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;
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;
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）
/* 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();
}
}
size_t xPortGetFreeHeapSize( void )
{
return xFreeBytesRemaining;
}
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。
以上就是FreeRTOS动态内存分配管理heap_2示例的详细内容，更多关于FreeRTOS动态内存管理heap_2的资料请关注其它相关文章！
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