CSCI 3500: Studio 1

Hello, world!

"It's a dangerous business, Frodo, going out of your door. You step into the road, and if you don't keep your feet, there is no knowing where you might be swept off to."

—Bilbo, The Fellowship of the Ring, Book 1, Chapter 3

Welcome to Operating Systems! Let's get started with two simple "Hello, world!" programs that demonstrate the difference between doing standard output with the C standard library versus Linux system calls.

In this studio, you will:

  1. Use the built-in Linux manual (man pages) to look up certain functions
  2. Write a "Hello, world!" program using the C standard library
  3. Write a "Hello, world!" program using Linux system calls

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 with the phrase Hello, world! 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. First we need a Linux environment. You can login locally to a Linux machine in the classroom or lab, or you can remotely connect to the department server at Alternatively, you can login locally and then also connect to hopper so that you can always access your work remotely. In general you will only ever need Linux terminal access for these exercises, though you're welcome to use NoMachine if you'd like the GUI desktop.

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

  2. Your first task is to write a "Hello, world!" program using the function fprintf() in the C standard library. To do so, pull up the function's documentation by typing "man fprintf" in the Linux terminal. This page tells you everything you need to know in order to use fprintf() in a program. As the answer to this exercise, give the #include header file(s) needed for this function (look right beneath the "SYNOPSIS" heading).

  3. Now look at the function signatures for both fprintf() and the printf(). What is the difference? (Signature is a name for the argument list.)

  4. Look at the first paragraph under the "DESCRIPTION" heading. What is the described difference between the printf() and fprintf() functions?

  5. Let's put our newfound knowledge to use! Quit out of the man page by pressing "q" and create a text file called "hello_fprintf.c". Finish the following empty program so that it prints a message of your choice.

    // Your name
    // The date
    // A short description of your program

    // Include file goes here

    int main( int argc, char* argv[] ){

    //Make a call to fprintf() here

    return 0;

    The first argument of fprintf() must specify an I/O stream. In this case you should use "stdout". This is a special stream that prints to the Linux console, but this function works for a variety of stream-oriented operations, such as writing to a file or sending data over a network. The second argument should be a string literal that gives your message, and should be terminated with "\n" (the newline character). For example, your string literal might look like:

    "Hello, world!\n"

    You can compile your program with the command:
    gcc -o hello_fprintf hello_fprintf.c

    Run your program by typing "./hello_fprintf". If your program runs correctly then copy and paste your terminal output as the answer to this exercise.

  6. Great! Now we want to accomplish the same thing but using a system call directly, as opposed to the C library. The particular system call we want to use is called write(). However, if you give the command "man write" you won't arrive at the correct documentation because there are multiple pages that might apply to such a common term. Instead, give the command "man man" to pull up the documentation for the manual system.

    As the answer to this exercise, give the section number for system calls.

  7. Now, use the answer to the last exercise to look up the documentation for the write() system call. The syntax you should use is:

    man <section number> write

    For example, the command "man 3 fprintf" would take you to the documentation page used previously, but explicitly states to look in the standard library section of the manual. As the answer to this exercise, give the header file which must be included to use the write() system call (again, look beneath the "SYNOPSIS" statement).

  8. Make a copy of your program called hello_write.c. The Linux terminal command to copy a file is cp. In this case, you should execute the command:

    cp hello_fprintf.c hello_write.c

    Leave the answer for this exercise blank.

  9. The fprintf() function doesn't actually implement the code that allows your program to print to the terminal, it relies on the operating system to do that. The OS provides a system call called write() that gives this functionality, and fprintf() calls this function on your behalf. However, you can call write() directly, which is what we will do now.

    Modify your program to use write() instead of fprintf(). There are two differences between the functions. The first is that you need to use a file descriptor instead of a file stream. In other words, replace the built-in variable "stdout" with the built-in variable "STDOUT_FILENO". Both variables refer to the same "thing" but in different ways.

    The second difference is that write() operates on a character buffer (a vector of characters) rather than a string literal. You can declare a character buffer as such:

    char buffer[] = "Hello, world!\n";

    You will also need to tell write() how large the character buffer is. When counting how many characters is in your buffer, don't forget the newline character '\n'. Note that the whole newline character escape sequence ('\n') counts as one character. Once you are finished, copy and paste your program output as the answer to this exercise.

  10. Try changing the third argument of write() to be much larger than your character buffer (say, 100 or 1000). What happens? Why do you think this is?

  11. The function fprintf() is provided by the C standard library and is guaranteed to exist for any standards-compliant C language implementation- even on other operating systems. The write() system call is not guaranteed to exist as it is provided by the operating system itself. Many system calls (but not all) have C library versions.

    Thinking as a software developer, speculate a situation when you would want to use a C library function and another situation when you would want to use an OS system call.

Optional Enrichment Exercises

  1. No optional exercises