Pointer to Pointer to Void Function Parameter

Question

I have a driver that I am modifying that was written by someone else in C. Since I need to add some C++ function calls I am now building the driver with C++ compiler versus C compiler (in Visual Studio 2012). When I changed to C++ I received a lot of build errors, all of which I have been able to fix except for a few. The few errors are all related to calls to the same function. I have searched the web extensively to try to find a solution to this issue without any success. So below are the components to this issue that I hope will allow someone to help me with this issue.

First, here is the function that is being called. You may be wondering why the original S/W developer created their own custom free memory function when they could have just used the free() function in the standard C library. I do not know why this was but since it was working just fine, I hesitate to change it.

void freeMemory(void **pBuffer)
{
    BOOL result;
    ASSERT(pBuffer != NULL);

    // see if buffer is already been freed
    if (*pBuffer == NULL)
    {   
        return;
    }

    result = VirtualFree(
        *pBuffer,                   // LPVOID lpAddress,   // address of memory
        0,                          // SIZE_T dwSize,      // size of memory
        MEM_RELEASE                 // DWORD dwFreeType    // operation type
        );
    // see of we were able to successfully free the memory  
    if (result == FALSE)
    {   
        ASSERT(FALSE);
        return;
    }

    // Mark the buffer pointer as now being free
    *pBuffer = NULL;
}  

So the next piece to this puzzle is a #define as follows:

#define FREE(pBuffer) freeMemory(&(pBuffer))

Finally, below is one of the many calls to this FREE function:

FREE(Buffer);

In this example, "Buffer" is a pointer to an unsigned char.

unsigned char *Buffer;

For reference purposes, the error that I am receiving, for this particular example, is "cannot convert parameter 1 from 'unsigned char *' to 'void **'"

It's been a long time since I have done much with straight C or C++ and pointers were never my strong suite. Based on the error, my assumption was that this error is related to not providing a cast in the function call, but considering how the free() function is used from the standard C library, it doesn't seem like this should be necessary. Any help on what I need to do with respect to how the FREE function is called to eliminate these errors would be greatly appreciated.

Answer 1

The freeMemory is used to create a safe free, since you can call it twice for the same pointer with no harm. I don't like this, it will only hide your mistake, but is very common to see it around.

The cause of this error is explained here, as pointed by Quentin.

An alternative for you code is:

#define FREE(pBuffer) \
{ \
    void *tmp; \
    tmp = pBuffer; \
    freeMemory(&tmp); \
    pBuffer = tmp; \
}

Answer 2

The problem I had when running variations of your code, was with inconsistent usage of VirualFree and VirtualAlloc, i.e. in your provided code example I could see no indication of VirtualAlloc() usage, which should be used in conjunction with VirtualFree().

Also, when I tried to assign memory to a specific address using VirtualAlloc(), It always failed, so I let the system specify the address. Below is a code snippet that demonstrates allocating and freeing page memory.

EDIT This now addresses your macro question, It uses the macro definition suggestion from Jonaton (with the two extra ;).

Adapted from here: (this built and ran without error, demonstrating usage of VirtualAlloc() used in conjunction with VirtualFree())

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>             

#define PAGES 100            

LPTSTR lpNxtPage;               
DWORD dwPages = 0;              
DWORD dwPageSize;

void freeMemory(void **pBuffer);

#define FREE(pBuffer) \
{ \
    void *tmp; \
    tmp = pBuffer; \
    freeMemory(&tmp); \
    pBuffer = tmp; \
}

void main(void)
{
    LPVOID lpvBase;               
    LPTSTR lpPtr;                 
    BOOL bSuccess;                
    DWORD i;                      
    SYSTEM_INFO sSysInfo;         

    GetSystemInfo(&sSysInfo);     


    dwPageSize = sSysInfo.dwPageSize;


    lpvBase = VirtualAlloc(
                     NULL,                 
                     PAGES*dwPageSize, 
                     MEM_RESERVE,          
                     PAGE_NOACCESS);       
    if (lpvBase == NULL )//do nothing

    lpPtr = lpNxtPage = (LPTSTR) lpvBase;


   // bSuccess = VirtualFree(
   //                    lpvBase,       
   //                    0,             
   //                    MEM_RELEASE); 
    FREE(lpvBase)  //replace in-line VirtualFree with macro

    ;
}


void freeMemory(void **pBuffer)
{
    BOOL result;
    assert(pBuffer != NULL);

    // see if buffer is already been freed
    if (*pBuffer == NULL)
    {   
        return;
    }

    result = VirtualFree(
        *pBuffer,                   // LPVOID lpAddress,   // address of memory
        0,                          // SIZE_T dwSize,      // size of memory
        MEM_RELEASE                 // DWORD dwFreeType    // operation type
        );
    // see of we were able to successfully free the memory  
    if (result == FALSE)
    {   
        assert(FALSE);
        return;
    }

    // Mark the buffer pointer as now being free
    *pBuffer = NULL;
}