3.6 Read-only Variables, Messages and Arrays

Since CHARM++ does not allow global variables for keeping programs portable across a wide range of machines, it provides a special mechanism for sharing data amongst all objects. Read-only variables, messages and arrays are used to share information that is obtained only after the program begins execution and does not change after they are initialized in the dynamic scope of main::main() function. They can be accessed from any chare on any processor as ``global'' variables.When a variable is declared as read only,it is PUPped so that it can be accessed from any chare on any processor. Large data structures containing pointers can be made available as read-only variables using read-only messages or read-only arrays. Read-only variables, messages and arrays can be used just like local variables for each processor, but the user has to allocate space for read-only messages using new to create the message in the main function of the mainchare.

Read-only variables, messages, and arrays are declared by using the type modifier readonly, which is similar to const in C++. Read-only data is specified in the .ci file (the interface file) as:

 readonly Type ReadonlyVarName;

The variable ReadonlyVarName is declared to be a read-only variable of type Type. Type must be a single token and not a type expression.

 readonly message MessageType *ReadonlyMsgName;

The variable ReadonlyMsgName is declared to be a read-only message of type MessageType. Pointers are not allowed to be readonly variables unless they are pointers to message types. In this case, the message will be initialized on every processor.

 readonly Type ReadonlyArrayName [arraysize];

The variable ReadonlyArrayName is declared to be a read-only array of type Type. Type must be a single token and not a type expression.

Read-only variables, messages and arrays must be declared either as global or as public class static data, and these declarations have the usual form:

 Type ReadonlyVarName;
 MessageType *ReadonlyMsgName;
 Type ReadonlyArrayName [arraysize];

Similar declarations preceded by extern would appear in the .h file.

Note: The current CHARM++ translator cannot prevent assignments to read-only variables. The user must make sure that no assignments occur in the program.