Question

DPI imposes the following additional restrictions on the representation of SystemVerilog data types.

• SystemVerilog types that are not packed and that do not contained packed elements have C compatible representation.
• Basic integer and real data types are represented as defined in Annex E.6.4.
• Enumeration types are represented as the types associated with them. Enumerated names are not available on C side of interface.
• Representation of packed types is implementation-dependent.
• Unpacked arrays embedded in a structure have C compatible layout regardless of the type of elements. Similarly, standalone arrays passed as actuals to a sized formal argument have C compatible representation.
• Standalone array passed as an actual to an open array formal
• if the element type is scalar or packed then the representation is implementation dependent
• otherwise the representation is C compatible. Therefore an element of an array shall have the same representation as an individual value of the same type. Hence, an array’s elements can be accessed from C code via normal C array indexing similarly to doing so for individual values.
• The natural order of elements for each dimension in the layout of an unpacked array shall be used, i.e., elements with lower indices go first. For SystemVerilog range [L:R], the element with SystemVerilog index min(L,R) has the C index 0 and the element with SystemVerilog index max(L,R) has the C index abs(L-R).

NOTE—This does not actually impose any restrictions on how unpacked arrays are implemented; it only says an array that does not satisfy this condition shall not be passed as an actual argument for a formal argument which is a sized array; it can be passed, however, for a formal argument which is an unsized (i.e., open) array. Therefore, the correctness of an actual argument might be implementation-dependent. Nevertheless, an open array provides an implementation-independent solution; this seems to be a reasonable trade-off.