modifying a function parameter (a pointer) from within the application

Question

This is a question based on C code for Win32. In one of my functions I have the following code:

void SeparateStuff()
{
    HGLOBAL hMem;
    IStream* pStream;
    Status result;
    unsigned char* pBytes;
    DWORD size;
    FILE* fp;

    if (hMem = GlobalAlloc(GMEM_MOVEABLE, 0))
    {

     if (CreateStreamOnHGlobal(hMem, FALSE, &pStream) == S_OK)
     {
  // save the buffer to a file 
  result = Scramble(pStream);  
  pStream->Release();

  if (result == Ok)
  { 
   if (pBytes = (unsigned char *) GlobalLock(hMem))
   {
                                size = GlobalSize(hMem);
                                // pBytes has the buffer now
                                // in this case write it to a file
                                if(fp = fopen("c:\\SomeFile.bin", "wb"))
                                {
                                    fwrite(pBytes, size, 1, fp);
                                    fclose(fp);
                                }
       }
      }

     }
     else
     {
      printf("error CreateStreamOnHGlobal. Last err: %d\n",  GetLastError());
      return;
     }
    }
    else
    {
     printf("error GlobalAlloc. Last err: %d\n", GetLastError());
     return;
    }
}

In this case the unsigned char* buffer is being saved to a file. Everything is fine. Now, I need to modify this code so the unsigned char* is returned. Something like:

void SeparateStuff(unsigned char* ReturnedBytes)

Where ReturnedBytes will be modified inside the function. I tried copying the pointer, i tried copying the data inside the pointer. I tries a lot of different approaches. It doesn't matter what I do I either get garbage in the calling function or an exception. The calling function (the main() for example) has to pass to SeparateStuff() also an unsigned char*.

How do I do that?

thanks!

Answer 1

You can use one of the following syntaxes:

unsigned char* SeparateStuff(); //Returns a pointer
void SeparateStuff(unsigned char** ppBuffer); //Pointer to the output buffer

Or in C++:

void SeparateStuff(unsigned char*& pBuffer) //Reference to the output buffer

Answer 2

SeparateStuff(&ReturnedBytes);

void SeparateStuff(unsigned char** ReturnedBytes)
{
    ...
}

Answer 3

Try this:

void SeparateStuff(unsigned char **handleToBuffer)
{
    ...

    if (*handleToBuffer = (unsigned char *) GlobalLock(hMem))
    {
        ...
    }
    else
    {
        return;
    }
    ...
}

Please make sure that all arguments are properly initialized. Also you have to guarantee that what GlobalLock returns stays there.