2.8 Memory accessing methods

GNU Smalltalk provides methods to query its own internal data structures. You may determine the real memory address of an object or the real memory address of the OOP table that points to a given object, by using messages to the Memory class, described below.

Method on Object: asOop

Returns the index of the OOP for anObject. This index is immume from garbage collection and is the same value used by default as an hash value for anObject (it is returned by Object’s implementation of hash and identityHash).

Method on Integer: asObject

Converts the given OOP index (not address) back to an object. Fails if no object is associated to the given index.

Method on Integer: asObjectNoFail

Converts the given OOP index (not address) back to an object. Returns nil if no object is associated to the given index.

Other methods in ByteArray and Memory allow to read various C types (doubleAt:, ucharAt:, etc.). These are mostly obsoleted by CObject which, in newer versions of GNU Smalltalk, supports manually managed heap-backed memory as well as garbage collected ByteArray-backed memory.

Another interesting class is ObjectMemory. This provides a few methods that enable one to tune the virtual machine’s usage of memory; many methods that in the past were instance methods of Smalltalk or class methods of Memory are now class methods of ObjectMemory. In addition, and that’s what the rest of this section is about, the virtual machines signals events to its dependents exactly through this class.

The events that can be received are

returnFromSnapshot

This is sent every time an image is restarted, and substitutes the concept of an init block that was present in previous versions.

aboutToQuit

This is sent just before the interpreter is exiting, either because ObjectMemory quit was sent or because the specified files were all filed in. Exiting from within this event might cause an infinite loop, so be careful.

aboutToSnapshot

This is sent just before an image file is created. Exiting from within this event will leave any preexisting image untouched.

finishedSnapshot

This is sent just after an image file is created. Exiting from within this event will not make the image unusable.