好记性不如铅笔头

ARM, FreeRTOS, 操作系统

FreeRTOS代码阅读笔记:heap_2.c

FreeRTOS中对于内存的管理当前一共有5种实现方式(作者当前的版本是8.2.1),均在【 \Source\portable\MemMang 】下面,这里笔记下。 

heap_2.c:

/*
 * A sample implementation of pvPortMalloc() and vPortFree() that permits
 * allocated blocks to be freed, but does not combine adjacent free blocks
 * into a single larger block (and so will fragment memory).  See heap_4.c for
 * an equivalent that does combine adjacent blocks into single larger blocks.
 *
 * See heap_1.c, heap_3.c and heap_4.c for alternative implementations, and the
 * memory management pages of http://www.FreeRTOS.org for more information.
 */
/*
一种非常简单的内存实现方式,和heap_1.c的实现差不多,内部也是一个大号的数组,不过实现了内存的回收,但是没有实现空闲内存的整合,因此
会产生内存碎片
*/
 
 
#include <stdlib.h>

/* Defining MPU_WRAPPERS_INCLUDED_FROM_API_FILE prevents task.h from redefining
all the API functions to use the MPU wrappers.  That should only be done when
task.h is included from an application file. */
#define MPU_WRAPPERS_INCLUDED_FROM_API_FILE

#include "FreeRTOS.h"
#include "task.h"

#undef MPU_WRAPPERS_INCLUDED_FROM_API_FILE

/* A few bytes might be lost to byte aligning the heap start address. */
#define configADJUSTED_HEAP_SIZE	( configTOTAL_HEAP_SIZE - portBYTE_ALIGNMENT )
//由于可能会出现内存对齐问题,数组起始地址可能会前移,因此实际的数组大小可能小于申请的大小,这里为了简单,直接向前移动了portBYTE_ALIGNMENT个字节。

/*
 * Initialises the heap structures before their first use.
 */
static void prvHeapInit( void );

//用来申请内存的大号数组
/* Allocate the memory for the heap. */
static uint8_t ucHeap[ configTOTAL_HEAP_SIZE ];

//空闲内存管理结构体,通过它来管理释放回来的内存
/* 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. */
	/* 链表形式存储,保存指向下一块空闲内存的结构体,注意这里是单向链表 */
	size_t xBlockSize;						/*<< The size of the free block. */
	/* 该块空闲内存的大小,注意这里BlockLink_t其实也是放在它对应的空余内存的头部的,但是空闲内存的大小并没有考虑放入的BlockLink_t */
} BlockLink_t;

/* 考虑到字节对齐后BlockLink_t的大小 */
static const uint16_t heapSTRUCT_SIZE	= ( ( sizeof ( BlockLink_t ) + ( portBYTE_ALIGNMENT - 1 ) ) & ~portBYTE_ALIGNMENT_MASK );
#define heapMINIMUM_BLOCK_SIZE	( ( size_t ) ( heapSTRUCT_SIZE * 2 ) )

/* BlockLink_t是以单向链表方式储存的,这里存储下链表的头和尾  */
/* Create a couple of list links to mark the start and end of the list. */
static BlockLink_t xStart, xEnd;

/* Keeps track of the number of free bytes remaining, but says nothing about
fragmentation. */
/* 当前剩余的空闲内存总大小 */
static size_t xFreeBytesRemaining = configADJUSTED_HEAP_SIZE;

/* 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.
 */
/*
工具宏,将一个BlockLink_t的指针插入到单向链表中,单向链表以空闲内存大小为序排列,内存小的在头部
*/ 
 
#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;									\
}
/*-----------------------------------------------------------*/

void *pvPortMalloc( size_t xWantedSize )
{
BlockLink_t *pxBlock, *pxPreviousBlock, *pxNewBlockLink;
static BaseType_t xHeapHasBeenInitialised = pdFALSE;
void *pvReturn = NULL;

	vTaskSuspendAll();////首先暂停当前所有执行的任务
	{
		/* If this is the first call to malloc then the heap will require
		initialisation to setup the list of free blocks. */
		if( xHeapHasBeenInitialised == pdFALSE )
		{//如果是第一次执行该函数,那么先调用下初始化函数
			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 )
		{
			/* 空余内存的头部要放一个BlockLink_t来管理,因此这里需要人为的扩充下申请的内存大小 */
			xWantedSize += heapSTRUCT_SIZE;
			
			/* 保证字节对齐 */
			/* 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. */
			//从空余内存链表的头部开始找,如果该空余内存的大小>xWantedSize,就从这块内存中抠出一部分内存返回,剩余的内存生成新的BlockLink_t插入链表中
			
			pxPreviousBlock = &xStart;
			pxBlock = xStart.pxNextFreeBlock;
			//从链表头部开始查找大小符合条件的空余内存
			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 )
			{
				/* 找到了,就把该块内存返回给用户,注意内存的头部有BlockLink_t,需要偏移掉 */
				/* Return the memory space - jumping over the BlockLink_t structure
				at its start. */
				pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + heapSTRUCT_SIZE );
				
				//把这块内存从链表中删除
				/* This block is being returned for use so must be taken out of the
				list of free blocks. */
				pxPreviousBlock->pxNextFreeBlock = pxBlock->pxNextFreeBlock;
				
				//如果剩下的内存大小符合要求,就把它放到空余内存链表中
				/* 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 ) );
				}
				//更新剩余内存总大小
				xFreeBytesRemaining -= pxBlock->xBlockSize;
			}
		}

		traceMALLOC( pvReturn, xWantedSize );
	}
	( void ) xTaskResumeAll();//恢复执行
 //如果定义了钩子函数,那么申请失败时就调用钩子函数
	#if( configUSE_MALLOC_FAILED_HOOK == 1 )
	{
		if( pvReturn == NULL )
		{
			extern void vApplicationMallocFailedHook( void );
			vApplicationMallocFailedHook();
		}
	}
	#endif

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

void vPortFree( void *pv )
{
uint8_t *puc = ( uint8_t * ) pv;
BlockLink_t *pxLink;
/*
释放很简单,把它重新放回到空余内存链表中就可以了。
*/
	if( pv != NULL )
	{
		/* The memory being freed will have an BlockLink_t structure immediately
		before it. */
		puc -= heapSTRUCT_SIZE;//这里向前偏移,重新找回BlockLink_t

		/* 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 ) );//把BlockLink_t放回链表
			xFreeBytesRemaining += pxLink->xBlockSize;//更新剩余内存总大小
			traceFREE( pv, pxLink->xBlockSize );
		}
		( void ) xTaskResumeAll();
	}
}
/*-----------------------------------------------------------*/

size_t xPortGetFreeHeapSize( void )
{
	return xFreeBytesRemaining;
}
/*-----------------------------------------------------------*/

void vPortInitialiseBlocks( void )
{
	/* This just exists to keep the linker quiet. */
}
/*-----------------------------------------------------------*/

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

    //保证pucAlignedHeap也是在按照指定内存要求对齐的,通过这里可以知道,初始化pucAlignedHeap时并不是一定等于&ucHeap[0]的,而是会根据字节对齐的要求,在&ucHeap[0]和&ucHeap[portBYTE_ALIGNMENT]之间,这就会导致ucHeap的前几个字节可能会被浪费到,这也是为什么会有一个configADJUSTED_HEAP_SIZE。
	/* Ensure the heap starts on a correctly aligned boundary. */
	pucAlignedHeap = ( uint8_t * ) ( ( ( portPOINTER_SIZE_TYPE ) &ucHeap[ portBYTE_ALIGNMENT ] ) & ( ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) ) );
	
	/* 静态变量的初始化其实就是链表的初始化,这里将所有的空余内存作为一个BlockLink_t,放入空闲列表中 */
	/* 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. */
	/*初始化链表的头部 */
	/* 个人感觉这里的代码写的不好,应该先对pxFirstFreeBlock赋值,然后将xStart.pxNextFreeBlock = pxFirstFreeBlock 就更好了 */
	xStart.pxNextFreeBlock = ( void * ) pucAlignedHeap;
	xStart.xBlockSize = ( size_t ) 0;
	
	/*初始化链表的尾部 */
	/* xEnd is used to mark the end of the list of free blocks. */
	xEnd.xBlockSize = configADJUSTED_HEAP_SIZE;
	xEnd.pxNextFreeBlock = NULL;

	/* 将pxFirstFreeBlock 放入空闲链表中 */
	/* 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;//注意这里的空余内存大小标记为configADJUSTED_HEAP_SIZE,并没有减去BlockLink_t的大小,因此在后面的内存申请中又人为的加了一个heapSTRUCT_SIZE。
	pxFirstFreeBlock->pxNextFreeBlock = &xEnd;
}
/*-----------------------------------------------------------*/

 

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