Warning: Padding of 4 bytes required before …

fortran does not have the global variable to share data among different subroutines, thus COMMON block will be mostly used.

However, it has restrictions:

• The common statement should appear together with the variable declarations, before the executable statements.

• Different common blocks must have different names (just like variables).

• A variable cannot belong to more than one common block.

• The variables in a common block do not need to have the same names each place they occur (although it is a good idea to do so), but they must be listed in the same order and MAY have the same type and size.

please take a look at the last restriction, word MAY (I added), which means if it happens for the parameters in COMMON block do not have the same size, in some circumstances for many compilers, the size of the smaller one will be extended and padded to be comparable with the size of the larger one, and at meanwhile the Warning information: Padding of 4 bytes required before ... will be printed out. (value 4 can be any number of size difference…)

codes to reproduce this warning info

      PROGRAM NICE
         COMMON /NOV/ FOO
      END

      SUBROUTINE GOOD
         IMPLICIT NONE
         REAL*8 FOO
         COMMON /NOV/ FOO
      END

as you can see, if the above codes are compiled, the Warning info will be prompted.

explanation

because no IMPLICIT NONE is specified, in PROGRAM NICE, the size of parameter FOO is set to default, 4 Bytes, however, in SUBROUTINE GOOD, its size is explicitly set to 8 Bytes, therefore, during the compiling, the FOO in NICE will be padded with additional 4 Bytes to match the size in GOOD.

All those warning info is totally acceptable!!