FreeRTOS-V10 源码分析——内存管理 (heap 1~5)

上篇：《FreeRTOS-V10 源码分析——队列(queue)》

前言

内存管理可以使用标准的C库malloc（）和free（）函数分配内存，但由于以下一个或多个原因，它们可能不合适或不合适：

• 它们并不总是在小型嵌入式系统上可用。
• 它们的实现可以相对较大，占用宝贵的代码空间。
• 它们很少是线程安全的。
• 它们不是确定性的；执行函数所需的时间将因调用而异。
• 可能会受到碎片的困扰。
• 它们会使链接器配置复杂化。
• 如果允许堆空间增长到其他变量使用的内存中，它们可能是难以调试的错误的根源

针对嵌入式系统FreeRTOS有自己的多种内存管理方式可以供选择： heap_1.c, heap_2.c, heap_3.c, heap_4.c and heap_5.c。当FreeRTOS需要RAM而不是调用malloc（）时，它调用pvPortMalloc（）。当RAM被释放时，内核调用vPortFree（），而不是调用free（）。pvPortMalloc（）与标准C库malloc（）函数具有相同的原型，vPortFree（）与标准C库free（）函数具有相同的原型。pvPortMalloc（）和vPortFree（）是公共函数，因此也可以从应用程序代码中调用。

一般常用的是heap4。

请对照源码一起查看。

FreeRTOS 官网以及源码下载： https://www.freertos.org/

1. heap1

概述

Heap_1.c实现了pvPortMalloc（）的一个非常基本的版本，并且没有实现vPortFree（）。从不删除任务或其他内核对象的应用程序有可能使用heap1。

源码分析

• pvPortMalloc

void *pvPortMalloc( size_t xWantedSize )
{
void *pvReturn = NULL;
static uint8_t *pucAlignedHeap = NULL;

	/* Ensure that blocks are always aligned to the required number of bytes. */
	#if( portBYTE_ALIGNMENT != 1 )
	{
		if( xWantedSize & portBYTE_ALIGNMENT_MASK )
		{
			/* Byte alignment required. */
			xWantedSize += ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) );
		}
	}
	#endif

	vTaskSuspendAll();
	{
		if( pucAlignedHeap == NULL )
		{
			/* Ensure the heap starts on a correctly aligned boundary. */
			pucAlignedHeap = ( uint8_t * ) ( ( ( portPOINTER_SIZE_TYPE ) &ucHeap[ portBYTE_ALIGNMENT ] ) & ( ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) ) );
		}

		/* Check there is enough room left for the allocation. */
		if( ( ( xNextFreeByte + xWantedSize ) < configADJUSTED_HEAP_SIZE ) &&
			( ( xNextFreeByte + xWantedSize ) > xNextFreeByte )	)/* Check for overflow. */
		{
			/* Return the next free byte then increment the index past this
			block. */
			pvReturn = pucAlignedHeap + xNextFreeByte;
			xNextFreeByte += xWantedSize;
		}

		traceMALLOC( pvReturn, xWantedSize );
	}
	( void ) xTaskResumeAll();

	#if( configUSE_MALLOC_FAILED_HOOK == 1 )
	{
		if( pvReturn == NULL )
		{
			extern void vApplicationMallocFailedHook( void );
			vApplicationMallocFailedHook();
		}
	}
	#endif

	return pvReturn;
}

先检查对应硬件是否需要字对齐，然后挂起所有任务，若空间足够则返回指向申请区域的指针；若申请失败则调用钩子函数（有定义configUSE_MALLOC_FAILED_HOOK = 1）

• vPortFree

void vPortFree( void *pv )
{
	/* Memory cannot be freed using this scheme.  See heap_2.c, heap_3.c and
	heap_4.c for alternative implementations, and the memory management pages of
	http://www.FreeRTOS.org for more information. */
	( void ) pv;

	/* Force an assert as it is invalid to call this function. */
	configASSERT( pv == NULL );
}

无操作

2. heap2

概述

Heap_2.c还可以通过细分由configTOTAL_Heap_SIZE确定维数的数组来工作。它使用最佳匹配算法来分配内存，与heap_1不同，它允许释放内存。同样，数组是静态声明的，因此即使在分配了数组中的任何内存之前，应用程序也会消耗大量RAM。最佳匹配算法确保pvPortMalloc（）使用的内存块大小与请求的字节数最接近。该管理方式不会合并相邻的空闲内存。

源码分析

• pvPortMalloc

void *pvPortMalloc( size_t xWantedSize )
{
BlockLink_t *pxBlock, *pxPreviousBlock, *pxNewBlockLink;
static BaseType_t xHeapHasBeenInitialised = pdFALSE;
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( xHeapHasBeenInitialised == pdFALSE )
		{
			prvHeapInit();
			xHeapHasBeenInitialised = pdTRUE;
		}

		/* 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 += heapSTRUCT_SIZE;

			/* Ensure that blocks are always aligned to the required number of bytes. */
			if( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) != 0 )
			{
				/* Byte alignment required. */
				xWantedSize += ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) );
			}
		}

		if( ( xWantedSize > 0 ) && ( xWantedSize < configADJUSTED_HEAP_SIZE ) )
		{
			/* Blocks are stored in byte order - traverse the list from the start
			(smallest) 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 we found the end marker then a block of adequate size was not found. */
			if( pxBlock != &xEnd )
			{
				/* Return the memory space - jumping over the BlockLink_t structure
				at its start. */
				pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + heapSTRUCT_SIZE );

				/* 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 );

					/* 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 ) );
				}

				xFreeBytesRemaining -= pxBlock->xBlockSize;
			}
		}

		traceMALLOC( pvReturn, xWantedSize );
	}
	( void ) xTaskResumeAll();

	#if( configUSE_MALLOC_FAILED_HOOK == 1 )
	{
		if( pvReturn == NULL )
		{
			extern void vApplicationMallocFailedHook( void );
			vApplicationMallocFailedHook();
		}
	}
	#endif

	return pvReturn;
}

/*-----------------------------------------------------------*/

static const uint16_t heapSTRUCT_SIZE	= ( ( sizeof ( BlockLink_t ) + ( portBYTE_ALIGNMENT - 1 ) ) & ~portBYTE_ALIGNMENT_MASK );

/* Define the linked list structure.  This is used to link free blocks in order
of their size. */
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;

/*-----------------------------------------------------------*/
static uint8_t ucHeap[ configTOTAL_HEAP_SIZE ];

static void prvHeapInit( void )
{
BlockLink_t *pxFirstFreeBlock;
uint8_t *pucAlignedHeap;

	/* Ensure the heap starts on a correctly aligned boundary. */
	pucAlignedHeap = ( uint8_t * ) ( ( ( portPOINTER_SIZE_TYPE ) &ucHeap[ portBYTE_ALIGNMENT ] ) & ( ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) ) );

	/* 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;

	/* xEnd is used to mark the end of the list of free blocks. */
	xEnd.xBlockSize = configADJUSTED_HEAP_SIZE;
	xEnd.pxNextFreeBlock = NULL;

	/* To start with there is a single free block that is sized to take up the
	entire heap space. */
	pxFirstFreeBlock = ( void * ) pucAlignedHeap;
	pxFirstFreeBlock->xBlockSize = configADJUSTED_HEAP_SIZE;
	pxFirstFreeBlock->pxNextFreeBlock = &xEnd;
}

挂起所有任务，申请的内存加上内存块头信息，进行内存对齐。内存块头信息包含了下一块内存区域的头信息指针*pxNextFreeBlock和该指针的指向区域空白空间大小xBlockSize。然后从头开始遍历所有空白内存块(空白内存按照空白大小升序排列，在vPortFree的时候加入链表时排序)，直至找到大小大于申请内存大小或者到达末端；找到适合的空白内存块后，将该快内存切割成两块（一块是申请大小，另一块是空白大小减去申请大小），然后返回申请内存指针，若为空调用内存申请失败钩子函数（有定义configUSE_MALLOC_FAILED_HOOK = 1）。

这里进行第一次申请内存会对堆进行一次初始化，对ucHeap[ configTOTAL_HEAP_SIZE ]进行链表的初始化。configTOTAL_HEAP_SIZE在FreeRTOSConfig.h中定义。

• vPortFree

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 -= heapSTRUCT_SIZE;

		/* This unexpected casting is to keep some compilers from issuing
		byte alignment warnings. */
		pxLink = ( void * ) puc;

		vTaskSuspendAll();
		{
			/* Add this block to the list of free blocks. */
			prvInsertBlockIntoFreeList( ( ( BlockLink_t * ) pxLink ) );
			xFreeBytesRemaining += pxLink->xBlockSize;
			traceFREE( pv, pxLink->xBlockSize );
		}
		( void ) xTaskResumeAll();
	}
}

/*
 * Insert a block into the list of free blocks - which is ordered by size of
 * the block.  Small blocks at the start of the list and large blocks at the end
 * of the list.
 */
#define prvInsertBlockIntoFreeList( pxBlockToInsert )								\
{																					\
BlockLink_t *pxIterator;															\
size_t xBlockSize;																	\
																					\
	xBlockSize = pxBlockToInsert->xBlockSize;										\
																					\
	/* Iterate through the list until a block is found that has a larger size */	\
	/* than the block we are inserting. */											\
	for( pxIterator = &xStart; pxIterator->pxNextFreeBlock->xBlockSize < xBlockSize; pxIterator = pxIterator->pxNextFreeBlock )	\
	{																				\
		/* There is nothing to do here - just iterate to the correct position. */	\
	}																				\
																					\
	/* Update the list to include the block being inserted in the correct */		\
	/* position. */																	\
	pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock;					\
	pxIterator->pxNextFreeBlock = pxBlockToInsert;									\
}

根据指针，找到指针前的内存头信息(puc -= heapSTRUCT_SIZE;)，将内存块按照内存大小升序加入到空白内存的链表。

3. heap3

void *pvPortMalloc( size_t xWantedSize )
{
void *pvReturn;

	vTaskSuspendAll();
	{
		pvReturn = malloc( xWantedSize );
		traceMALLOC( pvReturn, xWantedSize );
	}
	( void ) xTaskResumeAll();

	#if( configUSE_MALLOC_FAILED_HOOK == 1 )
	{
		if( pvReturn == NULL )
		{
			extern void vApplicationMallocFailedHook( void );
			vApplicationMallocFailedHook();
		}
	}
	#endif

	return pvReturn;
}
/*-----------------------------------------------------------*/

void vPortFree( void *pv )
{
	if( pv )
	{
		vTaskSuspendAll();
		{
			free( pv );
			traceFREE( pv, 0 );
		}
		( void ) xTaskResumeAll();
	}
}

就是调用库函数malloc和free

4. heap4

概述

heap4使用第一个匹配算法分配内存。与heap_2不同，heap_4将相邻的空闲内存块合并成一个较大的块，从而将内存碎片的风险降至最低。first fit算法确保pvPortMalloc（）使用第一个足够大的空闲内存块来保存请求的字节数。

源码分析

• pvPortMalloc

在申请内存上和heap2是一致的，但是是按照内存地址去检索下一个空白内存块，就不再赘述。

• vPortFree

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();
		}
	}
}

/*-----------------------------------------------------------*/

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();
	}
}

在释放内存的时候主要的区别在于prvInsertBlockIntoFreeList，按照内存地址升序加入到空白内存链表时，会进行一次前后内存块是否相邻的检查，相邻的话就进行合并。

5. heap5

概述

heap5用来分配和释放内存的算法与heap4使用的算法相同。与heap_4不同，heap_5不限于从单个静态声明的数组分配内存；heap_5可以从多个独立的内存空间分配内存。heap5是当运行FreeRTOS的系统所提供的RAM在系统的内存映射中不显示为单个连续（没有空间）块时非常有用。在使用前必须要先调用vPortDefineHeapRegions()进行内存块区域的定义。

源码分析

• vPortDefineHeapRegions

void vPortDefineHeapRegions( const HeapRegion_t * const pxHeapRegions )
{
BlockLink_t *pxFirstFreeBlockInRegion = NULL, *pxPreviousFreeBlock;
size_t xAlignedHeap;
size_t xTotalRegionSize, xTotalHeapSize = 0;
BaseType_t xDefinedRegions = 0;
size_t xAddress;
const HeapRegion_t *pxHeapRegion;

	/* Can only call once! */
	configASSERT( pxEnd == NULL );

	pxHeapRegion = &( pxHeapRegions[ xDefinedRegions ] );

	while( pxHeapRegion->xSizeInBytes > 0 )
	{
		xTotalRegionSize = pxHeapRegion->xSizeInBytes;

		/* Ensure the heap region starts on a correctly aligned boundary. */
		xAddress = ( size_t ) pxHeapRegion->pucStartAddress;
		if( ( xAddress & portBYTE_ALIGNMENT_MASK ) != 0 )
		{
			xAddress += ( portBYTE_ALIGNMENT - 1 );
			xAddress &= ~portBYTE_ALIGNMENT_MASK;

			/* Adjust the size for the bytes lost to alignment. */
			xTotalRegionSize -= xAddress - ( size_t ) pxHeapRegion->pucStartAddress;
		}

		xAlignedHeap = xAddress;

		/* Set xStart if it has not already been set. */
		if( xDefinedRegions == 0 )
		{
			/* 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 = ( BlockLink_t * ) xAlignedHeap;
			xStart.xBlockSize = ( size_t ) 0;
		}
		else
		{
			/* Should only get here if one region has already been added to the
			heap. */
			configASSERT( pxEnd != NULL );

			/* Check blocks are passed in with increasing start addresses. */
			configASSERT( xAddress > ( size_t ) pxEnd );
		}

		/* Remember the location of the end marker in the previous region, if
		any. */
		pxPreviousFreeBlock = pxEnd;

		/* pxEnd is used to mark the end of the list of free blocks and is
		inserted at the end of the region space. */
		xAddress = xAlignedHeap + xTotalRegionSize;
		xAddress -= xHeapStructSize;
		xAddress &= ~portBYTE_ALIGNMENT_MASK;
		pxEnd = ( BlockLink_t * ) xAddress;
		pxEnd->xBlockSize = 0;
		pxEnd->pxNextFreeBlock = NULL;

		/* To start with there is a single free block in this region that is
		sized to take up the entire heap region minus the space taken by the
		free block structure. */
		pxFirstFreeBlockInRegion = ( BlockLink_t * ) xAlignedHeap;
		pxFirstFreeBlockInRegion->xBlockSize = xAddress - ( size_t ) pxFirstFreeBlockInRegion;
		pxFirstFreeBlockInRegion->pxNextFreeBlock = pxEnd;

		/* If this is not the first region that makes up the entire heap space
		then link the previous region to this region. */
		if( pxPreviousFreeBlock != NULL )
		{
			pxPreviousFreeBlock->pxNextFreeBlock = pxFirstFreeBlockInRegion;
		}

		xTotalHeapSize += pxFirstFreeBlockInRegion->xBlockSize;

		/* Move onto the next HeapRegion_t structure. */
		xDefinedRegions++;
		pxHeapRegion = &( pxHeapRegions[ xDefinedRegions ] );
	}

	xMinimumEverFreeBytesRemaining = xTotalHeapSize;
	xFreeBytesRemaining = xTotalHeapSize;

	/* Check something was actually defined before it is accessed. */
	configASSERT( xTotalHeapSize );

	/* Work out the position of the top bit in a size_t variable. */
	xBlockAllocatedBit = ( ( size_t ) 1 ) << ( ( sizeof( size_t ) * heapBITS_PER_BYTE ) - 1 );
}

这里的初始化时将，各个分离的内存块通过链表连接到一起才能使用全部内存块。之前由于是单一的的内存块，直接通过编译器分配即可。

• pvPortMalloc

和heap4一致；

• vPortFree

和heap4一致；