Arrays inside structs in C

Question

I´m struggling to understand this concept: I have a fixed size definition:

(from http://msdn.microsoft.com/pt-br/library/aa931918.aspx)

typedef struct _FlashRegion {
  REGION_TYPE regionType;
  DWORD dwStartPhysBlock;
  DWORD dwNumPhysBlocks;
  DWORD dwNumLogicalBlocks;
  DWORD dwSectorsPerBlock;
  DWORD dwBytesPerBlock;
  DWORD dwCompactBlocks;
} FlashRegion, *PFlashRegion;

this FlashRegion struct, is used in this another struct: (from: http://msdn.microsoft.com/pt-br/library/aa932688.aspx)

typedef struct _FlashInfoEx {
  DWORD cbSize;
  FLASH_TYPEflashType;
  DWORD dwNumBlocks;
  WORD dwDataBytesPerSector;
   DWORD dwNumRegions;
  FlashRegion region[1]; 
} FlashInfoEx, *PFlashInfoEx;

The problem is, I can have a variable number of FlashRegions inside a FlashInfoEx. The function that I´m debugging does this somewhere in the code:

 memcpy (pFlashInfoEx->region,  g_pStorageDesc->pRegionTable,
         g_pStorageDesc->dwNumRegions *  sizeof(FlashRegion));

That means that it copies an amount of regions to pFlashInfoEx (that I pass in the call of the function);

So, the code will overwrite memory if dwNumRegions is bigger than one. If that is the case, Should I create a FlashRegion [FIXED_SIZE] in my code and somehow place/overwrite in FlashInfoEx->region? How do I do that?

Thanks, Marcelo

Answer 1

The concept is while FlashRegion looks like a fixed size structure, it is actually dynamically sized. The magic is done when allocating the structure - instead of calling (FlashRegion*)malloc(sizeof(FlashInfoEx)) or new FlashRegion, you call something like (FlashRegion*)malloc(sizeof(FlashInfoEx)+sizeof(FlashRegion)*(numRegions-1))

Answer 2

This is a fairly common C idiom for variably sized structures. In order to make this work, you need to ensure you allocate enough memory for the array size you want, usually something like

pFlashInfoEx = malloc(offsetof(PFlashInfoEx, region) + g_pStorageDesc->dwNumRegions *  sizeof(FlashRegion));

this interacts badly with trying to use 'new' in C++; you have to allocate the memory manually:

void *mem = ::operator new(offsetof(PFlashInfoEx, region) + g_pStorageDesc->dwNumRegions *  sizeof(FlashRegion));
pFlashInfoEx = new(mem) PFlashInfoEx;
for (int i = 1; i < g_pStorageDesc->dwNumRegions; i++)
    new (&pFlashInfoEx->region[i]) FlashRegion;