Computer Science 381
Programming Unix in C

Winter Immersion 2016, The College of Saint Rose

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In this lab, you will work through some examples focused on how C handles input and output, including to and from files.

Before you start, create a directory on a Unix or Mac system to contain your work for this lab. You may do your work on any C-capable Unix-like system (such as a lab Mac or mogul).

Readings from K&R

Skim through Chapters 2 and 3 of K&R. Most of what is there is identical to or at least very similar to what you likely know well from other programming languages you have used. Much of what we will look at this lab is in Chapter 7, though that chapter assumes you have seen some of the material in Chapters 4-6. Even so, it is useful to be able to use higher-level input and output right away, so we will consider much of the content of that chapter in this lab as well.

Command-line Parameters

You have likely seen Java applications that take command-line parameters (the String args[] parameter to the main method of a class). A C program that wishes to make use of command-line parameters must declare two parameters to the main function, traditionally named argc and argv.

The parameter argc to the main function is a count of how many command-line strings are included in argv, which is an array of strings.

See Example:
/home/cs381/examples/printargs

Note: argv[0] is not the first parameter, it is the program name itself, and this array entry for the program name is included in the value of argc.

Even when we enter numbers for command-line parameters, the operating system will provide them to your program as strings. So we need to be able to convert strings to a numeric equivalent.

See Example:
/home/cs381/examples/repeat

This is done with the overly complicated strtol function, which we use, then check error conditions. There's a lot here we have not yet seen.

• The man page for strtol tells us we need to include two additional header files, stdlib.h and limits.h.
• It also tells us about the parameters to strtol, which are the string which we would like to convert to a number, a pointer into the string at the point beyond which we matched a number (which we don't care about, so we pass in NULL), and the base to use for the conversion. We also see that the number is the return value.
• Error checking for strtol is messy - we need to check the variable errno, defined in errno.h, to see if an error condition was encountered. If so, errno will be a non-zero value and we print an error message and exit.
• We use fprintf instead of printf when printing the error message. This is because we want to give this output special significance. Rather than sending it to the standard output, which is what printf would do, we send it to standard error, by using fprintf and specifying stderr as the first parameter. Java supports the same idea: use System.err instead of System.out.
• Other than that, it works just like printf. We give it a format string. In this case, it includes one specifier, a %s, which means to expect an additional parameter which is a character string (well, really a pointer to a null-terminated array of char). Here, the string is argv[0], the first command-line parameter, which is always the name of the program. This labels the error message with the program name.
• Once we have detected the error, we don't want to continue, so we call the exit function with an error code of 1 to terminate execution. We could also use the call return 1;.
• Note that the error check here has two %s's, so we have two additional parameters to fprintf, both pointers to strings.

Formatted Keyboard Input

We have seen how to use the getchar function to get input from the keyboard or redirected from a file, one character at a time. But often, we'd like to read input as words or numbers.

C's standard mechanism for this is the scanf function.

See Example:
/home/cs381/examples/scanf-example

• scanf is a very strange thing. It will make a bit more sense once you have more experience with the printf function, but for now we can summarize what we see there as "read in an integer value (represented by the %d in the format string), and put it into the place pointed at by the address of x, then return the number of values that matched the input with the correct format." Similarly for the double value using a %lf in the format string.
• The scanf call forces us to think a bit about pointers, which are the key to understanding so much of how C works. scanf's parameters after the format string are always a list of pointers to a place in memory where there is room to put the values being read in. In this case, we want the int value to end up in the local variable x, so we have to take the address of the variable with the & operator. Don't worry, it will make better sense when you see more examples.
• The scanf to read in a string is a special case and does not require the & operator. This is because the name of a C string already is a pointer to the first element in the array. Again, much more on this when we study C pointers in more detail.
• Next, we check to make sure that the input to scanf did, in fact, represent an int value. If not, we print an error message and exit. Otherwise, we continue.

File I/O

Read Section 7.5 of K&R to learn about reading from and writing to files in C.

Programming Assignment Preview

Note: the programming assignment portion will be due as part of the next lab submission, but you are strongly encouraged to get as much of it done this week as possible.

There is collection of graph data files that represent various highway systems on this page. Each of the ".gra" files is in the format described under "Graph Data" here.

Write a program extremes.c that reads the "waypoints" portion of a graph file whose file name is specified as a command-line parameter and find the easternmost, westernmost, northernmost, and southernmost waypoints in the file. For each, print out the waypoint label and its latitude and longitude.

Notes:

• You can safely ignore the "road segments" portion of the input file for this assignment.
• You can safely assume that no waypoint label is longer than 256 characters.
• By including the \n in your fscanf format string, you can automatically move the input to the next line.
• You can copy one C string to another using the strcpy function, defined in string.h. If you have strings declared as

    char str1[256];
    char str2[256];
  

  the following would copy the contents of str1 to str2:

    strcpy(str2, str1);
  

• Valid values for latitudes range from -90 (southernmost) to 90 (northernmost) and for longitudes range from -180 (westernmost) to 180 (easternmost).
• If you were doing this in an object-oriented language like Java or C++, you might create an object to represent each waypoint and provide methods to compare them, print them, etc.. But here, you can just use three variables, a string (array of char) and two doubles to represent the current waypoint you are considering, and extra sets of strings and two doubles to keep track of each "most extreme" in some direction waypoint you've encountered so far.

This program will be worth 30 points, broken down as shown at the end of our next lab assignment.

Executables for reference solutions to all programs are available on mogul in /home/cs381/labs/io.

Submission

Please submit all required files as email attachments to terescoj AT strose.edu by Monday, December 28, 2015. Be sure to check that you have used the correct file names and that your submission matches all of the submission guidelines listed on the course home page. In order to email your files, you will need to transfer them from mogul to the computer from which you wish to send the email. There are a number of options, including the sftp command from the Mac command line.

Grading

This lab is graded out of 20 points.