Writing in the location outside of array

Question

I've just started learning programming. This is my first post. I'm reading a book "C Programming Language" by Kernighan and Ritchie, and I came across an example that I don't understand (section 1.9, p 30).

This program takes text as input, determines the longest line, and prints it. Char array line[MAXLINE] is declared, where MAXLINE is 1000. This should mean that the last element of this array has index of MAXLINE-1, which is 999. However, if you look at function getline, which is being passed line[] array as an argument (and MAXLINE as lim), it appears that if user input is a line longer than MAXLINE, i will be incremented until i = lim, that is, i = MAXLINE. Therefore, the statement line[i] = '\0' will be line[MAXLINE] = '\0'.

This looks wrong to me - how can we write to the line[MAXLINE] location, if the size of line[] is MAXLINE. Wouldn't it be writing into the location outside of the array?

The only explanation I can come up with is that when declaring char array[size], C language actually creates char array[size+1] array, where the last element is reserved for the NULL character. If so, this is pretty confusing, and isn't mentioned in the book. Can anyone confirm this, or explain what's going on?

#include <stdio.h>
#define MAXLINE 1000 /* maximum input line length */
int getline(char line[], int maxline);
void copy(char to[], char from[]);

/* print the longest input line */
main()
{
    int len;                           /* current line length */
    int max;                          /* maximum length seen so far */
    char line[MAXLINE];          /* current input line */
    char longest[MAXLINE];     /* longest line saved here */

    max = 0;

    while ((len = getline(line, MAXLINE)) > 0)
           if (len > max) {
           max = len;
           copy(longest, line);
           }
    if (max > 0) /* there was a line */
           printf("%s", longest);

return 0;
}

/* getline: read a line into s, return length */
int getline(char s[],int lim)
{
    int c, i;

    for (i=0; i < lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
        s[i] = c;
    if (c == '\n') {
        s[i] = c;
        ++i;
    }
    s[i] = '\0';

return i;
}

/* copy: copy 'from' into 'to'; assume to is big enough */
void copy(char to[], char from[])
{
    int i;
    i = 0;

    while ((to[i] = from[i]) != '\0')
        ++i;
}

Answer 1

This for loop appears to be doing the reading in getline:

for (i=0; i < lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
    s[i] = c;

It looks like i is incremented until it reaches lim - 1, not lim (where lim here is equal to MAXLINE in the case you were talking about). Hence, if the line is longer than MAXLINE, it stops after reading MAXLINE-1 characters, and tacks on the '\0' at the end like you expect.