Debug and HeapAlloc - c

Question

On trying to deal with heaps in WinApi I've got some strange results for HeapAlloc's actions. Lets consider the following code. The problem is that according to Microsoft Documentation on Windows API(next - Doc), I have to get two Success strings printed to the console. By I get an Error when trying to run this code with the Debud option in MSVC 2013. But the strangest thing is that when I try to run this code without Debug option, or run the compiled .exe file, I get the correct result.
#include <Windows.h>
#include <stdio.h>
int main()
{
LPSYSTEM_INFO sys;
HANDLE hNewHeap;
LPVOID ptr;
sys = (LPSYSTEM_INFO) HeapAlloc(GetProcessHeap(),
0,
sizeof(SYSTEM_INFO));
GetSystemInfo(sys);
printf("Page size: %u\n", sys->dwPageSize);//Here we get the
//'Page size: 4096' string
//printed to the console
hNewHeap = HeapCreate(0, 1, 1);
//That's easy. We create new heap object, getting its HADNLE descriptor.
//According to Doc, the initial heap size is set to page size, which is
//4096 on my computer, like maximum heap size is also done. So the heap
//size now is 4096.
ptr = HeapAlloc(hNewHeap, 0, 2624); //Here we allocate the memory
//block in our new heap, that might have 2624 bytes size.
if ( ptr ) printf("Success!\n");//Here we check if the HeapAlloc functio
//worked correctly and print the appropriate string.
else printf("Error!\n");
//On this time we get 'Success' string printed to the console and free
//allocated memory block
if ( ptr ) HeapFree(hNewHeap, 0, ptr);
ptr = HeapAlloc(hNewHeap, 0, 2525);//Here we try to allocate the memory
//block, which size is 2526. And, like previous time, we expect to get
//'Success'.
if ( ptr ) printf("Success!\n");
else printf("Error!\n");
//But we get 'Error' here!!!
if ( ptr ) HeapFree(hNewHeap, 0, ptr);
HeapDestroy(hNewHeap);
system("pause");
};
If you try the same with any less than 2624 number, you will not get an 'Error'. If you try to do this with any more that 2625 number, you WILL get an 'Error'. But we get an 'Error' only when the Debug option is on.
Can somebody explain me why is happening so?
P.S.: Sorry for bad English.
P.S.: The strange is also the number 2625 that do not correspond to any function or application size and that sometimes I get correct result, that is after restarting studio or making some changes in the code.(But only sometimes)

Answer 1

You are creating a fixed size heap. The documentation says:
The HeapCreate function rounds dwMaximumSize up to a multiple of the system page size and then reserves a block of that size in the process's virtual address space for the heap.
So your heap has a fixed size of 4096 bytes.
Subsequently your allocation from this heap fails because the heap is not large enough for the block being allocated. Again from the documentation:
The system uses memory from the private heap to store heap support structures, so not all of the specified heap size is available to the process. For example, if the HeapAlloc function requests 64 kilobytes (K) from a heap with a maximum size of 64K, the request may fail because of system overhead.
It is these heap support structures that are causing your confusion. You think that there is sufficient space in the heap for your second allocation, but you are not accounting for the heap support structures.
The documentation for HeapAlloc tells you to call GetExceptionCode on failure. Expect that to return STATUS_NO_MEMORY.