Thread: simple question about a char

say I have the following:

char c[20] = "adajhdqwuihdquiwh"

and in the middle of the code I want to reset c to an empty string again, how can I do this?

Originally Posted by vart

c[0] = '\0';

That is the easiest way, but might not work for everything. It will make the first character '/0' which will make it seem like an empty string. But if later on you change the first character then you ll have your previous string with the first character changed. A more safe way is to do this:

Code:

for (i=0; i<20; ++i) {
   c[i] = '\0';
}

so all the characters of the string are "empty". If you empty the string then write something you will have a string with that something only. For example:

Code:

char c[20] = "adajhdqwuihdquiwh";
c[0] = '/0'; //c[] is empty
c[0] = 'b'; c[1] = 'e';
printf("%s", c); //outputs "ceajhdqwuihdquiwh" which isn't want you want

on the other hand

Code:

char c[20] = "adajhdqwuihdquiwh";
for (i=0; i<20; ++i) {
   c[i] = '\0';              //c[] is empty
c[0] = 'b'; c[1] = 'e';
printf("%s", c); //outputs "ce" which is what you want

This is fine for most applications:

Code:

c[0] = '\0';

But if c stores something secret like a password and you don't want the risk of that information (or as least part of the information) to be found by accident, or by some hacker, you should nullify all the data like this:

Code:

memset( c, '\0', sizeof( c ) );

Originally Posted by foxman

I'm not buying this. If you later change the first character, and only the first character, then it's because you have good reason to do so and you know what you are doing. I find that vart method is perfectly safe.

But it is not perfectly safe, proven from some examples given. Lets say you work in a project and I tell you that this function empties a string. If you are an experience C programmer you would demand to know exactly what I mean by "empty string". But if not you will just assume that the string is empty. Which means that if you write one character you will have a string with one character. If you write two characters you will have a string with two characters.
Now, many will argue that whenever you write a string in C you should always put the '\0' at the end. So, if we assume that everybody follows that rule then emptying only the first character is perfectly safe. But that is just an assumption. You never know how someone might want to manipulate a string so it better to assume the worst and empty the whole string.
As for efficiency, it is obvious that is better to empty only the first char. But efficiency isn't everything.
And I suppose memset() is better to use than a loop.

From what I've learned about mainframes over the past few years, they don't use NUL terminated strings. They use buffers of specific sizes... So if you're communicating with, or programming on a mainframe, you may need to blank out the whole string with NULs. But that is probably only true if you're copying the string with memcpy() rather than strcpy().

But then those aren't C-strings.

Originally Posted by robwhit

But then those aren't C-strings.

You're right. But that's the best argument I could come up with (other than security) for why you might want to blank out the whole string.
For most applications, just setting the first char to '\0' should be good enough, but there are certain times when you should be blanking out the whole string.

Even though strings are assumed to end with '\0' that doesn't mean that everybody knows the correct terminology. The title says a "simple question about char" even though it asks how to empty a string. So of course I assume that the OP isn't familiar with what the correct definition of a string is in C. A lot of people thing a string is just an array of char.
-So if the actual question is emptying an array of char then you should empty everything in the array (except if you want something more specific). As you would do if you wanted to empty an array of int.
-If the actual question is emptying a string (a null terminated array of char) then you should just empty the first element.

Depending how you see an array of char will give you the correct solution.

A last thing is that when initializing an array of char as you did it is terminated by a '\0'. So in you case it will have 18 elements intialized. The 17 you put and the '\0' one.

>> A lot of people thing (sic) a string is just an array of char.

But a string is an array of char in C. Those people would be right.

Zeroing out memory can be done if you have a security concern, like cpjust suggests, and even then one could consider it a little overkill since RAM is not persistent memory.