Why doesn't this while loop go on forever? Beginner C question

Question

#define MAXLINE 1000
char pattern[] = "ould";
main()
{
    char line[MAXLINE];
    int found = 0;

    while (getline(line,MAXLINE) > 0)
       if (strindex(line, pattern) >= 0 ) {
          printf("%s", line);
          found ++;
       }
    return found
}

Why does that while loop go on forever? getline(line,1000) would, as I barely understand it, return the length of the line, and "append it" (?) to MAXLINE, making a number larger than 1000, depending on the line length... why would that ever dip below zero?

I read that getline returns -1 if "unsuccessful", when would that be? When would line not be read?

This is on page 69 of Kernighan. I've been skipping around the book, and I'm currently backtracking to see where I missed something.

"Describe what you tried, what you expected to happen, and what actually resulted. Minimum 20 characters."

Answer 1

The version of getline shown on page 69 of K&R Second Edition is defined as:

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

    i = 0;
    while (--lim > 0 && (c=getchar()) != EOF && c != '\n')
        s[i++] = c;
    if (c == '\n')
        s[i++] = c;
    s[i] = '\0';
    return i;
}

From this we can see that one of the conditions that must be met for the function to continue consuming characters from the stdin stream is that the returned character does not equal the constant value EOF .

(c=getchar()) != EOF

EOF , or end-of-file , indicates that there is no more data to be read from the stream. See What is EOF in the C programming language? for additional details.

In the event that getchar returns EOF the very first time it is called in getline , i will never increment beyond 0 , and the function will return zero.

This version of getline cannot return -1 .

The condition you have shown

while (getline(line,MAXLINE) > 0)

will cause this loop to stop when the return value is less than or equal to zero. In other words, when getline indicates it has read no data.

Answer 2

I'm assuming you are using the getline and strindex defined in the book:

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

    i = 0;
    while (--lim > 0 && (c=getchar()) != EOF && c != '\n')
        s[i++] = c;
    if (c == '\n')
        s[i++] = c;
    s[i] = '\0';
    return i;
}

/* strindex: return index of t in s, -1 if none */
int strindex(char s[], char t[])
{
    int i, j, k;

    for (i = 0; s[i] != '\0'; i++) {
        for (j=i, k=0; t[k] != '\0' && s[j] == t[k]; j++, k++)
            ;
        if (k > 0 && t[k] == '\0')
            return i;
    }
    return -1;
}

As you can see getline reads until it reaches end of file(EOF). If you are running the program on the command-line, your program is waiting for this signal from standard in, you can send this signal manually by pressing Control-d.

if you cat a file into the program:

$ cat test.txt | ./your program

the end of file value it sent, at the end of the file.