XL Fortran provides a standardized mechanism for interoperating with C based on the Fortran 2003 Standard. An entity is said to be interoperable if equivalent declarations of it can be made in the two languages. XL Fortran enforces interoperability for types, variables, and procedures. Interoperability with the C programming language allows portable access to many libraries and the low-level facilities provided by C and allows the portable use of Fortran libraries by programs written in C. XL The details of this implementation are discussed in this section.