HeapAlloc pointer usage question

[44mag]

I'm not sure what to do with the pointer received after allocating a block of mem:


.data?
buffer db 64 dup(?)
hMem DWORD ?
pMem DWORD ?

...
...

.elseif uMsg==WM_CREATE

   invoke HeapCreate,HEAP_GENERATE_EXCEPTIONS,64,NULL
   mov hMem,eax
   invoke HeapAllocate,hMem,HEAP_GENERATE_EXCEPTIONS,32
   mov pMem,eax


So I'm guessing now the addr of pMem points to an addr that points to an allocated block of memory?   :dazzled:  Or am I making it more complicated than it really is?  Is there a way I can just change the address of pMem to the addr it contains so I use it like my buffer array?  Thanks for any info!

[Twister]

Correct. The return value, if it is not null, is the beginning address of the memory block. Also, be sure to free the allocated memory after you are done using it

[qWord]

You have filled pMem with a pointer to the allocated data block. If you want to access the elements, this pointer must be loaded into a register:
e.g.:

Code Select Expand

mov esi,pMem
movzx eax,BYTE ptr [esi]
movzx edx,BYTE ptr [esi+1]
mov [esi],dl
mov [esi+1],al

BTW: do you really want to use HEAP_GENERATE_EXCEPTIONS? - this requires SEH.

[dedndave]

...also
there is probably no need for HeapCreate
each process has a heap already
unless you are playing complex "games" with the heap, just use that

Code Select Expand

        INVOKE  GetProcessHeap
        mov     hHeap,eax
        INVOKE  HeapAlloc,eax,NULL,<number of bytes>
        mov     hBlock,eax

;hBlock is the address of your memory block
;when you are done using the block, free it...

        INVOKE  HeapFree,hHeap,NULL,hBlock