I have a fortran program that has the structure given below. This program has been crashing with 'Segmentation fault' error, so I did a lot of digging into the root cause.
It turns out, in the subroutine SUB1, when the DO loop counter variable I reaches a value of 187606, the value of Z(608673), which was originally read as 28.29 from the external file, somehow inexplicably changes to 3.9809702045652999E-309, even though there has been no calculation that would change the values of any of the X, Y or Z arrays after being read from the external file.
I suspect many other array element values are also being set to arbitrary numbers by the program midway through the execution (probably around zero) - though it is impossible to explicitly check for this, given the large size of the array - but in essence this is what is causing the program to crash (these values are used elsewhere in the program).
So the main question is: Why are the array element values being reset arbitrarily by the program, although these arrays are SAVE'd in the MODULE VARIABLES and this module is being USE'd by all the relevant routines? Isn't the SAVE statement meant to preserve the values of the arrays?
Note: I tried running this code on [a] Ubuntu Linux Dell laptop with 16 GB of RAM (no other memory hungry program running) and [b] a Windows laptop with 16 GB RAM. On both these machines I used the gfortran compiler and the same problem occurs. For small values of N, the code seems to run fine - only very large arrays are a problem.
File vars.f90
MODULE VARIABLES
......
......
INTEGER :: N
REAL(8), DIMENSION(:), ALLOCATABLE :: X,Y,Z
SAVE N
SAVE X,Y,Z
.....
.....
... (several other variable declarations) ....
END MODULE VARIABLES
File allocate.f90
MODULE ALLOCATE_ARRAYS
CONTAINS
SUBROUTINE ALLOC_ARR
USE VARIABLES
USE DEALLOCATE_ARRAYS
.....
ALLOCATE(X(N),Y(N),Z(N), STAT=IALLOC)
IF (IALLOC /= 0) THEN
WRITE(6,*)' ERROR while allocating arrays X Y Z'
CALL DEALLOC_ARRAYS
STOP
ENDIF
.....
END SUBROUTINE ALLOC_ARR
END MODULE ALLOCATE_ARRAYS
File deallocate.f90
MODULE DEALLOCATE_ARRAYS
CONTAINS
SUBROUTINE DEALLOC_ARR
USE VARIABLES
.....
IF (ALLOCATED(X)) DEALLOCATE(X)
IF (ALLOCATED(Y)) DEALLOCATE(Y)
IF (ALLOCATED(X)) DEALLOCATE(X)
...
END SUBROUTINE DEALLOC_ARR
END MODULE DEALLOCATE_ARRAYS
File main.f90
PROGRAM PROG1
USE VARIABLES
USE ALLOCATE_ARRAYS
USE DEALLOCATE_ARRAYS
.....
N = 18666800 ![in practice this is read from an external input file]
CALL ALLOC_ARR !Allocate X,Y,Z each to be of length N
!Read all values of X(1:N), Y(1:N) and Z(1:N) from an external file] ...
OPEN(11,FILE=datafile.txt,STATUS='OLD')
DO I = 1,N
READ(11,*,IOSTAT=ISTAT) X,Y,Z
IF ( ISTAT /= 0 ) THEN
WRITE(6,*)' *** SERIOUS WARNING ***'
WRITE(6,*)' Something went wrong while trying to read numbers X Y Z at I = ',I
READ(5,*)
ENDIF
ENDDO
CLOSE(11)
CALL SUB1
.....
CALL DEALLOC_ARRAYS
END PROGRAM PROG1
SUBROUTINE SUB1
USE VARIABLES
USE ALLOCATE_ARRAYS
USE DEALLOCATE_ARRAYS
.....
DO I = 1,N
write(6,*)' X(608673) Y(608673) Z(608673) = ',X(608673),Y(608673),Z(608673) ![this statement only inserted for tracing the bug]
CALL SUB2(I,X(I),Y(I),Z(I))
ENDDO
END SUBROUTINE SUB1
SUBROUTINE SUB2(IPASS,XPASS,YPASS,ZPASS)
USE VARIABLES
USE ALLOCATE_ARRAYS
USE DEALLOCATE_ARRAYS
[carry out some calculations that use the values of IPASS, XPASS, YPASS, ZPASS, but never change their values]
END SUBROUTINE SUB2
File datafile.txt (small sample):
.....
.....
10879.544935 1200.249974 28.290163
10914.193168 205.374638 236.847393
23872.837623 3634.498293 23721.923293
.....
.....
