FAQ-0239 Why is a cast used from jlong to TInt64 in the PowerInfo example?


FAQ-0239 Why is a cast used from jlong to TInt64 in the PowerInfo example?

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Classification:	Java	Category:	JNI
Created:	06/29/99	Modified:	09/02/2001
Number:	FAQ-0239
Platform:	ER5

Question:
In the PowerInfo example in the SDK, a TInt64 variable is the return variable to the TimeOnPower() method. This is explicitly cast to a jlong before it is returned. Yet the JNI documentation from Sun indicates that no such casting should be necessary for a long variable in C or C++. Why is the cast used? And is such a cast necessary for other basic types?

Answer:
The cast is needed because TInt64 is a class, whereas jlong is a primitive type similar to int or double. There is no TInt64 constructor which takes a jlong, nor any TInt64 function which returns a jlong, but the representations in memory are identical so the cast allows us to treat either value as though it is the other.

jlong jvalue;
TInt64 int64value;

int64value = jvalue; // illegal - no suitable constructor
int64value = *REINTERPRET_CAST(jlong*,&jvalue); // nasty, but works

All of the other Java arithmetic types are simple typedefs for standard C++ primitive types, and have corresponding EPOC typedefs:

Further information about jlong

EPOC does not currently have a name for the primitive 64-bit integer type, and in fact the ANSI C and C++ standards do not include 64-bit integers. Both MSVC and GCC support 64-bit integers as non-standard extensions and the EPOC Runtime for Java makes use of that support where possible.

In particular, the compilers will be able to support code such as

jlong j1=100;
jlong j2=37;
j1 = j1+j2; // MSVC and GCC compilers generate appropriate inline assembler
but not

j1 = j1/j2; // OK with MSVC, fails with GCC
because the GCC compiler will assume the existence of a helper function which EPOC does not provide. The problem will show up as a link-time error in MARM builds, similar to the problem with certain operations on TReal32 and TReal variables - see "What is __fixunssfsi?" in the EPOC C++ Knowledgebase.