Fortran 77 Tutorial

4. How to use Fortran on the department Unix computers

Practical details

You will need an account on our Unix network. If you don't have one, see the department system administrator. Also, you will need to ask the department office for a key code so that you can gain access to the computer lab.

We will be using Fortran under the Unix operating system. If you have no previous experience with Unix, you will have to learn the basics on your own. However, you will be provided with a tutorial introduction to Unix for reference.

Source code, object code, compiling, and linking

A Fortran program consists of plain text that follows certain rules (syntax). This is called the source code. You need to use an editor to write (edit) the source code. A commonly used editor in Unix "vi", but it isn't very user friendly. The "text editor" program on our Sun workstations is easy to use. (Access it by right-clicking with the mouse, going to the "programs" menu, and choosing "text editor".)

When you have written a Fortran program, you should save it in a file that has the extension .f or .for. Before you can execute the program, you must translate it into machine readable form. This is done by a special program called a compiler. The Unix command which runs the Fortran 77 compiler is f77. The output from the compilation is named a.out by default, but you can choose another name if you wish. Once the program has successfully been compiled, and an executable file such as a.out has been created, you may run the program by simply typing the name of the executable file, for example a.out. (This explanation is a bit oversimplified. Really, the compiler translates source code into object code and the linker/loader makes this into an executable file.)

Examples:

Refer back to Section 3, and write the program file circle.f. Then compile and run the program.

If you need to have several executables at the same time, it is a good idea to give them (different!) descriptive names. This can be accomplished using the -o option. For example,

    f77 circle.f -o circle.out

will compile the file circle.f and save the executable in the file circle.out (rather than the default a.out). Please note that object codes and executables take a lot of disk space, so you should delete them when you are not using them. (The remove command in Unix is rm.)

Optional Topic

In the previous examples, we have not distinguished between compiling and linking. These are two different processes but the Fortran compiler performs them both, so the user usually does not need to know about it. But in the next example we will use two source code files.

    f77 circle1.f circle2.f

This will generate three files, the two object code files circle1.o and circle2.o, plus the executable file a.out. What really happened here, is that the Fortran compiler first compiled each of the source code files into object files (ending in .o) and then linked the two object files together into the executable a.out. You can separate these two steps by using the -c option to tell the compiler to only compile the source files:

    f77 -c circle1.f circle2.f
    f77 circle1.o circle2.o

Compiling separate files like this may be useful if there are many files and only a few of them need to be recompiled. In Unix there is a useful command called make which is usually used to handle large software packages with many source files. These packages come with a makefile and all the user has to do is to type make. Writing makefiles is a bit complicated so we will not discuss this in this tutorial.

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