Fortran Wiki
iso_fortran_env

iso_fortran_env is an intrinsic module that provides the following constants relating to the Fortran environment:

character_storage_size
the size, in bits, of the character storage unit.
error_unit
the unit number used for error reporting (stderr).
file_storage_size
the size, in bits, of the file storage unit.
input_unit
the unit number used for input (stdin).
iostat_end
the value that is assigned to the iostat= specifier if an end-of-file condition occurs during a read statement.
iostat_eor
the value that is assigned to the iostat= specifier if an end-of-record condition occurs during a read statement.
numeric_storage_size
the size, in bits, of the numeric storage unit.
output_unit
the unit number used for output (stdout).