Why isn't it just "Block", why the extra "+ 1"? Isn't Block the thing you want to return?
Here is some of the relevant the code from handmade_asset.cpp:
Here is some of the relevant the code from handmade_asset.cpp:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | internal void *
AcquireAssetMemory(game_assets *Assets, memory_index Size)
{
void *Result = 0;
for(;;)
{
asset_memory_block *Block = FindBlockForSize(Assets, Size);
if(Block)
{
Block->Flags |= AssetMemory_Used;
Assert(Size <= Block->Size);
Result = (u8 *)(Block + 1);
memory_index RemainingSize = Block->Size - Size;
memory_index BlockSplitThreshold = 4096; // TODO(casey): Set this based on the smallest asset?
if(RemainingSize > BlockSplitThreshold)
{
Block->Size -= RemainingSize;
InsertBlock(Block, RemainingSize, (u8 *)Result + Size);
}
else
{
// TODO(casey): Actually record the unused portion of the memory
// in a block so that we can do the merge on blocks when neighbors
// are freed.
}
break;
|
If I remember correctly then that is because asset_memory_block is stored in memroy just before the actual asset data. Here's a diagram:
Block + 1 is simply way how to get pointer to actual asset data. Doing + 1 for pointer is same as adding sizeof(type) to it's address in bytes.
1 2 3 4 5 6 | +-----------------------+-------------+ | asset_memory_block | asset data | +-----------------------+-------------+ ^ ^ | | +--- Block pointer +-- Result pointer |
Block + 1 is simply way how to get pointer to actual asset data. Doing + 1 for pointer is same as adding sizeof(type) to it's address in bytes.
"Block" points to the header struct for the block -- which we use to track the size for freeing later. The actual memory we're allocating follows directly *after* the block header. "Block + 1" increments the pointer by the size of the header, giving us a pointer to the memory itself.