FreeRTOS动态内存分配管理heap_2示例

目录

• heap_2.c
• 内存堆管理
• 分配
• 初始化内存堆
• 把新构造的结构体插入空闲链表
• 释放
• 还剩空闲字节数
• 适用范围、特点

heap_2.c

内存堆管理

heap_2和heap_1一样是开辟一个大数组作为堆空间供用户使用，但是采用单项不循环链表来管理内存的分配释放，主要思想是用链表把内存块串起来，数据结构如下

/* Define the linked list structure.  This is used to link free blocks in order
of their size. */
typedef struct A_BLOCK_LINK
{
   //指向下一个空闲内存块管理结构体
	struct A_BLOCK_LINK *pxNextFreeBlock;	/*<< The next free block in the list. */
	//记录申请的字节数，包括链表占用所占字节数
	size_t xBlockSize;						/*<< The size of the free block. */
} BlockLink_t;

与引入链表管理而带来的相关变量如下

//链表结构体对齐后所占字节数
static const uint16_t heapSTRUCT_SIZE	= ( ( sizeof ( BlockLink_t ) + ( portBYTE_ALIGNMENT - 1 ) ) & ~portBYTE_ALIGNMENT_MASK );
//2倍链表结构体对齐后所占字节数，这作为一个阈值，在分配时起作用
#define heapMINIMUM_BLOCK_SIZE	( ( size_t ) ( heapSTRUCT_SIZE * 2 ) )
/* Create a couple of list links to mark the start and end of the list. */
//定义2个局部静态全局结构体变量用于管理
static BlockLink_t xStart, xEnd;

还剩空闲字节数

/* Keeps track of the number of free bytes remaining, but says nothing about
fragmentation. */
static size_t xFreeBytesRemaining = configADJUSTED_HEAP_SIZE;

分配

void *pvPortMalloc( size_t xWantedSize )
{
BlockLink_t *pxBlock, *pxPreviousBlock, *pxNewBlockLink;
static BaseType_t xHeapHasBeenInitialised = pdFALSE;
void *pvReturn = NULL;
//挂起调度器，防止函数重入
	vTaskSuspendAll();
	{
		/* If this is the first call to malloc then the heap will require
		initialisation to setup the list of free blocks. */
		//第一次调用会初始化内存堆
		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 )
		{
		    //用户分配字节数+管理结构体占用字节数
			xWantedSize += heapSTRUCT_SIZE;

			/* Ensure that blocks are always aligned to the required number of bytes. */
			//总的字节数再做此字节对齐
			if( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) != 0 )
			{
				/* Byte alignment required. */
				xWantedSize += ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) );
			}
		}
        //待分配字节数大于0且小于总共堆字节数
		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指向头链表
			pxPreviousBlock = &xStart;
			//pxBlock指向第一个开始空闲块
			pxBlock = xStart.pxNextFreeBlock;
			//当pxBlock所管理的空闲块字节数小于待分配的
			//且没有遍历到空闲块管理链表尾部则一直遍历
			while( ( pxBlock->xBlockSize < xWantedSize ) && ( pxBlock->pxNextFreeBlock != NULL ) )
			{
				//pxPreviousBlock这里是保存当前空闲块管理结构体,为了后面找到返回的内存地址
				pxPreviousBlock = pxBlock;
				//指向下一个空闲块管理结构体
				pxBlock = pxBlock->pxNextFreeBlock;
			}
			/* If we found the end marker then a block of adequate size was not found. */
			//pxBlock不等于结尾说明找到符合大小的空闲块
			if( pxBlock != &xEnd )
			{
				/* Return the memory space - jumping over the BlockLink_t structure
				at its start. */
				//pvReturn用作返回给用户，这里要偏移一个空闲块管理结构体占用内存大小
				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. */
				//因为pxPreviousBlock->pxNextFreeBlock指向的空闲块被分配了，
				//所以要把pxPreviousBlock->pxNextFreeBlock指向的空闲块移除出去，
				//也就是pxPreviousBlock->pxNextFreeBlock指向pxBlock->pxNextFreeBlock
				//也就是跳过分配出去的那个块
				pxPreviousBlock->pxNextFreeBlock = pxBlock->pxNextFreeBlock;

				/* If the block is larger than required it can be split into two. */
				//这里判断，
				//如果将要分配出去的内存块大小xBlockSize比分配出去的还要大heapMINIMUM_BLOCK_SIZE(2倍管理结构体)
				//为了节约就把再分成2块，一块返回给用户，
				//一块构造一个新的空闲管理结构体后插入空闲链表
				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. */
					//注意这里xWantedSize是管理结构体和和真正需要字节数之和
					//所以是在pxBlock基础上偏移xWantedSize作为新的管理结构体
					pxNewBlockLink = ( void * ) ( ( ( uint8_t * ) pxBlock ) + xWantedSize );

					/* Calculate the sizes of two blocks split from the single
					block. */
					//pxNewBlockLink新的管理结构体大小
					//是待分配pxBlock->xBlockSize-xWantedSize
					pxNewBlockLink->xBlockSize = pxBlock->xBlockSize - xWantedSize;
					//更新pxBlock->xBlockSize大小为xWantedSize
					pxBlock->xBlockSize = xWantedSize;

					/* Insert the new block into the list of free blocks. */
					//把新构造的空闲管理结构体按xBlockSize大小升序插入到空闲链表
					prvInsertBlockIntoFreeList( ( pxNewBlockLink ) );
				}
                //还剩空闲字节数要减去分配出去的
				xFreeBytesRemaining -= pxBlock->xBlockSize;
			}
		}

		traceMALLOC( pvReturn, xWantedSize );
	}//解挂调度器
	( void ) xTaskResumeAll();
//如果定义了申请失败钩子函数，这里将执行
	#if( configUSE_MALLOC_FAILED_HOOK == 1 )
	{
		if( pvReturn == NULL )
		{
			extern void vApplicationMallocFailedHook( void );
			vApplicationMallocFailedHook();
		}
	}
	#endif
//返回给用户
	return pvReturn;
}

其中xFreeBytesRemaining初始化如下

/* Keeps track of the number of free bytes remaining, but says nothing about
fragmentation. */
static size_t xFreeBytesRemaining = configADJUSTED_HEAP_SIZE;

初始化内存堆

static void prvHeapInit( void )
{
BlockLink_t *pxFirstFreeBlock;
uint8_t *pucAlignedHeap;
	/* Ensure the heap starts on a correctly aligned boundary. */
    //与heap1操作相同，确保portBYTE_ALIGNMENT字节对齐，实际使用的首址是pucAlignedHeap
	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. */
	//空闲链表结构体头部初始化,pxNextFreeBlock指向实际使用的首址pucAlignedHeap
	xStart.pxNextFreeBlock = ( void * ) pucAlignedHeap;
	//空闲链表结构体头部没有可用内存，所以xBlockSize是0
	xStart.xBlockSize = ( size_t ) 0;
	/* xEnd is used to mark the end of the list of free blocks. */
	//空闲链表结构体尾部初始化,xBlockSize=configADJUSTED_HEAP_SIZE仅仅是为了后面的升序排列,不代表可以空闲字节数
	xEnd.xBlockSize = configADJUSTED_HEAP_SIZE;
	//空闲链表结构体尾部初始化,pxNextFreeBlock指向NULL表示结尾
	xEnd.pxNextFreeBlock = NULL;
	/* To start with there is a single free block that is sized to take up the
	entire heap space. */
	//第一个空闲块,pxFirstFreeBlock,即上面xStart指向的pucAlignedHeap
	pxFirstFreeBlock = ( void * ) pucAlignedHeap;
	//可以空闲内存为configADJUSTED_HEAP_SIZE
	pxFirstFreeBlock->xBlockSize = configADJUSTED_HEAP_SIZE;
	//指向空闲链表结构体尾部
	pxFirstFreeBlock->pxNextFreeBlock = &xEnd;
}

初始化后的示意图如下
这里注意xBlockSize是包括管理结构体占用内存大小的（出来xStart和xEnd之外，这2个做排序用）

把新构造的结构体插入空闲链表

/* STATIC FUNCTIONS ARE DEFINED AS MACROS TO MINIMIZE THE FUNCTION CALL DEPTH. */
/*
 * 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;																	\																					\
    //这里获得新构造的空闲结构体成员xBlockSize大小等下用于升序插入
	xBlockSize = pxBlockToInsert->xBlockSize;										\																					\
	/* Iterate through the list until a block is found that has a larger size */	\
	/* than the block we are inserting. */											\
	//从头开始找到要插入的位置
	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. */																	\
	//插入
	pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock;					\
	pxIterator->pxNextFreeBlock = pxBlockToInsert;									\
}

释放

释放就很简单了，就是偏移下地址后直接插入空闲链表

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. */
		//偏移回地址
		puc -= heapSTRUCT_SIZE;
		/* This unexpected casting is to keep some compilers from issuing
		byte alignment warnings. */
		pxLink = ( void * ) puc;
       //挂起调度器
		vTaskSuspendAll();
		{
			/* Add this block to the list of free blocks. */
		    //插入空闲链表
			prvInsertBlockIntoFreeList( ( ( BlockLink_t * ) pxLink ) );
			//剩余空闲内存增加
			xFreeBytesRemaining += pxLink->xBlockSize;
			traceFREE( pv, pxLink->xBlockSize );
		}//解挂调度器
		( void ) xTaskResumeAll();
	}
}

还剩空闲字节数

size_t xPortGetFreeHeapSize( void )
{
	return xFreeBytesRemaining;
}

适用范围、特点

适用于需要释放的场合，且每次申请释放的内存都是固定大小的，因为释放时不会合并相邻空闲内存块，所以如果每次申请释放都是随机的，到最后即使剩余内存大于要想要分配，由于有很多小的内存碎片导致最终分配失败。

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