C - Limit the string length

Question

(Sorry for my bad english !)

I wrote a program that asks you to type a password no longer than a certain number, eight characters in this case. The characters that pass the limit will be cut out from the array:

#include <stdio.h>
#define MAXCHAR 8

main()
{
    char password[MAXCHAR];
    int i;
    char c;

    printf("Insert password: MAX 8 CHARS!\n\n");
    for(i = 0; i <= MAXCHAR; i++){
        c = getchar();

        if(i == MAXCHAR){
            break;
        }
        else{
            password[i] = c;
        }
    }

    printf("%s\n", password);
}

So the program works BUT there is a "strange" problem. If the limit IS EIGHT and I type a password longer than eight characters (Example: P455w0rds98) the output will be like this:

P455w0rd☺

So it puts a smiley at the end and I don't know why. It happens only if a the limit is established at eight.

Solution

You must specify the length to print or terminate the string. Otherwise, you will invoke undefined behavior. Try this, in which the latter method is implemented.

#include <stdio.h>
#define MAXCHAR 8

int main(void)
{
    char password[MAXCHAR + 1]; /* allocate one more element for terminating null-character */
    int i;
    char c;

    printf("Insert password: MAX 8 CHARS!\n\n");
    for(i = 0; i <= MAXCHAR; i++){
        c = getchar();

        if(i == MAXCHAR){
            break;
        }
        else{
            password[i] = c;
        }
    }
    password[MAXCHAR] = '\0'; /* terminate the string */

    printf("%s\n", password);
}

Some people say that the if(i == MAXCHAR){ break; } part doesn't look good, so here is another code example:

#include <stdio.h>
#define MAXCHAR 8

int main(void)
{
    char password[MAXCHAR + 1]; /* allocate one more element for terminating null-character */
    int i;

    printf("Insert password: MAX 8 CHARS!\n\n");
    /* read exactly 8 characters. To improve, breaking on seeing newline or EOF may be good */
    for(i = 0; i < MAXCHAR; i++){
        password[i] = getchar();
    }
    password[MAXCHAR] = '\0'; /* terminate the string */
    getchar(); /* to match number of call of getchar() to the original: maybe for consuming newline character after 8-digit password */

    printf("%s\n", password);
}

Comments

[Claudio Pisa]

@BruceDavidWilner Why would you say such a thing ? "...computer programming may not be for you." Damn you destroyed me, i was just a rookie when a posted this. What you said is not encouraging at all.

[Sourav Ghosh]

for(i = 0; i <= MAXCHAR; i++)..are you sure? is not it, like 9 elements? you can get rid of the whole break; thingy....

[Eugene Sh.]

@SouravGhosh there is a hack inside the loop to work around it :) Update: Yeah, you have seen it. A classic example of putting a patch without understanding the root cause.

[MikeCAT]

@SouravGhosh 9 characters will be read but only 8 characters + null-character will be stored. It may be weird specification, but I don't think it is invalid.

[Eugene Sh.]

@MikeCAT And it will mislead the user, thinking his password is 9 characters.

[MikeCAT]

@EugeneSh. It may be 8 characters + newline, but password with 7 characters or less isn't allowed here... What does "MAX" mean?

[Eugene Sh.]

@MikeCAT Based on your code it's 8+newline. Anyway, this whole if-break thing is a nonsense.

[tofro]

fgetsof MAXCHAR characters actually reads MAXCHAR - 1 characters and uses the last one for the string end. It will also stop reading after the MAXCHAR-1th character has been seen, regardless of whether it was a newline or not. So with fgets you might run into the problem of accepting maximum 6 chars+newline or 7 chars (with a newline still remaining in the stream), which makes it even harder to hit the exact maximum character count.

[Sourav Ghosh]

@tofro and why do you think that? 1) MAXCHAR needs to be adjusted, it's not a fixed value. 2) You have to check and remove the trailing newline, if any, then check for the string length, what's the issue there? 3) with a newline still remaining in the stream that is not supposed to be of much concern here, is not it?

[tofro]

@SouravGosh The main thing is: fgets will not wait for confirmation of the password by the user once he has supplied the maximum amount of characters - Which is at least irritating, but can lead to all sorts of errors. Works, but I would never use that in production code.

[Claudio Pisa]

I got two question now: 1) Why "if(i == MAXCHAR){ break; }" doesn't look good ? 2)Ok but why that goddamn smile appears only if the limit is eight ? o.O And why does it appear always ?

[MikeCAT]

@ClaudioPisa 1) Thinking again, I don't think the code is so bad because using it you can merge two calls of getchar() into one. 2) By chance if you invoke undefined behavior.

[MikeCAT]

@EugeneSh. Why do you think this whole if-break thing is a nonsense.?