How does C know the end of my string?

Question

I have a program in which I wanted to remove the spaces from a string. I wanted to find an elegant way to do so, so I found the following (I've changed it a little so it could be better readable) code in a forum:

char* line_remove_spaces (char* line)
{
    char *non_spaced = line;
    int i;
    int j = 0;
    for (i = 0; i <= strlen(line); i++)
    {
        if ( line[i] != ' ' )
        {
            non_spaced[j] = line[i];
            j++;
        }
    }
    return non_spaced;
}

As you can see, the function takes a string and, using the same allocated memory space, selects only the non-spaced characters. It works!

Anyway, according to Wikipedia, a string in C is a "Null-terminated string". I always thought this way and everything was good. But the problem is: we put no "null-character" in the end of the non_spaced string. And somehow the compiler knows that it ends at the last character changed by the "non_spaced" string. How does it know?

Answer 1

This does not happen by magic. You have in your code:

for (i = 0; i <= strlen(line); i++)
              ^^

The loop index i runs till strlen(line) and at this index there is a nul character in the character array and this gets copied as well. As a result your end result has nul character at the desired index.

If you had

for (i = 0; i < strlen(line); i++)
              ^^

then you had to put the nul character manually as:

for (i = 0; i < strlen(line); i++)
{
    if ( line[i] != ' ' )
    {
        non_spaced[j] = line[i];
        j++;
    }
}
// put nul character
line[j] = 0;

Answer 2

Others have answered your question already, but here is a faster, and perhaps clearer version of the same code:

void line_remove_spaces (char* line)
{
  char* non_spaced = line;

  while(*line != '\0')
  {
    if(*line != ' ')
    {
      *non_spaced = *line;
      non_spaced++;
    }

    line++;
  }

  *non_spaced = '\0';
}