CSCI 2510: Studio - 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

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

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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 code and answers file to this git respository.

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Required Exercises

1. First we need a computing environment, and I will assume you are using our department Linux server as our default environment. You are welcome to do your studios someplace else, but I can only provide limited support for those alternatives. I would suggest installing the Windows Subsystem for Linux (WSL) on Windows. If you are on Mac, the built-in terminal will work. You will also need to find a text editor, and you should install the GNU C Compiler, usually known as GCC.

   If you want to use the department Linux resources, then you can login locally to a Linux machine in the classroom or lab, or you can remotely connect to the department server at hopper.slu.edu. In general you will only ever need Linux terminal access for these exercises, though you're welcome to use a 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. (If you are not using the Linux environment, you can also type "man fprintf" into a search engine and you will find the same materials posted online.) 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 hello_fprintf.c -o hello_fprintf

   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" explicitly states to look in the standard library section of the manual. As it so happens, this command takes us to the documentation page we saw previously.

   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() is a byte-oriented function rather than a string-oriented function. The fprintf() function was able to figure out how many characters to write on its own, but you will need to tell write() how many characters to output. 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. For example, the string "dog\n" has four characters.

   Once you are finished, copy and paste your new program as the answer to this exercise.

10. Try changing the third argument of write() to be much larger than your character buffer (say, 1000 or 10000). What do you think happens when you do this? Try running the code- does anything usual show up? If so, 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

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