Program crashing when attempting to convert array into a string with null character

Question

I'm filling an array with 40 pseudo-random capital letters and then converting the array into a string by terminating it with a null character at the 41st element, but my program crashes when I do the latter. Here is the code:

char s1[41];
int i;

for (i = 0; i < 41; i++)
{
    s1[i] = ((rand() % 26) + 'A');
}
s1[i] = '\0';

puts(s1);

The program runs just fine if I print out each character in the array one at a time without the null assignment statement, but everything fails when the null assignment is included. I am required to convert the array into a string and then use the puts function. Why is this happening? Is a string not simply an array of characters terminated with a '\0' (null)? What is causing this? Is this a compiler error? Attached is a screenshot of the error message.

I am using Microsoft Visual Studio Express 2013, if it matters.

Off by one. You are writing right behind the array, to s1[41]. — AlexD
– AlexD, Commented Apr 28, 2015 at 19:41
Unrelated, but you might want to look into random number generation. (rand() % 26) = veeery bad. — Greenflow
– Greenflow, Commented Apr 28, 2015 at 19:43
You're lucky this program crashes. If it hadn't, this bug might have gone unnoticed for a long time! — Mr Lister
– Mr Lister, Commented Apr 28, 2015 at 19:44
@gldraphael He does that to nul terminate the string, it isn't there by default unless s1 had static storage duration. — nos
– nos, Commented Apr 28, 2015 at 20:09

sfjac · Accepted Answer · 2015-04-28 19:46:38Z

1

To clarify the comments, if it's not obvious to you: i==41 after exiting the loop, so s1[i] is writing outside the bounds of the array. This is undefined behavior and, as mentioned, you're lucky it crashed.

answered Apr 28, 2015 at 19:46

sfjac

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MV94 · Accepted Answer · 2015-04-28 19:53:23Z

1

Just realized that I was assigning the null character to a location outside of the array. I needed to decrement i.

answered Apr 28, 2015 at 19:53

MV94

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2 Comments

user2249683 Over a year ago

No: make it char s1[41 + 1];

David C. Rankin Over a year ago

Don't decrement i, change your loop limit. for (i = 0; i < 40; i++) will write your 40 values (i.e. indexes 0-39, then the 41st value goes in s[i] = 0; where i is 40.

Caleb Gallegos · Accepted Answer · 2015-04-28 19:46:49Z

0

Make 41 to 40 then try again

i < 40

Usually trial and error with this number can give you different results

   char s1[41];
int i;



for (i = 0; i < 40; i++)
{
    s1[i] = ((rand() % 26) + 'A');
}
s1[i] = '\0';

puts(s1);

answered Apr 28, 2015 at 19:46

Caleb Gallegos

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