Parameter passing on the server side generally follows the same rules as for the client side. Additionally, every operation receives a trailing parameter of type
Ice::Current. For example, the
name operation of the
Node interface has no parameters, but the corresponding
name method of the servant interface has a single parameter of type
Current. We will ignore this parameter for now.
On this page:
Server-Side Mapping for Parameters in C++11
For each parameter of a Slice operation, the C++ mapping generates a corresponding parameter for the virtual member function in the skeleton. Parameter passing on the server side follows these rules:
- in-parameters are passed by value only
- out-parameters are passed by reference
- return values are passed by value
- optional parameters are enclosed in
Compared to the client side rules, the only difference is for in-parameters: on the client side, they are mapped to value or const reference (depending on the parameter type), while on the server side they are always passed by value.
On the client side, you allocate these in-parameters and Ice only needs to read them, so const reference is fine for parameters like strings and vectors. On the server side, Ice allocates these parameters and then relinquishes them to your servant: getting these parameters by value allows your servant to adopt (move) them.
To illustrate the rules, consider the following interface that passes string parameters in all possible directions:
The generated skeleton class for this interface looks as follows:
As you can see, there are no surprises here. For example, we could implement
op as follows:
This code is in no way different from what you would normally write if you were to pass strings to and from a function; the fact that remote procedure calls are involved does not impact on your code in any way. The same is true for parameters of other types, such as proxies, classes, or dictionaries: the parameter passing conventions follow normal C++ rules and do not require special-purpose API calls or memory management.
Thread-Safe Marshaling in C++
The marshaling semantics of the Ice run time present a subtle thread safety issue that arises when an operation returns data by reference. For C++ applications, this can affect servant methods that return instances of Slice classes or types referencing Slice classes.
The potential for corruption occurs whenever a servant returns data by reference, yet continues to hold a reference to that data. For example, consider the following Slice:
And the following servant implementation:
Suppose that a client invoked the
getGrid operation. While the Ice run time marshals the returned class in preparation to send a reply message, it is possible for another thread to dispatch the
clearValues operation on the same servant. This race condition can result in several unexpected outcomes, including a failure during marshaling or inconsistent data in the reply to
getGrid. Synchronizing the
clearValues operations does not fix the race condition because the Ice run time performs its marshaling outside of this synchronization.
Solution 1: Copying
One solution is to implement accessor operations, such as
getGrid, so that they return copies of any data that might change. There are several drawbacks to this approach:
- Excessive copying can have an adverse affect on performance.
- The operations must return deep copies in order to avoid similar problems with nested values.
- The code to create deep copies is tedious and error-prone to write.
Solution 2: Copy on Write
Another solution is to make copies of the affected data only when it is modified. In the revised code shown below,
_grid with a copy that contains empty values, leaving the previous contents of
This allows the Ice run time to safely marshal the return value of
getGrid because the
values array is never modified again. For applications where data is read more often than it is written, this solution is more efficient than the previous one because accessor operations do not need to make copies. Furthermore, intelligent use of shallow copying can minimize the overhead in mutating operations.
Solution 3: Marshal Immediately
Finally, a third approach is to modify the servant mapping using metadata in order to force the marshaling to occur immediately within your synchronization. Annotating a Slice operation with the
marshaled-result metadata directive changes the signature of the corresponding servant method, if that operation returns mutable types. The metadata directive has the following effects:
- For an operation
opthat returns one or multiple values and at least one of those values has a mutable type, the Slice compiler generates an
OpMarshaledResultclass and the return type of the servant method becomes
- The constructor for
OpMarshaledResulttakes an extra argument of type
Current. The servant must supply the
Currentin order for the results to be marshaled correctly.
The metadata directive has no effect on the proxy mapping, nor does it affect the servant mapping of Slice operations that return
void or return only immutable values.
You can also annotate an interface with the
marshaled-result metadata and it will be applied to all of the interface's operations.
After applying the metadata, we can now implement the
Grid servant as follows:
Here are more examples to demonstrate the mapping:
Review the generated code below to see the changes that the presence of the metadata causes in the servant method signatures: