Currently, gfortran automatically aligns data in COMMON blocks, as in the following example:
implicit none integer(kind=4) :: n real(kind=8) :: p common /foo/ n,p end This gives the warning: COMMON 'foo' at (1) requires 4 bytes of padding at start This automatic padding can result in unexpected results, as demonstrated by the following code: subroutine one() implicit none integer :: n1,n2,n3,n4 common /foo/ n1,n2(2),n3,n4 print *, n3, n4 end subroutine one program prog implicit none integer :: n1,n2,n3 real(8) :: r1 common /foo/ n1,r1,n2,n3 n2 = 2 n3 = 3 call one() end program prog The output of this program is "0 2" instead of "2 3". Therefore gfortran should have an option to suppress automatic alignment. Preferrably this option should be enabled by default. See also http://gcc.gnu.org/ml/fortran/2008-09/msg00221.html -- Summary: alignment of data in COMMON blocks Product: gcc Version: 4.4.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: fortran AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: janus at gcc dot gnu dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37486
