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Lab 12 - Inputs and Arguments

From now on, we’ll assume that you start lab by connecting to the CS portal and that you are familiar with the command line environment. If you have not been practicing with the terminal, we strongly encourage reviewing Lab 1.

We will also assume that you ran the setup script from that lab and have all modules (including clang and git) by default.

The goal of this lab is to give you some practice working with the string.h and stdio.h libraries.

Lab Goals

After this lab, you should:

  1. Gain increasing familiariarity with C, including function arguments, printf, and scanf
  2. Be able to read and parse command-line arguments in C


You will write a program that will do the following:

  1. Read 2 string from standard-in, concatenate them and print the result to the console.
  2. Read 2 intergers from standard-in, add them and print the result to the console.
  3. Read 2 floats from standard-in, add them and print the result to the console.
  4. Use a command line argument that is supplied when you run your program.

Helpful functions

Let’s begin by looking at some functions that you’ll find helpful in this lab.


The printf function allows you to print to the console. Below is the function prototype for printf.

int printf(const char *format, ...) 

The first parameter is a pointer to a string. This parameter provides a template of the values that follow. printf expects that this string to include some key specifies. Here as some example specifiers.

Code Meaning
%c character
%d or %i base-10 int – use %ld for a long
%e or %E scientific notation float – use %le for a double
%f #.######-notation float – use %lf for a double

If you wanted to print a char we would pass "%c" as the first parameter. You can read more about the printf from its manual page by running

man 3 printf

We need to include the section number (3) of the library calls section of the manual because there is also a command-line tool named printf in section 1 of the manual.

The second printf parameter ... represents a variable number of parameters. This means that we can place one or more parameters in it’s spot. For example, the following code snippet prints out the value 3. Here we only placing one parameter in the ...’s position.

printf("%d", 3); 

We could also print multiple values by adding more parameters. Look at the next example. Notice that we also updated the first parameter to include additional format specifiers.

print(" Number 1 %d , and the other number %d", 3, 7) 

This string acts as template. The printf method will search for the specifiers and replace them with the values of the parameters that follow.


The scanf method allows you to read user input from the console, and has a simpilar prototype to printf.

int scanf(const char *format, ...)

You will begin by writing a program that prompts the user to enter two strings. The program then concatenates them, and prints the result:

Enter First String: Hello
Enter Second String: Bot

We’ll do this using scanf, the formatted string entry library function from stdio.h, and strcat, the string concatenation library function from string.h.

#include <stdio.h>
#include <string.h>

int main() {
    char string1[10];
    char string2[10];
    printf("Enter First String: ");
    scanf("%s", string1);
    printf("Enter Second String: ");
    scanf("%s", string2);
    printf("%s\n", string1);

Both scanf and printf are varargs functions with a “format string” as their first argument. The format string looks for special codes beginning %s to represent different data types to parse (scanf) or display (printf). %s means “a string”: a word for scanf, any char * for printf. Hence

scanf("%s", string1);

means “read input, discarding whitespace, until you find a word and store the word as a null-terminated string starting at the address string1, overwriting whatever was already there.”

The above code has a stack buffer overflow vulnerability. To see this, try typing a very long word when prompted. What happens?

scanf and printf both allow some additional information between the % and the s (or other letter that specifies a type). Of note here is the length limit:

scanf("%10s", string1); // reads at most 10 characters

Note you need padding for the null byte: scanf("%10s", string1) can put 11 bytes into string1.

strcat also has a stack buffer overflow error, as it modified the buffer associated with first parameter. Try entering 2 strings that are twelve-charaters each.

Task 1

Modify the program so that it only reads the 10 chars for the strings. Modify the string1 array (buffer) so that has enough space to hold the concantinated result.

Test your progress.

Try an example that reads two 10 char words (example 0123456789) and prints the concatenated result. Your code should concatenate the strings with strcat, not simply print them next to one another.

Task 2

Extend the program so that it also reads integers and floats. We begin by adding a prompt that ask the user to enter the value’s type. Users should enter “I” for integers, “F” for floats and “S” for Strings. Note your program should be case sensitive.

You’ll likely need to check man 3 scanf to learn how to read the different datatypes in.

What types of values do you want to add: F
Enter your first value: 0.2
Enter your second value: 0.5 

Once you’ve entered your values, add the numbers of concatenate the strings and print the result:

The results is : 0.700000

Try printing the sum of two very small numbers, like 0.0000000001 or 1e-8. Notice the error in the answer? That’s an error in formatting, not arithmetic. You could fix it with %e instead of %f in your printf, but then 1+2 would be displayed as 3.000000e+00, which is rather ugly.

For extra kudos, look through the manual page for printf to find a formatting flag you can use that will show 1+2 without an e but also show numbers close to 0 with an e.

Test your progress


  1. Adding two integers.
  2. Adding two floats
  3. Concatenating two strings.

Task 3

Command line arguments are values passed in after the executiable. Consider the following example

./a.out a b c 

Here a, b and c are all command line arguments. Let’s write a program that reads these values:

#include <stdio.h>

int main(int argc, const char* argv[]) {
    for(int i = 0; i < argc; i++){
        printf("Argument [%d]: %s\n", i, argv[i]); 
    return 0;

When you run the program with the command line parameters above you should get the following output.

$ ./a.out a b c
Argument [0]: ./a.out 
Argument [1]: a 
Argument [2]: b 
Argument [3]: c

Notice the first argument (argument 0) is the name of the program itself.

Let’s extend your program so you can pass in a command-line argument that specifics the type values that you want to enter. For simplicity will only support one type, integers.

So if a user wants to enter integers they’ll run the program like this:

a.out -i 

If a users start the program like this, your program should skip the step of prompting the user for the type. If another type of flag is supplied your program should print invalid flag supplied. If no flag is supplied your program should prompt the user to enter the type.

You might may find the strcmp function useful. Read the manual page for strcmp by running the following command.

man strcmp

Check-off with a TA

To check-off this lab, show a TA your completed program running with:

  1. No flag supplied. Should ask for a type first, then two numbers or strings depending on the type given. Show an example contactination two strings.
  2. -i flag supplied. Should ask for two integers. Show an example adding two integers.
  3. -k flag supplied. Should say “invalid flag supplied” and return without doing anything else.

Copyright © 2023 John Hott, portions Luther Tychonievich.
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