Thread: Function readlines() from K&R

Hi!

Please, help me to get through this function.
Here is what I do and don't understand in it.
First the function together with get_line function.

Code:

 
/* readlines: read input lines */
int readlines(char *lineptr[], int maxlines)
{
    int len, nlines;
    char *p, line[MAXLEN];
    nlines = 0;
    
    while ((len = get_line(line, MAXLEN)) > 0)
    {
        if (nlines >= maxlines || (p = alloc(len)) == NULL)
        {
            return -1;
        }
        else
        {
            line[len - 1] = '\0'; /* delete newline */
            strcpy(p, line);
            lineptr[nlines++] = p;
        }
    }
    return nlines;
}
/* getline: read a line into s, return length */

int get_line (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;
}

Let's assume the first input of "Hello world".

Readlines()
1) defines pointer to char p, and character array line[]
2) first iteration of while loop:
- calls getline()
- if there was an input getline() returns the value
of i, which is assigned to the variable len
- nlines is set to 0, it's a counter (index)
- loop checks if len is bigger then 0; len = 12
(Hello world + '\n' + '\0'), condition is true
- if statement allocates memory to p by calling alloc
function
- then if statement checks its conditions; they are
false
- move to else part, and set line[11] to '\0'
- copy bytes from line to p by strcpy(p, line)
- make lineptr[0] point to p[0], now
the memory stored for whatever lineptr should point to
is filled with "Hello world"
- nlines is incremented and is now equals 1
- loop has finished its first iteration

3) second iteration of while loop starts the processes all
over again, by calling get_line function, which in turn
overwrites all values stored in line[], and after get_line
is done with its job, line[] has new input text stored in
it. Then goes the same process as in the first iteration.
By overwriting values of line[] the get_line allows to get
a few input lines.
Is this assumption correct?

Thanks!

If by assumption, you mean fact, then yes, your analysis is essentially correct.