Lab 4: File Input and Output

Note that you have until Tuesday night to complete this lab.

Goals for this lab

By the time you have completed this lab, you should:

  • know what a FILE stream is
  • know the difference between stdin, stdout, and stderr
  • be able to read from files on disk
  • be able to parse and validate an input stream
  • have reinforced using the List ADT

The purpose of this lab is to have you demonstrate reading and parsing file input streams.

Lab Pairing

For this lab you may work with a partner. To facilitate your submissions as a pair you MUST pair up using the submission system. When you visit the project page from where you can view all your submission you should see a link to a page where you can join a group. Of the two people in the group, one of you must invite the other, and the other must confirm the invitation. Once you group up, any submissions that either you, or your partner have already made will be visible to each other in addition to all future submissions.

Lab Preparation

At this point, everyone should have a COE account. If you do not, you need to make that a priority and should partner up with someone who already has an account.

Before you begin, let’s prepare a cs24 directory in your home directory if you don’t already have one. In that directory we’ll create a subdirectory for the files for this lab. Note that these instructions assume you are using either a CSIL machine or one of the lab machines:

After logging in, create the directory (assuming you haven’t already done so):

mkdir -p cs24/lab4

Change the permissions of the cs24 directory such that only your account can access the files inside this directory. This step is incredibly important. If omitted, other students can peek at your work and you may be held partially responsible. It should also go without saying that you may not give other students access to your account. If you’ve done that in the past please change your password at this time.

chmod 700 cs24

Change into the lab4 directory and copy the files you will need for this lab:

cd cs24/lab4
cp -r ~bboe/public_html/cs24_f13/code/lab4/* .

The provided files

In your directory you will find the following files:

  • linked_list.o and list.h – The combination of these files allow you to make use of struct List and the associated functions.

  • Makefile - This file specifies how the group_by binary should be compiled.

  • tests/MIXED_INPUT - A sample input file which contains both valid lines and invalid lines.

  • tests/MIXED_OUTPUT - The combined output (stderr and stdout) that should be produced when running group_by on the MIXED_INPUT input file.

  • tests/SAMPLE_INPUT - A simple input file that contains only valid input lines.

  • tests/SAMPLE_OUTPUT - The output that should be produced when running group_by on the SAMPLE_INPUT input file.

Your task

Your task is to write a program, group_by.c, that:

  1. Reads lines from an input stream (either from stdin or a named file)
  2. Parses valid lines for [AGE] [NAME]
  3. Discards invalid lines with the appropriate error message
  4. Adds the name to the end of a list for the associated age
  5. For each valid age (1-99 inclusive) output the age and list of names associated with that age if there are any.
  6. Deallocates any memory allocated before exiting.

For instance, if the input is the contents of SAMPLE_INPUT:

57 George Washington
61 John Adams
57 Thomas Jefferson
57 James Madison
58 James Monroe
57 John Quincy Adams
61 Andrew Jackson
54 Martin Van Buren
68 William Harrison
51 John Tyler

then the output should be:

Age 51
  0: John Tyler
Age 54
  0: Martin Van Buren
Age 57
  0: George Washington
  1: Thomas Jefferson
  2: James Madison
  3: John Quincy Adams
Age 58
  0: James Monroe
Age 61
  0: John Adams
  1: Andrew Jackson
Age 68
  0: William Harrison

Compiling and executing your program

Type make to compile your program. On successful compilation the file group_by should be created.

The program you create needs to execute in two ways. The first is simply via:

$ ./group_by

With no arguments, your program should read lines from the standard input stream (stdin).

The second way is by providing a named file as the only argument to the program:

$ ./group_by some_input_filename

In this case your program should (1) attempt to read lines from the named file some_input_filename and gracefully fail (exactly as described below) if the file cannot be opened.

With any other number of arguments your program should output Usage: group_by [FILENAME]\n to the standard error stream.

Standard FILE streams

In previous labs we have utilized command line arguments to give our programs input. Another common way to take input in a program is through an input stream, likewise all program output is handled through an output stream. Simply, a stream is a pointer to a FILE structure, or FILE * that can be readable and/or writable. An input stream is a stream that is readable and an output stream is a stream that is writable.

Any time you have the code #include <stdio.h> three global streams are made available. Those are:

  • stdin - A read-only stream (from the program’s point of view) that represents input to the program. Under normal execution this stream is provided via keyboard input.

  • stdout - A write-only stream that is used to produce normal program output. You implicitly use the stdout stream every time you make a call to printf.

  • stderr - A write-only stream that is typically used to produce error output. Under normal program execution the stderr stream is printed to the terminal along with the stdout stream.

Outputting to writable streams

We already know how to output to stdout using printf, but how would we output to stderr? It turns out that printf(...) is essentially the same as fprintf(stdout, ...) [ref]. Thus if we wanted to output “hello world\n” to stderr we could write:

fprintf(stderr, "hello world\n");

Likewise let’s assume we had another writable stream assigned to the variable FILE *my_stream. We could provide output to that stream by:

fprintf(my_stream, "hello world\n");

Three other useful functions for writing to stream are fputc, for writing single characters (includes '\0'); fputs, for writing complete c-strings without %? replacement; and fwrite, for writing a fixed amount of data from a buffer (includes '\0').

WARNING: As the clang compiler will warn you, you should never call any form of printf with a c-string that is not hard-coded in your program code. If you only want to output the contents of a c-string (let’s call it str) to stream stream, use either fputs(str, stream) or fprintf(stream "%s", str). Using fprintf(stream, str) makes your program vulnerable to format string exploits.

Inputting from readable streams

There are a number of functions to read input from readable streams.


The simplest is fgetc that reads in one character at a time. Astute readers will not that rather than return a char type, this function returns an int type. The reason is that fgetc needs some value to indicate that an error has occurred and both 0 and -1 (255 unsigned) are valid characters. Thus fgetc returns an integer and uses the additional bytes for error conditions. For instance fgetc will return the magic value EOF when the stream has no more data. EOF stands for end-of-file.

The following is a simple program that echos, or repeats the data it reads from the standard input stream to the standard output stream.

#include <stdio.h>
int main() {
  int c;
  while ((c = fgetc(stdin)) != EOF) {
    fputc(c, stdout);
  return 0;

When running this program, you can terminate the keyboard input stream (thus triggering EOF) by pressing ctrl+d (you may need to press it twice if you’ve already input characters on the line).


Another useful function is fgets. fgets attempts to read an entire line (up to and including a newline, '\n') into a pre-allocated char array (often referred to as a buffer) and then null-terminates the buffer so that it can be treated as a c-string. NOTE: fgets will read null-terminating characters, '\0', just like any other character.

The following is a nearly functionally identical example of the former echo example, using fgets and fputs rather than fgetc and fputc:

#include <stdio.h>
#define BUFFER_SIZE 64
int main() {
  char buf[BUFFER_SIZE];
  while (fgets(buf, BUFFER_SIZE, stdin) != NULL) {
    fputs(buf, stdout);
  return 0;

There are three primary differences between this example and the former. The obvious one is that the former example reads character-by-character, where as the second example attempts to read line-by-line.

The second has to do with the BUFFER_SIZE constant defined. An array, by definition, has a fixed sized, however, a line can be arbitrarily long. Thus to prevent buffer overflows, fgets requires the programmer to specify the size of the buffer to copy the data to, and copies in at most that size - 1 bytes. The last byte is used to copy the null-terminating character.

The final difference is that fgets returns NULL when there is no more data to read, i.e., it is at the end of the stream.

Other read functions

In addition to fgetc and fgets is fread. fread is the complement function to fwrite which allows read an fixed amount of data into a buffer (includes ‘\0’).

Working with named files

So far we’ve only looked at working with stdin, stdout, and stderr. Often it is useful to read or write to a named file. In C, we can think of the FILE structure and its associated functions as an ADT for handling input and output. Thus, just like our list ADT, there are similar constructor and destructor functions for named files. Those are fopen and fclose respectively.

Echo from a named file

Below is a simple example of the character-by-character echo program, this time echoing data from the named file foobar rather than standard input:

#include <stdio.h>
int main() {
  FILE *input = fopen("foobar", "r");
  int c;
  while ((c = fgetc(input)) != EOF) {
    fputc(c, stdout);
  return 0;

Note that we use "r" as the second argument to fopen to indicate open the file for reading only. Also, note that the program will segfault if the file foobar does not exist relative to the present working directory at runtime. Just like our list_construct function, fopen will return NULL if the FILE structure cannot be created, or (opened in this case).

Gracefully handling fopen errors

There are actually a number of reasons why fopen might fail. The most obvious is the file does not exist (when opening for reading), but another failure case is that the user does not have permission to open the file. In C there is a convenient perror function to output (to standard error) an appropriate error message.

Let’s update our example to gracefully handle errors with opening the file:

#include <stdio.h>
int main() {
  FILE *input = fopen("foobar", "r");
  if (input == NULL) {
    return 1;
  int c;
  while ((c = fgetc(input)) != EOF) {
    fputc(c, stdout);
  return 0;

Now if when you run the program, the file foobar does not exist the following will be output to the standard error stream:

foobar: No such file or directory

Likewise, if foobar exists, but it is not readable, the following will be output:

foobar: Permission denied

Echo to a named file

The previous example took input from a named file and output it to the standard output stream. The following example, takes input from the standard input stream (normally keyboard) and outputs it to the named file “foobar”. Note that you should read the full fopen description to see what happens when you open files for writing that already exist and how to get the desired behavior.

#include <stdio.h>
int main() {
  FILE *output = fopen("foobar", "w");
  if (output == NULL) {
    return 1;
  int c;
  while ((c = fgetc(stdin)) != EOF) {
    fputc(c, output);
  return 0;

Remember that to close the keyboard input stream you should hit ctrl+d on your keyboard.

Reading and parsing lines

You should use fgets to read in each line of the file. You are to assume that no lines are longer than 63 characters (include the newline, if available) thus a buffer of size 64 is sufficient to store the line data plus the null terminating character that fgets will automatically add. NOTE: You do NOT need to check for lines longer than 63 characters (though I encourage you to think about how you might do that).

Each valid line is of the format AGE SPACE NAME. Is a base-10 number in the range 1-99 (inclusive). Note that 0001 and 00000000099 are valid ages and should be treated just like 1 and 99 respectively, but you shouldn’t have to do anything special to handle those numbers (if you are you’re making things too complicated for yourself). The SPACE is a single literal SPACE, ' ', character. Finally, NAME is all characters following the SPACE character up to the new line character '\n' (if one exists).

Any lines which do not match this format should result in a message to the standard error stream containing the line that resulted in the error. The exact format string message you are to use is:

"Ignored invalid line: %s\n"

See the MIXED_INPUT and MIXED_OUTPUT file for examples of invalid output.

Splitting c-strings

A very useful function for parsing c-strings is the strtok function. Below is an example program that splits up the individual terms (separated by either a space character, ' ', or a new line character, '\n').

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void output_terms(char *msg) {
  char *orig = strdup(msg);  // Save a copy as strtok is destructive
  char *token_arg = msg;  // Non-null only for the first call to strtok
  char *token;
  int index = 0;
  // Loop over each token (split on either space or new line characters)
  while ((token = strtok(token_arg, " \n")) != NULL) {
    token_arg = NULL;  // Subsequent strtok calls require this to be NULL
    printf("Token %d: %s\n", index++, token);
  printf("The original message was: %s\n", orig);

int main() {
  char line1[] = "1 23 456 789\n";
  char line2[] = "hello world";  // Note that this does not end with '\n'
  return 0;

Note that strtok is destructive thus we use strdup to make a copy of the original string in order to use it afterwards.

Storing lists of names by ages

Aside from the task of parsing data, this is the primary challenge for this lab (don’t discuss this part outside of your group except with the instructor and TAs). You are provided with a complete implementation of the List ADT, thus allowing you to #include "list.h".

You should store each name in a list corresponding to a single age, and use the appropriate output function to output the contents of the list. The question you must answer is how do you manage a list for each age such that after reading all the input, the names associated with each age are output in age order? Good luck!

Submitting the assignment

Only one person in a group need submit the assignment but make sure that both you and your partner can view the submission on the submission site. If you cannot, you need to complete the making a group process. Please review the submission instructions as needed. Note that you may resubmit this assignment as many times as necessary up until the deadline.


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