Here is an example of modular programming; compilation is performed
in three alternative ways:

  1. From all sources:  g95 mod1.f90 mod2.f90 main.f90 -o main  

  2. From object files: g95 mod1.o mod2.o main.f90 -o main

  3. From a library:    g95 main.f90 -o main -L. -lmods

The example sources are main.f90, mod1.f90, mod2.f90

main.f90
--------------------------
PROGRAM MAIN  
  
  USE mod1
  USE mod2
  
  PRINT *, Message
  
  call sub1(A)
  call sub2(A)
  call sub3(A)
  
END PROGRAM MAIN
---------------------------


mod1.f90
---------------------------
MODULE mod1

CONTAINS

  subroutine sub1(X)
  print *,X
  end subroutine sub1

  subroutine sub2(X)
  print *,X**2
  end subroutine sub2

END MODULE mod1
---------------------------


mod2.f90
---------------------------
MODULE mod2

  REAL :: A=3.0
  CHARACTER(LEN=36) :: Message="This is an example of using modules."

CONTAINS
  
  subroutine sub3(X)
  print *,X**3
  end subroutine sub3
  
END MODULE mod2
---------------------------

The program can be compiled as follows.

From all source files:

  $ g95 mod1.f90 mod2.f90 main.f90 -o main

Or we can compile the modules to objects:

  $ g95 -c mod1.f90
  $ g95 -c mod2.f90
  (these commands generate the object files mod1.o and  mod2.o,
  also mod1.mod and mod2.mod).

and compile by linking the objects

  $ g95 mod1.o mod2.o main.f90 -o main
    (the files mod1.mod and mod2.mod are needed for linking)

Or the module objects can be placed inside a library:

  $ ar rcvf libmods.a mod1.o mod2.o

    ar rcvf libmods.a another.o  - to add another object
    ar tv libmods.a              - to list the contents of the library
    ar dv libmods.a mod2.o       - to delete object mod2.o
    nm libmods.a                 - to list the  subprograms it contains

and compiled by linking the library

  $ g95 main.f90 -o main -L. -lmods 
    (the files mod1.mod and mod2.mod are still needed for linking)

02/10/2006