Programming Visual Basic for the Palm OS

Memory Management Functions

Let’s review some of the common Palm OS SDK functions for managing memory.

To allocate memory, use MemHandleNew as follows:

typedef struct GlobalVarsType
{
   Int16   nOpenCount;     // library open count    
   Int16   nContextCount;  // number of contexts using this library
   //Other library global variables
}GlobalVarsType;

UInt32 nMemSize = sizeof(GlobalVarsType);
//Allocate global memory
MemHandle gHandle = MemHandleNew(nMemSize);

To lock a chunk of memory for read/write access, use the following sequence.

MemPtr gLockPtr;
gLockPtr = MemHandleLock(gHandle);
//read from or write to global memory
MemHandleUnlock(gHandle);

Alternatively, since MemHandleLock returns a MemPtr, you may use the following MemHandleLock/MemPtrUnlock pair:

MemPtr gLockPtr;
gLockPtr = MemHandleLock(gHandle);
//read from or write to global memory
MemPtrUnlock(gLockPtr);

To set the owner ID of memory so it belongs to the OS, you must first lock the memory chunk, set the owner ID, and then unlock the memory chunk. You typically set the owner ID immediately after creating the memory:

MemHandle gHandle = MemHandleNew(nMemSize);
MemPtr gLockPtr = MemHandleLock(gHandle);
//Set the owner to the OS, so it is not freed automatically.
MemPtrSetOwner(gLockPtr, 0);
               
MemPtrUnlock(gLockPtr);

When your library is done with its global memory—which is typically when the last application unloads it from memory—the library should free its memory using MemHandleFree, as follows:

//Our library is being closed by ...