GNU Fortran supports some of the new features of the Technical Specification (TS) 29113 on Further Interoperability of Fortran with C. The wiki has some information about the current TS 29113 implementation status. In particular, the following is implemented.
See also Further Interoperability of Fortran with C.
OPTIONALattribute is allowed for dummy arguments of
RANKintrinsic is supported.
ASYNCHRONOUSattribute is compatible with TS 29113.
DIMENSION(..)). However, the array descriptor of the TS is not yet supported.