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Fortran Assignment of 2D array

  1. Oct 11, 2018 #1
    Hi guys,
    On with my Fortran adventures...I'm trying to accomplish a simple task: assign a 2D array with some reals. Here's an excerpt from the code:

    Code (Fortran):

    subroutine get_energy_and_grad(natoms_,coords,ereal,gradmat)

        include 'cbka.blk'
        include 'opt.blk'
        integer, intent(in)    :: natoms_
        double precision, intent(in)    :: coords(3*natoms_)
        real*8, intent(out)    :: ereal    ! total energy
        real*8, dimension(3,3), intent(out)    :: gradmat

        call vlist
        call srtbon1
        call encalc
        ereal = estrc ! total energy

    !    write (*,*) 'natoms_ is: ', natoms_

        write (*,*) 'size of gradmat is:', size(gradmat)
        write (*,*) 'size of d is:', size(d)
        write (*,*) 'shape of gradmat is:', shape(gradmat)
        write (*,*) 'shape of d is:', shape(d)

        gradmat = reshape((/ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 /),(/3, 3/))

        write(*,*) 'gradmat is: ', gradmat(2,3)

    end subroutine get_energy_and_grad
    This just won't assign the 1.0-9.0 reals into gradmat and throws out the following:

    Code (Text):

     size of gradmat is:           9
     size of d is:        7500
     shape of gradmat is:           3           3
     shape of d is:           3        2500

    Program received signal SIGSEGV: Segmentation fault - invalid memory reference.

    Backtrace for this error:
    #0  0x7F3B8F78EE08
    #1  0x7F3B8F78DF90
    #2  0x7F3B8F0D54AF
    #3  0x40151C in __reaxff_interface_MOD_get_energy_and_grad
    #4  0x402C5E in MAIN__ at main.f90:?
    Segmentation fault (core dumped)
    I'm obviously doing something really stupid, but I just can't figure out what's wrong... need help!

    Last edited: Oct 11, 2018
  2. jcsd
  3. Oct 11, 2018 #2


    Staff: Mentor

    Here's a tutorial on the reshape function:


    I don't see what wrong but a memory error often indicates referencing an element outside the array bounds.

    Perhaps, you could try being more explicit and breaking the statement up a bit:

    Code (Fortran):

     real*8, dimension (1:9) :: b = (/ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 /)

     gradmat = reshape( b , (/3, 3/) )

  4. Oct 11, 2018 #3
    Thanks jedishrfu, but it doesn't work either. I suspect there's something wrong with my declarations....
    I post here the full code if this helps

    Main program

    Code (Fortran):

    program main

    use reaxff_interface
    implicit none

    integer natoms_
    real*8 ereal
    real*8, allocatable :: coords(:,:)
    real*8, allocatable :: gradvec(:,:)

    call get_energy_and_grad(gradvec)

    write (*,*) "This is main. grad(3,3) is : ", gradvec(3,3)

    end program main

    Code (Fortran):

        module reaxff_interface
        implicit none

    subroutine get_energy_and_grad(grad)
        include 'cbka.blk'
        include 'opt.blk'
        real*8, dimension(3,3), intent(out)     :: grad
        real*8, dimension (1:9) :: b

        b = (/ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 /)
        grad = reshape( b , (/3, 3/) )
    end subroutine get_energy_and_grad

    end module reaxff_interface
    Code (Text):

    Program received signal SIGBUS: Access to an undefined portion of a memory object.

    Backtrace for this error:
    #0  0x7F4FB566DE08
    #1  0x7F4FB566CF90
    #2  0x7F4FB4FB44AF
    #3  0x7F4FB574563F
    #4  0x7F4FB5748154
    #5  0x7F4FB5748D3E
    #6  0x402DBF in MAIN__ at main.f90:?
    Bus error (core dumped)
  5. Oct 13, 2018 #4


    Staff: Mentor

    I see two or maybe three problems:
    1. In your three call statements, none of the subroutine calls is written correctly. The syntax for a call statement is
    call SubName(argument1, argument2, ...). If a subroutine has no parameters, the parentheses still must be used. See http://pages.mtu.edu/~shene/COURSES/cs201/NOTES/F90-Subprograms.pdf, on page titled "Subroutines: 2/2".
    You don't show the definitions for the three subroutines, so I can't tell if any of them takes any arguments.

    2. What is d? There is no declaration for d in either the parameter list or in the declarations for local variables, so d apparently is a global variable. It's very bad practice for a function of subroutine to access global variables. At the very least, doing this makes debugging much more difficult.

    3. I'm not sure that reshape will take a list of constants as its first argument. Per the GNU Fortran Compiler man page for reshape (https://gcc.gnu.org/onlinedocs/gfortran/RESHAPE.html), it appears to me that the first argument has to be a variable, but I'm not certain of this.
  6. Oct 15, 2018 #5
    Thanks Mark44 for your help!
    I managed to solve the problem by properly declaring and allocating coords and gradvec variables outside the get_energy_and_grad subroutine.

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