Before beginning: Read this lab tutorial; it has some useful tips for programming in the C environment.
In this project, you’ll build a few different UNIX utilities, simple versions of the commonly used cat and grep commands. We’ll call each of them a slightly different name to avoid confusion; for example, instead of cat, you’ll be implementing wcat (i.e., “Wooster” cat). You can find the GitHub classroom link to the assignment in the course Moodle page.
Objectives:
While the project focuses upon writing simple C programs, you can see from the above that even that requires a bunch of other previous knowledge, including a basic idea of what a shell is and how to use the command line on some UNIX-based systems (e.g., Linux or macOS), how to use an editor such as emacs, and of course a basic understanding of C programming. If you do not have these skills already, this is not the right place to start.
Summary of what gets turned in:
The program wcat is a simple program. Generally, it reads a file as specified by the user and prints its contents. A typical usage is as follows, in which the user wants to see the contents of main.c, and thus types:
$ ./wcat main.c
#include <stdio.h>
...
As shown, wcat reads the file main.c and prints out its contents. The “./” before the wcat above is a UNIX thing; it just tells the system which directory to find wcat in (in this case, in the “.” (dot) directory, which means the current working directory).
To create the wcat binary, you’ll be creating a single source file, wcat.c, and writing a little C code to implement this simplified version of cat. To compile this program, you will run the included Makefile with the command:
$ make
This will make a single executable binary called wcat which you can then run as above.
You’ll need to learn how to use a few library routines from the C standard library (often called libc) to implement the source code for this program, which we’ll assume is in a file called wcat.c. All C code is automatically linked with the C library, which is full of useful functions you can call to implement your program. You can learn more about the C library here and here.
For this project, we recommend using the following routines to do file input and output: fopen(), fgets(), and fclose(). Whenever you use a new function like this, the first thing you should do is read about it – how else will you learn to use it properly?
On UNIX systems, the best way to read about such functions is to use what are called the man pages (short for manual). In our HTML/web-driven world, the man pages feel a bit antiquated, but they are useful and informative and generally quite easy to use.
To access the man page for fopen(), for example, just type the following at your UNIX shell prompt:
$ man fopen
Then, read! Reading man pages effectively takes practice; why not start learning now?
We will also give a simple overview here. The fopen() function “opens” a file, which is a common way in UNIX systems to begin the process of file access. In this case, opening a file just gives you back a pointer to a structure of type FILE, which can then be passed to other routines to read, write, etc.
Here is a typical usage of fopen():
FILE *fp = fopen("main.c", "r");
if (fp == NULL) {
printf("cannot open file\n");
exit(1);
}
A couple of points here. First, note that fopen() takes two arguments: the name of the file and the mode. The latter just indicates what we plan to do with the file. In this case, because we wish to read the file, we pass “r” as the second argument. Read the man pages to see what other options are available.
Second, note the critical checking of whether the fopen() actually succeeded. This is not Java where an exception will be thrown when things goes wrong; rather, it is C, and it is expected (in good programs, i.e., the only kind you’d want to write) that you always will check if the call succeeded. Reading the man page tells you the details of what is returned when an error is encountered; in this case, the macOS man page says:
Upon successful completion fopen(), fdopen(), freopen() and fmemopen() return a FILE pointer.
Otherwise, NULL is returned and the global variable errno is set to indicate the error.
Thus, as the code above does, please check that fopen() does not return NULL before trying to use the FILE pointer it returns.
Third, note that when the error case occurs, the program prints a message and then exits with error status of 1. In UNIX systems, it is traditional to return 0 upon success, and non-zero upon failure. Here, we will use 1 to indicate failure. Side note: if fopen() does fail, there are many reasons possible as to why. You can use the functions perror() or strerror() to print out more about why the error occurred; learn about those on your own (using… you guessed it… the man pages!).
Once a file is open, there are many different ways to read from it. The one we’re suggesting here to you is fgets(), which is used to get input from files, one line at a time.
To print out file contents, just use printf(). For example, after reading in a line with fgets() into a variable buffer, you can just print out the buffer as follows:
printf("%s", buffer);
Note that you should not add a newline (\n) character to the printf(), because that would be changing the output of the file to have extra newlines. Just print the exact contents of the read-in buffer (which, of course, many include a newline).
Finally, when you are done reading and printing, use fclose() to close the file (thus indicating you no longer need to read from it).
Details
The second utility you will build is called wgrep, a variant of the UNIX tool grep. This tool looks through a file, line by line, trying to find a user-specified search term in the line. If a line has the word within it, the line is printed out, otherwise it is not.
Here is how a user would look for the term foo in the file bar.txt:
$ ./wgrep foo bar.txt
this line has foo in it
so does this foolish line; do you see where?
even this line, which has barfood in it, will be printed.
Details