According the Fortran standard, valid Fortran names consist of a letter
between A to Z, a to z, digits 0,
1 to 9 and underscores (_) with the restriction
that names may only start with a letter. As vendor extension, the
dollar sign ($) is additionally permitted with the option
-fdollar-ok, but not as first character and only if the
target system supports it.
By default, the procedure name is the lower-cased Fortran name with an
appended underscore (_); using -fno-underscoring no
underscore is appended while -fsecond-underscore appends two
underscores. Depending on the target system and the calling convention,
the procedure might be additionally dressed; for instance, on 32bit
Windows with stdcall, an at-sign @ followed by an integer
number is appended. For the changing the calling convention, see
see GNU Fortran Compiler Directives.
For common blocks, the same convention is used, i.e. by default an
underscore is appended to the lower-cased Fortran name. Blank commons
have the name __BLNK__.
For procedures and variables declared in the specification space of a
module, the name is formed by __, followed by the lower-cased
module name, _MOD_, and the lower-cased Fortran name. Note that
no underscore is appended.