Writing Unix Utilities in C (not C++ or C#)

my-cat

The program my-cat 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 my-cat.c, and thus types:

prompt> ./my-cat my-cat.c #include  ... 

As shown, my-cat reads the file my-cat.c and prints out its contents. The "./" before the my-cat above is a UNIX thing; it just tells the system which directory to find my-cat in (in this case, in the "." (dot) directory, which means the current working directory).

To create the my-cat binary, you'll be creating a single source file, my-cat.c, and writing a little C code to implement this simplified version of cat. To compile this program, you will do the following:

$ gcc -o my-cat my-cat.c -Wall -Werror $ 

This will make a single executable binary called my-cat which you can then run as above.

You'll need to learn how to use a few library routines from the C standard library to implement the source code for this program, which we'll assume is in a file called my-cat.c. All C code is automatically linked with the C library, which is full of useful functions you can call to implement your program. Learn more about the C library at https://en.wikipedia.org/wiki/C_standard_library

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.

On UNIX systems, the best way to read about such functions is to use what are called the man pages (short for manual). To access the man page for fopen(), for example, just type the following at your UNIX shell prompt:

$ man fopen 

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("my-cat.c", "r"); if (fp == NULL) { printf("cannot open file "); 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 ( ) 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

• Your program my-cat can be invoked with one or more files on the command line; it should just print out each file in turn.
• In all non-error cases, my-cat should exit with status code 0, usually by returning a 0 from main() (or by calling exit(0)).
• If no files are specified on the command line, my-cat should just exit and return 0. Note that this is slightly different than the behavior of normal UNIX cat (if you'd like to, figure out the difference).
• If the program tries to fopen() a file and fails, it should print the exact message "my-cat: cannot open file" (followed by a newline) and exit with status code 1. If multiple files are specified on the command line, the files should be printed out in order until the end of the file list is reached or an error opening a file is reached (at which point the error message is printed and my-cat exits)