Introduction to C
C as a Language
C is a general programming language that is static typed, and has many functionalities such as lexical variable scope and recursion. It is somewhat low-level and can easily be compiled into machine languages that the computer can easily understand. It is one of the most widely used programming languages of all time and it also has popular derivatives such as C++, C#, and Objective-C. As with any new programming language, let’s write “Hello World” in our new language, C! We start with a blank file called hello.c
Hello World
Hello world is a simple enough program that we’re going to write it entirely, then take a look at what we’ve done. Here’s the code for hello.c
#include <stdio.h>
int main()
{
printf("Hello World\n");
return 0;
}
To make sure that it works, we must first compile our .c file into something that our computer can run. You need to havea compiler on your computer in order to do this. For this demo, and the rest of the posts in CS61C, we are going to use the gcc compiler.
To compile our hello.c file, we’re going to run:
gcc hello.c -o hello
Let’s deconstruct this command. First, we note that the Unix program that we’re running is gcc. The first argument, hello.c is the name of the file that we want to compile. We could stop here if we wanted but we added on a compiler flag, namely, -o. The -o flag for the gcc compiler allows you to specify an output file/name for the program. In this case, we are saying “compile the program hello.c into a program that the computer can run called hello.” If we omitted the -o hello part of the command, everything would still work but we would be left with a file called a.out that we can run. a.out is the default filename if one is not specified.
Once we have compiled our file with the command gcc hello.c -o hello, we can then run our file. To run our file, we enter the following into our Unix prompt:
$ ./hello
Hello World
If we did everything right, we should see the text ‘Hello World’. ./ means to execute the program (in this case we’re executing a program called ‘hello’).
Let’s take a moment to deconstruct our actual C program that we wrote, hello.c.
Deconstucting our first C Program
First, at the top of the file, we included the line:
#include <stdio.h>
which allows us to later on use the function printf. We say that printf is declared in this header file. Header files are contain declarations of classes, variables, and functions. It is similar to an import statement in Python or include in Ruby.
The second part:
int main()
means we are declaring a function main that returns an int. Every C file needs a main function or else nothing will run. The reason why it returns an int and not something else is because the Unix machine expects to receive an exit code from the program so that it knows what to do next.
Within the body of the main() function, we have the following:
printf("Hello World\n");
This line (with printf being imported from <stdio.h>) prints the text “Hello World\n” to standard out. In C, nothing is implicit so we need to explicitly state that we want a newline character \n to appear after our text.
The last line:
return 0;
is the exit code that we were talking about earlier. Since the main function is required to return an int, we return 0 because that represents the exit code for success. We want to signal to Unix that the file that we are running was a success.
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