatalError · Accepted Answer · 2012-03-12 17:47:11Z

1

%s with scanf() will discard any leading whitespace, of which it considers your Enter keypress. If you want to be able to accept an "empty" string, you'll need to accept your input in another way, perhaps using fgets():

printf("Enter the website URL:\n");
fgets(str, SIZE_OF_STR, stdin);
if(!strcmp(str,"\n")) {
    printf("Empty URL");
    exit(2);
}

Keep in mind the above code does not consider EOF, which would leave str unchanged.

answered Mar 12, 2012 at 17:47

FatalError

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Comments

orustammanapov · Accepted Answer · 2012-03-12 17:44:56Z

0

shouldn't you check for '\n' - new line? Enter will represented as a new line character '\n'

answered Mar 12, 2012 at 17:44

orustammanapov

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