CSCI 3500: Studio 4

Error Checking

Any non-trivial program requires you to call functions provided by either the operating system or system libraries. These functions almost always provide a return value, indicating whether an operation succeeded or failed. Checking these return values are vital to producing robust code, and greatly simplify debugging.

Checking return values appropriately will be a requirement for lab 2 and all subsequent labs!

In this studio, you will:

1. Look up return values in a function's man page
2. Check return values and handle errors appropriately
3. Print appropriate error messages with perror()

Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete.

As you work through these exercises, please record your answers in a text file. When finished, submit your work by sending your text file and source code to dferry_submit@slu.edu with the phrase Error Checking in the subject line.

Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.

Required Exercises

1. As the answer to the first exercise, list the names of the people who worked together on this studio.

2. Create a new file called open.c. Write a short program that accepts a single command line argument. This argument should be interpreted as a file name that your program attempts to open. Once the file is opened, your program should print the contents of the file to standard output. For example, the command "./open mars.txt" should open the file mars.txt and print it to the console. Use the open() function documented at man 2 open.

   Hint: start with your program from studio 2, called copy.c. Modify this program to open a file, and then modify the read() statement to read from your file descriptor rather than standard input. Alternately, start with your program from Lab 1.

   As the answer to this exercise, copy and paste your working code.

3. What happens when your argument specifies a file that doesn't exist?

4. Create a text file called perm.txt and fill it with some short message. Execute the command "chmod -rw perm.txt". This command removes read and write permissions from the file. Now try to execute your program on this file- what happens?

5. Look at the man page for open(). Around line 230 you will see a section called "RETURN VALUE". What value does open() return in the event of an error?

6. If you haven't already, insert an if statement directly after your call to open() to check for errors. Your error detection should check the criteria in the man page exactly. For example, if the reported error value for open() is -1, then check with the conditional "return_value == -1" rather than something like "return_value < 0". As the answer to this exercise, copy and paste the conditional you check.

7. The function open() also sets a special variable called errno. Many system calls and library functions will set this variable when they execute. If an error occurs, this variable tells the system what happened. You can print a helpful error message with the function perror().

   Insert a call to perror() inside your if statement from the last exercise. The only argument to this function is a short message that should describe the circumstances of the call. For example, "Error opening file" or "Error reading input".

   As the answer to this exercise, copy and paste your call to perror().

8. Finally, your program should exit gracefully when an error occurs. Insert a call to return or exit() with a negative value inside your if statement, but after perror(). Leave this answer blank.

9. Now, what happens when the argument to your program specifies a file that doesn't exist?

10. What happens when the argument specifies the file without read or write permissions- perm.txt?

11. Look at the following manual pages and determine what value(s) are returned in the event of an error, and whether these functions set errno:
    • man 2 open
    • man 2 close
    • man 2 read
    • man 2 write
    • man 3 printf

12. Give two reasons why error checking in this manner simplifies software development.

Optional Enrichment Exercises

1. The special variable errno is documented at man errno. There are a huge number of possible error statuses. Some of these are generic (e.g. insufficient permissions), while others are very specific (e.g. network host is unreachable).

   Having detailed error reporting presents the possibility that your programs can detect an error and attempt to self-correct, rather than quitting or crashing. For example, a common error status is EAGAIN or EBUSY, both of which indicate that an OS resource is currently unavailable. Rather than quitting, your program could simply wait and try again later.

   Browse through man errno and think about how you might handle some of the errors that can arise.