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Example: making a library archive
In this example a number of subroutines are incorporated info a library archive. Routines from the library are linked into the main program during the linking process.
It is often convenient to maintain object files in a library. By doing so, you can reduce the number of .o files you need to keep in a directory, which can have many advantages.
The example project
Let's say we have four routines that we want to maintain in a library and a main program (libex.f90) that will call those routines. In this example, the routines simply perform an operation on two real numbers and print the result. We'll make a library file, named operators.a, that will contain the subroutines. After we create the library file, we will delete the object files (*.o).
Here's what the files look like:
% ls add.f90 libex.f90 multiply.f90 divide.f90 makefile subtract.f90
! FILENAME: add.f
SUBROUTINE add(x,y)
IMPLICIT NONE
REAL, INTENT(IN) :: x, y
PRINT *, "The sum of", x, " and ", y, "is ", x+y
RETURN
END SUBROUTINE add
!FILENAME: divide.f
SUBROUTINE divide(x,y)
IMPLICIT NONE
REAL, INTENT(IN) :: x, y
IF(y==0.) THEN
PRINT *, "Can not divide by zero!"
RETURN
END IF
PRINT *, x, "divided by", y, "is ", x/y
RETURN
END SUBROUTINE divide
!FILENAME: multiply.f
SUBROUTINE multiply(x,y)
IMPLICIT NONE
REAL, INTENT(IN) :: x, y
PRINT *, "The product of", x, " and ", y, "is ", x*y
RETURN
END SUBROUTINE multiply
!FILENAME: subtract.f
SUBROUTINE subtract(x,y)
IMPLICIT NONE
REAL, INTENT(IN) :: x, y
PRINT *, "The difference, ", x, " - ", y, "is ", x-y
RETURN
END SUBROUTINE subtract
!FILENAME: libex.f
! An example program that illustrates calling routines
! from a user-built library of object modules.
PROGRAM libex
IMPLICIT NONE
REAL:: a=2.0,b=3.0
CALL add(a,b)
CALL multiply(a,b)
CALL subtract(a,b)
CALL divide(a,b)
END PROGRAM libex
The makefile
The following simple makefile can be used to maintain the archive and produce the executable. make has a special syntax for dealing with libraries. The library is defined on the line
$(LIB) : $(LIB)(add.o) $(LIB)(multiply.o) \ $(LIB)(divide.o) $(LIB)(subtract.o)
The the rule for producing the library is the following (note that the .o files are removed immediately after updating the library).
.f90.a :
$(CF) -c $<
$(AR) -r $@ $*.o
/bin/rm -f $*.o
This rule says
- If the .f file has been modified since the .a file was modified, compile the .f file using $(CF) -c.
- Take the corresponding .o file and replace it in the proper library (as defined above) using $(AR) (The ar command builds and maintains libraries, see man ar).
- Remove the .o file.
CF = xlf90
LD = xlf90
LIBOBJS = add.o multiply.o divide.o subtract.o
LIB = operators.a
EXE = libex
OBJS = libex.o
AR = ar
$(EXE): $(OBJS) $(LIB)
$(CF) -o $(EXE) $(OBJS) $(LIB)
$(LIB) : $(LIB)(add.o) $(LIB)(multiply.o) \
$(LIB)(divide.o) $(LIB)(subtract.o)
.f.a :
$(CF) -c $<
$(AR) -r $@ $*.o
/bin/rm -f $*.o
.f.o :
$(CF) -c $<
clean :
/bin/rm $(OBJS)
clobber :
/bin/rm $(EXE) $(LIB) $(OBJS)
The Output
% make
xlf90 -c libex.f
** libex === End of Compilation 1 ===
1501-510 Compilation successful for file libex.f.
xlf90 -c add.f
** add === End of Compilation 1 ===
1501-510 Compilation successful for file add.f.
ar -r operators.a add.o
ar: Creating an archive file operators.a.
/bin/rm -f add.o
xlf90 -c multiply.f
** multiply === End of Compilation 1 ===
1501-510 Compilation successful for file multiply.f.
ar -r operators.a multiply.o
/bin/rm -f multiply.o
xlf90 -c divide.f
** divide === End of Compilation 1 ===
1501-510 Compilation successful for file divide.f.
ar -r operators.a divide.o
/bin/rm -f divide.o
xlf90 -c subtract.f
** subtract === End of Compilation 1 ===
1501-510 Compilation successful for file subtract.f.
ar -r operators.a subtract.o
/bin/rm -f subtract.o
xlf90 -o libex libex.o operators.a
s00505% ./libex
The sum of 2.000000000 and 3.000000000 is 5.000000000
The product of 2.000000000 and 3.000000000 is 6.000000000
The difference, 2.000000000 - 3.000000000 is -1.000000000
2.000000000 divided by 3.000000000 is 0.6666666865
