CS201 – Winter 2008 Assignment 1 Due Jan 15, 2008
Exercise 5-13, K&R page 118
Write your own version of the tail utility, as specified in this problem.
Simplifications:
• Your program will not be tested with extremely long lines of text, nor with
non-text files, so you do not have to deal with pathological input data.
You can pick a number, such as 4096, and assume all lines of text in the
file are shorter than that. However, your program must deal with long
files; i. e., many lines of text.
• The program takes input only from its standard input, so you do not have
to open any files.
• Some of the example code in the book uses their own input function
"getline," but you can use the library function fgets to read lines of text.
Requirements are specified below. These will be used by the grader in grading your
submission, so use them as guidelines in testing your program before submitting it.
Among some of the important requirements:
• Your program must read the input file only once. That is, it cannot scan
through the file to count the lines and then read it again up to where it
wants to start printing.
• Your program must store each line of text that it is going to print in an
array of bytes that is just long enough to hold that line. That is, not in a
two-dimensional array of fixed size.
Attacking the problem
The major design issue is how to store the data in memory. The example function
readlines on page 109 of K&R shows how to read lines from a file, allocate memory, and
store the data. To read a line from a file, you can use the library function fgets; you don't
have to duplicate the function getline from the book.
The twist for this problem is that we do not want to store the whole file, only the last N
lines. Here is a suggested way to do it. You can do it differently if you want to.
Once we know how many lines we need, let's say nlines, we can dynamically allocate an
array of nlines pointers to characters. As we read each line of text into a buffer, we can
dynamically allocate an array for that line of text, pointed to by one of our pointers. Here
is a suggested way to allocate an array of pointers to characters:
char **linearray;
...
if((linearray = calloc(nlines, sizeof(char *))) == 0){
fprintf(stderr, "Couldn't allocate array\n");
exit(1);
}
