Include files and modules

Your program can be contained in various source files, all stored in the same directory or organised in some convenient directory tree. The details of the organisation depend on personal taste, arrangements made by the group of developers you belong to, or simply the history of the program. Whatever the directory structure is, you will encounter a situation where the compiler needs assistence in order to compile a particular source file:

• Fortran (and other languages) has the possibility to include an external file. While this feature has become a bit less useful with the advent of modules, it still has its uses. Quite often, such “include files” are stored in a directory separated from the directories containing the source files, because they are used in several locations. Quite often the name of that directory is “include”.
• As we have seen, compiling source code that defines one or more modules, leads to the compiler generating so-called “module intermediate files” (with the extension “.mod”). The compiler needs access to these files to be able to read the interfaces and variables and what not, and based on this information, actually compile the source code that uses the various modules.

Compilers support options like -I to indicate where these include files and module intermediate files are to be found. Suppose we store the two files of our tabulate program in the following directory structure:

tabulate/
    main/
        tabulate.f90
    sub/
        functions.f90

Compiling the file “functions.f90” with the commands

$ cd sub
$ gfortran -c functions.f90

leads to this structure:

tabulate/
    main/
        tabulate.f90
    sub/
        functions.f90
        user_functions.mod
        functions.o

To successfully compile and subsequently build the program we need to tell the compiler where it can find the file “user_functions.mod”:

$ cd main
$ gfortran -c tabulate.f90 -I ../sub
$ gfortran -o tabulate tabulate.o ../sub/functions.o

The result:

tabulate/
    main/
        tabulate.f90
        tabulate.o
        tabulate (or tabulate.exe on Windows)
    sub/
        functions.f90
        functions.o
        user_functions.mod

Notes:

• The details differ per compiler. Sometimes the -I option should be followed by a space and then the name of the directory, sometimes the directory should come consecutively.
• By default the module intermediate files (.mod) are put in the same directory as the object files. When your program code is organised in different subdirectories, they will get scattered over the directory tree, complicating the compilation process. Luckily, many compilers allow you to specify the output location for these files. For gfortran this is -J, for instance: -J../include (so that the .mod files could all appear in the same directory)
• For large programs, consisting of many source files, it is important to think through what organisation to use.