Lab Exercise 2: Compiling & running C; Input/Output

Duration: 1 session


To encourage you to find out about some of the basic features of the C language and the facilities that support their use.

Useful C on-line course themes etc.:

Learning outcomes

On successful completion of this exercise, a student will:



For this lab exercise you should do all your work in your COMP26120/ex2 directory.

Part 1: Compiling and Running C Programs

For this part, you don't have to do much programming, but you do need to write notes about what you do for future reference in the file howto.

Copy the starting files into your COMP26120/ex2 directory from /opt/info/courses/COMP26120/problems/ex2 (use this path) These files are: HelloWorld.c, SalaryAnalysis.c,, salary-data.txt, makefile, and howto

Step 1A: Hello World

Compile HelloWorld.c using the command (the > is the command-line prompt - you don't need to type it):
  > make HelloWorld
You should see a single line of output:
  gcc -g -std=c99 -Wall HelloWorld.c -o HelloWorld
Now run the resulting program using the command:
  > HelloWorld
(or, depending on your $PATH variable, you may need to use ./HelloWorld )
Again, you should see a single line of output:
  Hello world!

You just used the make command, which gets information from your makefile, to compile a C program. make used gcc (the Gnu C Compiler) to create a binary program HelloWorld, which can then be run directly without the use of an interpreter (i.e. unlike Java programs, you don't have to use e.g. java HelloWorld to run your compiled program).

Use the man command (e.g. type man gcc) to find out about the command parameters -g, -std=c99, -o and -Wall that make gave to gcc. Write notes about what you discover in howto.

Edit HelloWorld.c to comment out the #include line and recompile it. Write down the error-messages you get, to help you identify this sort of problem in the future.

Use the man command to find out about printf - if you type man printf (try it!) you will get output starting: PRINTF(1) User Commands PRINTF(1) which tells you that this is not what you were looking for - instead, you need to type man 3 printf to get information about the function in the C library: PRINTF(3) Linux Programmer's Manual PRINTF(3)
Before you proceed, write notes about what you have done in howto so that you will be able to cope with similar things in later exercises.

Step 1B: Salary Analysis

Compile using javac and run it with the command:
java SalaryAnalysis salary-data.txt
to see the output you should expect from the C version of this program.

Compile SalaryAnalysis.c using make as in step 1A (i.e. make SalaryAnalysis ). You should see an error message from ld saying:
"undefined reference to `lround'".
gcc has compiled the program without problems, but then called on the linker, ld, to add pre-compiled code from the libraries to your compiled program, and ld cannot find the library code for lround. Look at the synopsis section from:
man lround
The first line says what #include to use (check this in the C code), and the last line says what compiler and linker flags to use - you need to have a linker flag of -lm
To get this, edit the makefile to remove the # (the comment symbol) from the start of the line:
and use make again to recompile the program. Now run it using:
SalaryAnalysis salary-data.txt
and compare the output with that from the Java version. What differences are there?
Write notes about all of this in howto, to help you identify and deal with these sorts of problem in the future.

Part 2: C Input/Output

You should write a single program, part2.c, for all steps of this part. You should probably keep a working back-up each time you progress to a new step.

The steps become harder, with less hints, as you progress. If you are running out of time, don't try to complete all steps - instead get everything marked that you can by the deadline, and try to prepare better for the next lab exercise. (Ensure that you use submit to prove that you finished in time.)

Step 2A

Write a C program to read characters one-by-one from standard input (you can use ctrl-D to terminate the input), convert all upper-case characters to lower-case and all lower-case characters to upper-case, and write the result to standard output e.g.:
Hello World!
would become:
You should also count how many characters you have read, and how many of those you have converted in each direction, and output the totals at the end e.g.:
Read 13 characters in total, 8 converted to upper-case, 2 to lower-case

You will need to use getchar and putchar for the individual characters, as well as printf for the final character counts.
You may want to use some of the functions in ctype.h, such as tolower and isupper (e.g. use man ctype.h).

Step 2B

Edit your program so that the input is read from a file opened from within your program. For the time being, you can use a fixed file-name such as "input".
Hint: Stream Manipulation

Remember to check that the file is correctly opened. Test this by running the program when there is no input file available.
Hint: refer to man fopen and SalaryAnalysis.c

Step 2C

Edit your program so that the output is written to a file, using a fixed file-name such as output.

Remember to check that the file is correctly opened. Test this by running the program when a file of that name exists, but is write protected e.g. chmod u-w output

Step 2D

Edit your program so that, instead of using fixed filenames such as "input" and "output", both file-names are read from standard input when the program is run (i.e. use scanf or similar; I don't want you to use command-line parameters).

Marking Process

You must use labprint and submit as normal. They will look for howto and part2.c
The marks are awarded as follows:

Part 1:
2 marks - sensible notes in howto (including using gcc, make, man, ld etc., and using C libraries)
Part 2:
2 marks - Step 2A: upper-case and lower-case, and counts
2 marks - Step 2B: input from a file
2 marks - Step 2C: output to a file
2 marks - Step 2D: reading the file-names from standard input
Total 10