TotalView Exercise 2

Assignment

In this assignment, use TotalView to locate an error in the following program. The program is crashing with a segmentation fault.

      PROGRAM SEG

      DOUBLE PRECISION VEC(10)
      INTEGER I

      DO 10 I=1,10
        VEC(I) = 2.D0*I
10    CONTINUE

      I=1000000

      PRINT *, VEC(I)
      PRINT *, 'All Done'

      STOP
      END

Details

cut and paste the code into the file seg.f

ifort -g -o seg seg.f

./seg

A segmentation fault results when a program tries to access a memory segment that has not been allocated to the program.

Tips

Run TotalView, select "New Program", and load seg.
Click the "Go" button. The program will stop and point to the line where the segmentation violation occured.
Look at the variables and determine why the seg. fault occurs.

A solution

The index chosen (100000) is outside the boundries of the array. In fact, it is so far outside, the print statement is trying to print from a piece of memory outside the program. This causes a segmentation fault when the attempt is made to access the memory.