Arrays inside structs in C

Question

Hello everyone.

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;

Answer 3

The interace that you are using is using the struct hack. This means that you need to manually dynamically allocate enough storage for the structure as if it was declared with region being an array of more than 1 FlashRegion.

For this interface, it needs to be enough space for at least and array of dwNumRegions FlashRegions.

Something like offsetof(FlashInfoEx, region) + sizeof FlashRegion * n bytes where n is a number that you need to subsequently pass to FMD_GetInfoEx or whatever function you are using.

Answer 4

...see this answer on legacy design patterns using a simple example in a previous question.