Compiling and Executing Your Program

In this class, we will be using the GNU compiler gcc(1) to compile programs. This handout tells you how to do this on the CSIF systems, and then how to execute the program.

In this handout, what you type on the computer is in bold Courier typeface and what the computer outputs is in Roman Courier typeface. The “%” is the CSIF’s shell prompt.

Creating Your Program

Our 0-th homework assignment is to write a program that will print “hello, world!”, in C. We begin by creating a file — let’s call it hello.c – that contains the program. We need this to be a text file, because the input to the compiler is text; so, we can’t produce a Microsoft Word file and use it. It also has to end in “.c”. Otherwise, the compiler will not recognize it as a C program file, and it will complain. This type of file is called a “source code file” or “source file”.

You can create this file in any text editor you like. On the CSIF, the two text editors used most widely are vim(1) (or vi(1), a less advanced version of the same editor) and emacs(1), which is far more powerful and complex. If you haven’t used a text editor on Linux, I recommend you start with vim. Jonathan Vronsky has written a very brief guide to vim; look in the Handouts area to find it. To see a very good, but much more detailed, tutorial on this editor, use the command vimtutor:

% vimtutor

Go ahead and create your program file hello.c. Make sure it is on the CSIF system before you go to the next section.

Compiling Your Program

Now it’s time to compile your program. We will be using the gcc(1) compiler. Type the following to the shell prompt:

% gcc -ansi -pedantic -Wall hello.c

• -ansi: use ANSI standard C (this is the C90 standard)
• -pedantic: require strict conformance to ANSI standard C. This disallows some extensions.
• -Wall: warn about questionable constructions. This flags some code that is likely to cause problems.

If your program is in more than one file, for example the main routine is in main.c and functions are in files func1.c, func2.c, and func3.c, simply list all the source files in the compile command:

% gcc -ansi -pedantic -Wall main.c func1.c func2.c func3.c

At this point, your directory should contain a file named a.out. Look for it by running the command ls and checking that the file is one of those listed.

Executing Your Program

If you didn’t see any error messages in the previous step, the file a.out is the compiled program. Type

% a.out
Hello, world!

Naming Your program

By default, the compiler puts compiled programs in the file a.out. If you want your program to be called something else, you can do this in two ways. First, you can tell the compiler to call the output something other than a.out by using the “-o” option:

% gcc -ansi -pedantic -Wall hello.c -o hello

The second way is to rename the file a.out. Once it is created, you can do this using the mv(1) command:

% mv a.out hello

make and Makefiles

Typing the full compile command each time can be annoying. More importantly, when you begin writing more complex programs that occupy more than one file, it can be difficult to remember how to create a compiled program from them. To solve this problem, we use a program called make.

make executes a series of commands in a file called a Makefile (or makefile). These commands are selected based on which source files have been modified. For now, as we are only dealing with one source file, our Makefile will be very simple.

Store the following in a file named Makefile:

hello:
    gcc -ansi -pedantic -Wall hello.c -o hello

Now delete the file hello if it exists:

rm hello

% make
gcc -ansi -pedantic -Wall hello.c -o hello

Now let’s look at a couple of other results. Without deleting the file hello, execute the make command again:

% make
make: ’hello’ is up to date.

Let’s say you forget to put a horizontal tab at the beginning of the second line, and instead put blanks there. Here’s what happens:

% make
Makefile:2: *** missing separator (did you mean TAB instead of 8 spaces?).
Stop.