6.2.4 Volatile COMMON blocks
Some Fortran compilers, including g77, let the user declare COMMON with the VOLATILE attribute. This is invalid standard Fortran syntax and is not supported by gfortran. Note that gfortran accepts VOLATILE variables in COMMON blocks since revision 4.3.
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https://gcc.gnu.org/onlinedocs/gcc-9.3.0/gfortran/Volatile-COMMON-blocks.html