Lecture 7 – Feb 14th, 2023

Setup

Log in to clyde.
Create a directory and cd into it.
Copy ~rhoyle/pub/cs241/c-intro/hello.c to your directory.

Task

Open hello.c in your editor and read it. Try to guess what it will print out. Compile the code and run it by entering the following commands in the terminal.
```
$ clang -Wall -std=c11 -o hello hello.c
$ ./hello
```
Did it print what you expected?
Run the hello program again, this time passing it some names as arguments (try to guess what will happen before you run it). You don’t need to recompile the program, just run it again.
```
$ ./hello Adam Bob Cynthia
```
Modify hello.c such that the “Hello” messages are printed in the reverse order.

Since you have modified hello.c, you need to recompile the program. So once again, run
```
$ clang -Wall -std=c11 -o hello hello.c
$ ./hello Adam Bob Cynthia
```
If you get an error or warning message, read it closely and then try to fix the error/warning.

If your modified program printed out the line
```
Hello (null)!
```
then you have likely tried to print argv[argc] which has the special value NULL. This is an off-by-one error. You should fix this.
Lastly, we can use the
```
char *getenv(char const *environment_variable);
```
function to get the value of environment variables (as a string). In particular, getenv("USER") returns the user name of the user running the program.

In order to use getenv, the function needs to be declared before we can use it in main.c. Fortunately, getenv is part of the C standard library and is declared in the stdlib.h header.

Modify hello.c as follows. First, add the line
```
#include <stdlib.h>
```
to the top of the file.

Next, modify the first printf() line so that rather than printing Hello world!, it prints Hello followed by the user’s user name. Look closely at the other printf() line for inspiration. Rather than printing out argv[idx], you’ll want to print out getenv("USER").

Compile and run the program. (After editing a file, it’s easiest to press up in the terminal until you get to the clang line and then you can just press enter rather than typing out the whole clang -Wall -std=c11 ... line each time.)