Question

Formal and actual arguments of both imported tasks or functions and exported tasks or functions are bound by the principle “What You Specify Is What You Get.” This principle is binding both for the caller and for the callee, in C code and in SystemVerilog code. For the callee, it guarantees the actual arguments are as specified for the formal ones. For the caller, it means the function call arguments shall conform with the types of the formal arguments, which might require type-coercion on the caller side.

Another way to state this is that no compiler (either C or SystemVerilog) can make argument coercions between a caller’s declared formals and the callee’s declared the formals. This is because the callee’s formal arguments are declared in a different language than the caller’s formal arguments; hence there is no visible relationship between the two sets of formals. Users are expected to understand all argument relationships and provide properly matched types on both sides of the interface (see Annex E.6.2).

In SystemVerilog code, the compiler can change the formal arguments of a native SystemVerilog task or function and modify its code accordingly, because of optimizations, compiler pragmas, or command line switches. The situation is different for imported tasks and functions. A SystemVerilog compiler cannot modify the C code, perform any coercions, or make any changes whatsoever to the formal arguments of an imported task or function.

A SystemVerilog compiler shall provide any necessary coercions for the actual arguments of every imported task and function call. For example, a SystemVerilog compiler might truncate or extend bits of a packed array if the widths of the actual and formal arguments are different. Similarly, a C compiler can provide coercion for C types based on the relationship of the arguments in the exported task’s and function’s C prototype (formals) and the exported task’s and function’s C call site (actuals). However, a C compiler cannot provide such coercion for SystemVerilog types.

Thus, in each case of an inter-language function call, either C to SystemVerilog or SystemVerilog to C, the compilers expect but cannot enforce that the types on either side are compatible. It is therefore the user’s responsibility to ensure that the imported/exported function types exactly match the types of the corresponding tasks or functions in the foreign language.