What is the length of string when we don't explicitly initialize a character array with null character?

Question

I tried the following code on GNU GCC compiler and its giving me output as 26. But I don't understand how the code is working especially what actually strlen() function is doing.

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

using namespace std;

int main()
{
    char b[]={'G','E','E','K','S',' ','F','O','R',' ','F','U','N'};
    cout<<sizeof(b)<<endl;
    cout<<strlen(b)<<endl;
    return 0;
}

Answer 1

What is the length of string when we don't explicitly initialize a character array with null character?

The length of the string is the number of characters it contains. (Let us ignore for simplicity the fact that there is a difference between the number of graphemes, code points and code units, all of which are the length of the string depending on perspective. In the context of this answer, character == code unit).

The length of a null terminated string is the number of characters before the null terminator. If a string doesn't contain a null terminator, then it isn't a null terminated string.

 strlen(b)

b isn't a null terminated string. strlen requires that the argument points to a null terminated string. If the requirement isn't satisfied, the behaviour of the program is undefined.

But its working except that for giving o/p 13, its giving output 26

The behaviour is undefined. Possible behaviours include, none of which are guaranteed:

 - working
 - not working
 - random output
 - non-random output
 - the expected output
 - unexpected output
 - no output
 - any output
 - crashing at random
 - crashing always
 - not crashing
 - corruption of data
 - different behaviour, when executed on another system
 -                    , when compiled with another compiler
 -                    , on tuesday
 -                    , only when you're not looking
 - same behaviour in all of the above cases
 - anything else within the power of the computer (hopefully limited by the OS)

Undefined behaviour is undesirable in any program.

Answer 2

But I don't understand how the code is working especially what actually strlen() function is doing.

When you pass a string to strlen that is not null terminated, you are invoking undefined behavior. Don't count on any predictable behavior.

Answer 3

The Length of String or strlen refers to a C string that is as long as the number of characters between the beginning of the string and the terminating null character (without including the terminating null character itself). for example:

char TestStr[25] = "Test String";

if you use sizeof(TestStr) it will evaluate the 25 chars in the array but with strlen(TestStr) will evaluate only 11.

Hope this helps

Answer 4

Another case related to your issue:

int main()
{
  static const char fun_text[] = {'f', 'u', 'n', '\0', 'c', 'a', 's', 'e'};
  std::cout << strlen(fun_text) << std::endl;
  std::cout << sizeof(fun_text) << std::endl;
  return 0;
}

In the above case, the array size is 8.
However, the strlen function will return 3 because it runs into a '\\0' character before reaching the end of the array.

Summary

The strlen function returns length based on content.
The sizeof operator return the length of the array, regardless of content.