TotalView Exercise 3

Assignment

Use TotalView to locate an error in the following program. The program should print a sum of matrix components. The correct sum is 5.001E+7.

      PROGRAM WRONG
      COMMON /BLOCK/ VECA,VECB,VECA2
      DOUBLE PRECISION VECA(100,100), VECB(500), VECA2(100,100)
      INTEGER I
      DO 10 I=1,100
      DO 15 J=1,100
        VECA(I,J) =   (I-50.D0)**2+(J-50.D0)**2
        VECA2(I,J) =  2.D0*((I-50.D0)**2+(J-50.D0)**2)
15    CONTINUE
        VECB(10000-I*J) = 42.D0
10    CONTINUE
      SUM = 0
      DO 20 I=1,100
      DO 20 J=1,100
        SUM=SUM+VECA(I,J)+VECA2(I,J)
20    CONTINUE
      PRINT *, 'SUM should be 5.001E+07. SUM: ',SUM
      PRINT *, 'All Done'
      STOP
      END

Details

cut and paste the code into the file wrong.f

ifort -g -o wrong wrong.f

./wrong

Tips

Load the program in TotalView.
Double click on the common block named "block" in the stack frame
In the window for the common block that pops up, you will see a list of all the variables that make up the block. Double click on VECA2 to view the contents of the array.
When does the array VECA2 become corrupt?
What will fix the problem?

A solution

The index used to assign values to VECB goes out of the bounds for this array. The index only goes slightly outside the boundaries of the array, so it does not cause a segmentation fault. Instead, the loop starts assiging values to VECA2. Values in VECA2 were overwritten because VECA2 was in the next chunk of memory following VECB.

Change the index for VECB or comment out the line entirely to solve the problem.