目錄
FreeRTOS 中的 heap 4 內存管理，可以算是 heap 2 的增強版本，在 《FreeRTOS --（3）內存管理 heap2》中，我們可以看到，每次內存分配後都會產生一個內存塊，多次分配後，會產生很多內存碎片，在較爲複雜的場景（需要經常動態分配和釋放場景）下，幾乎是無法勝任；
所以就有了 heap 4，它相比 heap 2 來說，提供了相鄰空閒的內存塊合併的功能，一定程度上減少了內存碎片，使得釋放了的內存能夠再度合併稱爲較爲大的內存塊，以供有大內存塊的分配場景使用；
1、內存大小
和 heap 2 一樣，用於內存管理的內存大小來自於一個大數組，數組的下標就是整個需要被管理的內存的大小，這個是和具體芯片所支持的 RAM 大小相關：
configTOTAL_HEAP_SIZE
被管理的內存定義爲：
static uint8_t ucHeap[ configTOTAL_HEAP_SIZE ];
ucHeap 就是管理的對象；
這些基本的結構都是和之前的 heap2 保持一致，不再多說；
2、對齊
對齊的部分也是和 heap 2 一致，不在多說，更多的參考：《FreeRTOS --（3）內存管理 heap2》的對齊章節；
3、內存塊
內存塊的定義依然是：
typedef struct A_BLOCK_LINK
{
struct A_BLOCK_LINK *pxNextFreeBlock; /*<< The next free block in the list. */
size_t xBlockSize; /*<< The size of the free block. */
} BlockLink_t;
沒有任何差別，依然有 xStart 和 xEnd 來描述兩個 marker，和 heap 2 幾乎一樣《FreeRTOS --（3）內存管理 heap2》；這裏不再多說；
4、內存初始化
和 heap 2 一樣，內存初始化使用 prvHeapInit，在第一次調用內存分配的時候，檢查是否有初始化，否則進行 prvHeapInit 的調用，初始化相關的結構：
static void prvHeapInit( void )
{
BlockLink_t *pxFirstFreeBlock;
uint8_t *pucAlignedHeap;
size_t uxAddress;
size_t xTotalHeapSize = configTOTAL_HEAP_SIZE;
/* Ensure the heap starts on a correctly aligned boundary. */
uxAddress = ( size_t ) ucHeap;
if( ( uxAddress & portBYTE_ALIGNMENT_MASK ) != 0 )
{
uxAddress += ( portBYTE_ALIGNMENT - 1 );
uxAddress &= ~( ( size_t ) portBYTE_ALIGNMENT_MASK );
xTotalHeapSize -= uxAddress - ( size_t ) ucHeap;
}
pucAlignedHeap = ( uint8_t * ) uxAddress;
/* xStart is used to hold a pointer to the first item in the list of free
blocks. The void cast is used to prevent compiler warnings. */
xStart.pxNextFreeBlock = ( void * ) pucAlignedHeap;
xStart.xBlockSize = ( size_t ) 0;
/* pxEnd is used to mark the end of the list of free blocks and is inserted
at the end of the heap space. */
uxAddress = ( ( size_t ) pucAlignedHeap ) + xTotalHeapSize;
uxAddress -= xHeapStructSize;
uxAddress &= ~( ( size_t ) portBYTE_ALIGNMENT_MASK );
pxEnd = ( void * ) uxAddress;
pxEnd->xBlockSize = 0;
pxEnd->pxNextFreeBlock = NULL;
/* To start with there is a single free block that is sized to take up the
entire heap space, minus the space taken by pxEnd. */
pxFirstFreeBlock = ( void * ) pucAlignedHeap;
pxFirstFreeBlock->xBlockSize = uxAddress - ( size_t ) pxFirstFreeBlock;
pxFirstFreeBlock->pxNextFreeBlock = pxEnd;
/* Only one block exists - and it covers the entire usable heap space. */
xMinimumEverFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
xFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
/* Work out the position of the top bit in a size_t variable. */
xBlockAllocatedBit = ( ( size_t ) 1 ) << ( ( sizeof( size_t ) * heapBITS_PER_BYTE ) - 1 );
}
從編碼風格的角度上來說，看上去感覺和 heap 2 不是同一個人寫的（同樣的邏輯，不同的表達），不過沒關係，含義都是一樣的；
依然是初始化了 xStart 和 xEnd 兩個 marker；做了一些對齊處理後，將能夠分配的所有的空間一併掛到了 xStart->pxNextFreeBlock 鏈表；
和 heap 2 不一樣的是，heap 4 定義了一個標記，以表示內存是否有被使用，這裏定義了 xBlockAllocatedBit；如果是 32bit CPU 的話，這個 xBlockAllocatedBit = 0x01<<31；32 bit 的最高位，這顯然是安全的；
5、內存分配
內存分配，還是使用的 pvPortMalloc 接口，正確分配，返回可用的內存地址，出錯返回 NULL：
void *pvPortMalloc( size_t xWantedSize )
{
BlockLink_t *pxBlock, *pxPreviousBlock, *pxNewBlockLink;
void *pvReturn = NULL;
vTaskSuspendAll();
{
/* If this is the first call to malloc then the heap will require
initialisation to setup the list of free blocks. */
if( pxEnd == NULL )
{
prvHeapInit();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
/* Check the requested block size is not so large that the top bit is
set. The top bit of the block size member of the BlockLink_t structure
is used to determine who owns the block - the application or the
kernel, so it must be free. */
if( ( xWantedSize & xBlockAllocatedBit ) == 0 )
{
/* The wanted size is increased so it can contain a BlockLink_t
structure in addition to the requested amount of bytes. */
if( xWantedSize > 0 )
{
xWantedSize += xHeapStructSize;
/* Ensure that blocks are always aligned to the required number
of bytes. */
if( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) != 0x00 )
{
/* Byte alignment required. */
xWantedSize += ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) );
configASSERT( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) == 0 );
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
if( ( xWantedSize > 0 ) && ( xWantedSize <= xFreeBytesRemaining ) )
{
/* Traverse the list from the start (lowest address) block until
one of adequate size is found. */
pxPreviousBlock = &xStart;
pxBlock = xStart.pxNextFreeBlock;
while( ( pxBlock->xBlockSize < xWantedSize ) && ( pxBlock->pxNextFreeBlock != NULL ) )
{
pxPreviousBlock = pxBlock;
pxBlock = pxBlock->pxNextFreeBlock;
}
/* If the end marker was reached then a block of adequate size
was not found. */
if( pxBlock != pxEnd )
{
/* Return the memory space pointed to - jumping over the
BlockLink_t structure at its start. */
pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + xHeapStructSize );
/* This block is being returned for use so must be taken out
of the list of free blocks. */
pxPreviousBlock->pxNextFreeBlock = pxBlock->pxNextFreeBlock;
/* If the block is larger than required it can be split into
two. */
if( ( pxBlock->xBlockSize - xWantedSize ) > heapMINIMUM_BLOCK_SIZE )
{
/* This block is to be split into two. Create a new
block following the number of bytes requested. The void
cast is used to prevent byte alignment warnings from the
compiler. */
pxNewBlockLink = ( void * ) ( ( ( uint8_t * ) pxBlock ) + xWantedSize );
configASSERT( ( ( ( size_t ) pxNewBlockLink ) & portBYTE_ALIGNMENT_MASK ) == 0 );
/* Calculate the sizes of two blocks split from the
single block. */
pxNewBlockLink->xBlockSize = pxBlock->xBlockSize - xWantedSize;
pxBlock->xBlockSize = xWantedSize;
/* Insert the new block into the list of free blocks. */
prvInsertBlockIntoFreeList( pxNewBlockLink );
}
else
{
mtCOVERAGE_TEST_MARKER();
}
xFreeBytesRemaining -= pxBlock->xBlockSize;
if( xFreeBytesRemaining < xMinimumEverFreeBytesRemaining )
{
xMinimumEverFreeBytesRemaining = xFreeBytesRemaining;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
/* The block is being returned - it is allocated and owned
by the application and has no "next" block. */
pxBlock->xBlockSize |= xBlockAllocatedBit;
pxBlock->pxNextFreeBlock = NULL;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
traceMALLOC( pvReturn, xWantedSize );
}
( void ) xTaskResumeAll();
#if( configUSE_MALLOC_FAILED_HOOK == 1 )
{
if( pvReturn == NULL )
{
extern void vApplicationMallocFailedHook( void );
vApplicationMallocFailedHook();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif
configASSERT( ( ( ( size_t ) pvReturn ) & ( size_t ) portBYTE_ALIGNMENT_MASK ) == 0 );
return pvReturn;
}
內存分配部分的代碼和之前的 heap 2 的代碼幾乎完全一樣，這裏不做過多的解釋；
6、內存釋放
內存釋放是 vPortFree 接口：
void vPortFree( void *pv )
{
uint8_t *puc = ( uint8_t * ) pv;
BlockLink_t *pxLink;
if( pv != NULL )
{
/* The memory being freed will have an BlockLink_t structure immediately
before it. */
puc -= xHeapStructSize;
/* This casting is to keep the compiler from issuing warnings. */
pxLink = ( void * ) puc;
/* Check the block is actually allocated. */
configASSERT( ( pxLink->xBlockSize & xBlockAllocatedBit ) != 0 );
configASSERT( pxLink->pxNextFreeBlock == NULL );
if( ( pxLink->xBlockSize & xBlockAllocatedBit ) != 0 )
{
if( pxLink->pxNextFreeBlock == NULL )
{
/* The block is being returned to the heap - it is no longer
allocated. */
pxLink->xBlockSize &= ~xBlockAllocatedBit;
vTaskSuspendAll();
{
/* Add this block to the list of free blocks. */
xFreeBytesRemaining += pxLink->xBlockSize;
traceFREE( pv, pxLink->xBlockSize );
prvInsertBlockIntoFreeList( ( ( BlockLink_t * ) pxLink ) );
}
( void ) xTaskResumeAll();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
}
釋放的時候，將之前的內存通過調用 prvInsertBlockIntoFreeList 插入到 Free List 中，內存塊的合併就體現在這個函數；
6.1、合併
當釋放內存的時候調用到了 prvInsertBlockIntoFreeList，它實現了相鄰內存塊的合併工作，這也是 heap 4 與 heap 2 最大不同的地方：
static void prvInsertBlockIntoFreeList( BlockLink_t *pxBlockToInsert )
{
BlockLink_t *pxIterator;
uint8_t *puc;
/* Iterate through the list until a block is found that has a higher address
than the block being inserted. */
for( pxIterator = &xStart; pxIterator->pxNextFreeBlock < pxBlockToInsert; pxIterator = pxIterator->pxNextFreeBlock )
{
/* Nothing to do here, just iterate to the right position. */
}
/* Do the block being inserted, and the block it is being inserted after
make a contiguous block of memory? */
puc = ( uint8_t * ) pxIterator;
if( ( puc + pxIterator->xBlockSize ) == ( uint8_t * ) pxBlockToInsert )
{
pxIterator->xBlockSize += pxBlockToInsert->xBlockSize;
pxBlockToInsert = pxIterator;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
/* Do the block being inserted, and the block it is being inserted before
make a contiguous block of memory? */
puc = ( uint8_t * ) pxBlockToInsert;
if( ( puc + pxBlockToInsert->xBlockSize ) == ( uint8_t * ) pxIterator->pxNextFreeBlock )
{
if( pxIterator->pxNextFreeBlock != pxEnd )
{
/* Form one big block from the two blocks. */
pxBlockToInsert->xBlockSize += pxIterator->pxNextFreeBlock->xBlockSize;
pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock->pxNextFreeBlock;
}
else
{
pxBlockToInsert->pxNextFreeBlock = pxEnd;
}
}
else
{
pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock;
}
/* If the block being inserted plugged a gab, so was merged with the block
before and the block after, then it's pxNextFreeBlock pointer will have
already been set, and should not be set here as that would make it point
to itself. */
if( pxIterator != pxBlockToInsert )
{
pxIterator->pxNextFreeBlock = pxBlockToInsert;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
在 heap 2中，也有這個函數，但是它是按照內存塊的小到大進行排序，由於有相鄰內存塊合併的要求，所以在 heap 4 中，內存塊鏈表的組織形式，是按照他們的地址順序由小到大組織的；
首先通過 xStart 開始，獲取第一個空閒塊的地址，並比較它的下一個空閒塊地址和待插入空閒塊地址的大小，以便獲得合適的插入位置；
獲得合適的位置了以後，判斷待插入的這個空閒塊是否和前一個空閒塊相鄰，如果相鄰的話，就將兩個塊合併到一起：
/* Do the block being inserted, and the block it is being inserted after
make a contiguous block of memory? */
puc = ( uint8_t * ) pxIterator;
if( ( puc + pxIterator->xBlockSize ) == ( uint8_t * ) pxBlockToInsert )
{
pxIterator->xBlockSize += pxBlockToInsert->xBlockSize;
pxBlockToInsert = pxIterator;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
然後在判斷是否和後一個空閒塊也相鄰，如果相鄰，也將他們合併，當然這裏需要判斷是否是緊挨着 xEnd：
/* Do the block being inserted, and the block it is being inserted before
make a contiguous block of memory? */
puc = ( uint8_t * ) pxBlockToInsert;
if( ( puc + pxBlockToInsert->xBlockSize ) == ( uint8_t * ) pxIterator->pxNextFreeBlock )
{
if( pxIterator->pxNextFreeBlock != pxEnd )
{
/* Form one big block from the two blocks. */
pxBlockToInsert->xBlockSize += pxIterator->pxNextFreeBlock->xBlockSize;
pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock->pxNextFreeBlock;
}
else
{
pxBlockToInsert->pxNextFreeBlock = pxEnd;
}
}
else
{
pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock;
}
打個比方：
多次分配和釋放後，內存佈局如下所示：
如果要釋放中間那個內存，那麼就會觸發向上和向下的合併：
官方針對這個 heap 4 的官圖爲：
