Free Fortran Compilers G77

Free Fortran Compilers

There are a number of free Fortran 77 and 90 compilers available on the net. The one I have been using in my Fortran courses at York is GNU, which implements Fortran 77 and adds several Fortran 90 features. Thanks to Prof. Clive Page (Dept of Physics & Astronomy, University of Leicester, UK) for providing the compiler and for valuable advice on Fortran in general.
You can download the 1999 version of this compiler (version 2.95 of gcc) along with the SLATEC library (Version 4.1, July 1993), from this page. The package should run under all versions of Windows. All the needed files are packed in one zipped file (Fort99.zip) of about 6MB.
(If for some reason you need the older DOS/EMX version, which does not include a library and does not run under Windows XP, then you can download it from my old page.)

DOWNLOAD

• Create the directory \F
  The new folder must be immediately under the root of your hard disk. You can do this by double-clicking MyComputer, then double-clicking your hard drive (usually C:), and then selecting New Folderfrom the File menu and calling the folder F.
  
• Download the file Fort99.zip (5,820,239 bytes).
  You can do this by right-clicking the mouse on the above link, and choosing Save Target As.... In the Save As window that appears, locate the F folder, and save the file in it.
  
• Unzip the downloaded file into \F.
  Yon can do this by locating the file (starting from MyComputer) and simply double-clicking it to launch the zip/unzip program. Make sure to specify that all files should be extracted immediately under the F folder.
  
  Note: If the unzip program does not give you the option to specify the extraction location, let it extract the content to anywhere and then move the extracted folders (using cut and paste) to F. When done, you should see the four folders G77, SLATEC, MINE, and YORK appearing in F. 

That's it! You use the compiler through CLI, the command-line interface (aka terminal, command prompt, or DOS) after setting two environment variables, PATH and LIBRARY_PATH, as shown below.

USAGE

You store your programs in the \F\York directory, compile them using: f2exe, and create library object files using f2lib. Here is a very short program to test the compiler and the configuration:

  program Convert
  implicit none
  ! -----------------------------------------------Declare
  real*4 tempC, tempF, FACTOR
  integer*2 ZERO_SHIFT
  parameter (ZERO_SHIFT = 32, FACTOR = 5./9.)
  ! -----------------------------------------------Input
  print*, "Enter the temperature in Fahrenheit ..."
  read*, tempF
  ! -----------------------------------------------Compute
  tempC = FACTOR * (tempF - ZERO_SHIFT)
  ! -----------------------------------------------Output
  print*, "The corresponding Centigrade temperature is "
  print*, tempC, " degrees."
  end

Use any editor to create this program (simply copy and paste) and save it as a text file in the \F\York directory under the name test.for. You can, of course, use any editor you like as long as you can save the file in text format and with the extension you want. Notepad, for example, uses text but insists on using the txt extension (unless you override by double-quoting) while MS-Word insists on its propriety format (unless you explicitly override). I highly recommend using the Crimson editor, which can be downloaded from the on-line Lab-1 (see below).
To compile your program, start a CLI session (by launching the command prompt program, usually in the Accessories group) and issue these two commands:

        PATH=\F\G77\bin;%PATH%
        SET LIBRARY_PATH=\F\G77\lib

These set the environment so that your computer would know where the compiler and its libraries are located.

Note: these two commands must be issued every time you start a CLI session. You can optionally automate this step by adding these two variables to the system-wide environment using the Control Panel. 

You can now compile and run your program by typing:

  cd \F\York
  f2exe test
  test

If the first command returned an error then the directory was not created (or named) correctly. If the second command was not recognized, or complained that a library is missing, then the environment variables were not set correctly (you can issue the set command without any arguments to inspect all environment variables).
More information on using the compiler can be found in the on-line Labs at the Fortran@York site.

LANGUAGE

The \F\G77\doc directory has a detailed reference to the language, which is largly ANSI Fortran-77 but with some Fortran-90 features added (see below).
The above Fortran@York site contains a quick reference guide, lab, and SLATEC usage examples. If you are already familiar with Fortran then the following points may be all you need to know about this compiler:

1. Control Structures
   You can use either the old (goto-based) or the new (structured) control flow (or mix them in the same program). Support of the "ugly goto" is meant for existing code only, and any new development should avoid it.
2. Style
   You can write your source using either the old style code (column 7) or the newer free-form.
3. Compilation Command
   The above f2exe command is just a batch file that invokes g77, the "real" compilation command. The command:

       g77 -ffree-form prog.for -oprog.exe

   directs the compiler to compile the file prog.for and stores the output in the file prog.exe. The -ffree-form switch indicates free-form style (remove it if you are using the old style).
4. Comments
   In free-form style, use ! for both full-line and in-line comments. In the old style, use a "C" in column-1.
5. Statement Continuation
   In free-form style, you can continue a statement on the next line by ending it with an ampersand "&". In the old style, put a character in column-6.
6. Path Separator
   When referring to files (e.g. in the file=' ' parameter of the OPEN statement) use a forward slash "/" or two consecutive backslashes "\\" rather than a backslash to delimit directories. This is because the backslash "\" denotes an escape sequence in strings.
7. I/O Unit Numbers
   Not all unit numbers are allowed in the OPEN statement. In particular, unit 5 is "pre-connected" to the keyboard. Units 10 through 99 seem to work well with disk files.
8. Fortran-90 Features
   These include: Automatic arrays in subprograms, zero length strings, character constants may be enclosed in double quotes (") as well as single quotes, cycle and exit, the DOUBLE COMPLEX type, DO WHILE, the END decoration, KIND, IMPLICIT NONE, INCLUDE statements, list-directed and namelist I/O on internal files, binary, octal, and hex constants, `O' and `Z' edit descriptors, NAMELIST, OPEN specifiers STATUS='REPLACE', the FILE= specifier may be omitted in an OPEN statement if STATUS='SCRATCH' is supplied, relational operators <, <=, ==, /=, > and >= may be used instead of .LT., .LE., .EQ., .NE., .GT.and .GE. respectively, SELECT CASE (but not for character types).
9. Separate Compilation of Subprograms
   Your main program is recognized by the program statement, as in the Convert program above. The subprograms (functions and subroutines) can be included in the same file as the main program (in which case you can compile everything in one shot) or can be stored in separate file(s). It is recommended that you store general reusable subprograms in separate files so that you can reuse them (without recompiling them) in future projects. To compile a file that contains only subprograms (no program statement), use the f2lib command, which generates object files, one per sub, in the minedirectory, e.g.

       f2lib util

   will compile (without linking) the subprogram in util.for and store the output (an object file) in the file util.o. f2lib is just a batch file that invokes the g77 command with the -c (compile-only) switch, viz.

       g77 -c -ffree-form util.for -o..\mine\util.o

   A program that uses pre-compiled object files can be compiled (and linked to them) by simply referring to them in the compilation command:

       g77 -ffree-form prog.for -oprog.exe ..\mine\*.o

   The above command searches all object files in mine to resolve any missing reference in prog.for.
10. Separate Compilation of Subprograms, automated
    The supplied f2exe and f2lib batch files take care of separate compilation and delayed linking with object files and with the SLATEC subprograms. You don't have to directly issue the g77 command unless you use the old columnar style or you want to change one of the switches or directories.
11. Assembly Listing
    The -S (capital S) switch allows you to see a listing of the generated assembly code.