深入FreeRTOS内存管理：揭秘高效内存分配的秘密(源码分析)

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layout: post
title: “内存管理”
date: 2023-11-22 15:39:08 +0800
tags: FreeRTOS

---

内存管理

这一部分的使用以及各个文件的不同可以参考我的这一篇文章的有关内存的部分, 本篇的主要是实际的实现的分析

heap_1

static size_txNextFreeByte = ( size_t) 0;
static uint8_t*pucAlignedHeap = NULL;

xNextFreeByte用来定位下一个空闲的内存堆位置。真正的运作过程是记录已经被分配的内存大小，在每次申请内存成功后，都会增加申请内存的字节数目

pucAlignedHeap是一个指向对齐后的内存堆起始地址，我们使用一个数组作为堆内存，但是数组的起始地址并不一定是对齐的内存地址，所以我们需要得到FreeRTOS管理的内存空间对齐后的起始地址，并且保存在静态变量pucAlignedHeap中

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

    /* Ensure that blocks are always aligned. */
    #if ( portBYTE_ALIGNMENT != 1 )
    {
        if( xWantedSize & portBYTE_ALIGNMENT_MASK )
        {
            /* Byte alignment required. Check for overflow. */
            if( ( xWantedSize + ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) ) ) > xWantedSize )
            {
                //获取一个八字节对齐的大小
                xWantedSize += ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) );
            }
            else
            {
                xWantedSize = 0;
            }
        }
    }
    #endif /* if ( portBYTE_ALIGNMENT != 1 ) */

    vTaskSuspendAll();
    {
        if( pucAlignedHeap == NULL )
        {
            //第一次使用需要确定这一个的起始位置是对齐的
            /* Ensure the heap starts on a correctly aligned boundary. */
            pucAlignedHeap = ( uint8_t * ) ( ( ( portPOINTER_SIZE_TYPE ) & ucHeap[ portBYTE_ALIGNMENT - 1 ] ) & ( ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) ) );
        }
		//检测一下边界
        /* Check there is enough room left for the allocation and. */
        if( ( xWantedSize > 0 ) &&                                /* valid size */
            ( ( 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();

    return pvReturn;
}

void vPortInitialiseBlocks( void )
{
    /* Only required when static memory is not cleared. */
    xNextFreeByte = ( size_t ) 0;
}

size_t xPortGetFreeHeapSize( void )
{
    return( configADJUSTED_HEAP_SIZE - xNextFreeByte );
}

heap_2

采用一种最佳匹配算法, 会返回一块内存大小最接近的碎片的起始地址

它不能把相邻的两个小的内存块合成一个大的内存块，对于每次申请内存大小都比较固定的，这个方式是没有问题的

对于每次申请并不是固定内存大小的则会造成内存碎片

采用链表的数据结构记录空闲内存块，将所有的空闲内存块组成一个空闲内存块链表

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

    vTaskSuspendAll();
    {
        /* 第一次进来进行初始化 */
        if( xHeapHasBeenInitialised == pdFALSE )
        {
            prvHeapInit();
            xHeapHasBeenInitialised = pdTRUE;
        }

        if( xWantedSize > 0 )
        {
            /* 大小对齐一下, 每一块加一个控制结构体的大小 */
            xAdditionalRequiredSize = heapSTRUCT_SIZE + portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK );

            if( heapADD_WILL_OVERFLOW( xWantedSize, xAdditionalRequiredSize ) == 0 )
            {
                xWantedSize += xAdditionalRequiredSize;
            }
            else
            {
                xWantedSize = 0;
            }
        }

        /* 检测一下有的大小够不够 */
        if( heapBLOCK_SIZE_IS_VALID( xWantedSize ) != 0 )
        {
            if( ( xWantedSize > 0 ) && ( xWantedSize <= xFreeBytesRemaining ) )
            {
                /* 通过一个遍历找到一个大小合适的位置 */
                pxPreviousBlock = &xStart;
                pxBlock = xStart.pxNextFreeBlock;

                while( ( pxBlock->xBlockSize < xWantedSize ) && ( pxBlock->pxNextFreeBlock != NULL ) )
                {
                    pxPreviousBlock = pxBlock;
                    pxBlock = pxBlock->pxNextFreeBlock;
                }

                /* 有合适的位子 */
                if( pxBlock != &xEnd )
                {
                    /* 记录一下可以使用的地址 */
                    pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + heapSTRUCT_SIZE );

                    /* 把这个已经改变的地址先退出来 */
                    pxPreviousBlock->pxNextFreeBlock = pxBlock->pxNextFreeBlock;

                    /* 计算一下剩下的大小还可不可以用了 */
                    if( ( pxBlock->xBlockSize - xWantedSize ) > heapMINIMUM_BLOCK_SIZE )
                    {
                        /* 计算一下下一个地址的位置(xWantedSize是需求和控制结构体大小的和) */
                        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;

                    /* The block is being returned - it is allocated and owned
                     * by the application and has no "next" block. */
                    heapALLOCATE_BLOCK( pxBlock );
                    pxBlock->pxNextFreeBlock = NULL;
                }
            }
        }

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

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

    return pvReturn;
}

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

    /* 计算一下对齐的起始位置 */
    pucAlignedHeap = ( uint8_t * ) ( ( ( portPOINTER_SIZE_TYPE ) & ucHeap[ portBYTE_ALIGNMENT - 1 ] ) & ( ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) ) );

    /* 记录一下内存管理的起始位置 */
    xStart.pxNextFreeBlock = ( void * ) pucAlignedHeap;
    xStart.xBlockSize = ( size_t ) 0;

    /* 记录一下结束的位置 */
    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 = ( BlockLink_t * ) pucAlignedHeap;
    pxFirstFreeBlock->xBlockSize = configADJUSTED_HEAP_SIZE;
    pxFirstFreeBlock->pxNextFreeBlock = &xEnd;
}

#define prvInsertBlockIntoFreeList( pxBlockToInsert )          \
    {                                                          \
        BlockLink_t * pxIterator;                              \
        size_t xBlockSize;                                     \
        xBlockSize = pxBlockToInsert->xBlockSize;              \      
        /* 则一个循环的作用是找到需要插入的位置 */                 \
            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. */                                                                   \
                pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock;                   \
                    pxIterator->pxNextFreeBlock = pxBlockToInsert;           	                  \
}

void vPortFree( void * pv )
{
    uint8_t * puc = ( uint8_t * ) pv;
    BlockLink_t * pxLink;

    if( pv != NULL )
    {
        /* 找到对的位置 */
        puc -= heapSTRUCT_SIZE;
        pxLink = ( void * ) puc;
		//检测一下这个地址是不是合格的
        if( heapBLOCK_IS_ALLOCATED( pxLink ) != 0 )
        {
            if( pxLink->pxNextFreeBlock == NULL )
            {
                /* 这一个控制块不是被重复返回的 */
                heapFREE_BLOCK( pxLink );
                vTaskSuspendAll();
                {
                	/* Add this block to the list of free blocks. */
                    prvInsertBlockIntoFreeList( ( ( BlockLink_t * ) pxLink ) );
                    xFreeBytesRemaining += pxLink->xBlockSize;
                    traceFREE( pv, pxLink->xBlockSize );
                }
                ( void ) xTaskResumeAll();
            }
        }
    }
}

size_t xPortGetFreeHeapSize( void )
{
    return xFreeBytesRemaining;
}

heap_3

一个叫简单的封装

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

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


    return pvReturn;
}
void vPortFree( void * pv )
{
    if( pv != NULL )
    {
        vTaskSuspendAll();
        {
            free( pv );
            traceFREE( pv, 0 );
        }
        ( void ) xTaskResumeAll();
    }
}

heap_4

空闲块链表不是以内存块大小进行排序的，而是以内存块起始地址大小排序，内存地址小的在前，地址大的在后

void * pvPortMalloc( size_t xWantedSize )
{
    BlockLink_t * pxBlock;
    BlockLink_t * pxPreviousBlock;
    BlockLink_t * pxNewBlockLink;
    void * pvReturn = NULL;
    size_t xAdditionalRequiredSize;

    vTaskSuspendAll();
    {
        /* 检测一下有没有初始化 */
        if( pxEnd == NULL )
        {
            prvHeapInit();
        }

        if( xWantedSize > 0 )
        {
            /* 记录一下对齐的请求的大小并加上一个控制块的大小 */
            xAdditionalRequiredSize = xHeapStructSize + portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK );
			//防止溢出
            if( heapADD_WILL_OVERFLOW( xWantedSize, xAdditionalRequiredSize ) == 0 )
            {
                xWantedSize += xAdditionalRequiredSize;
            }
        }
        else
        {
            mtCOVERAGE_TEST_MARKER();
        }

        /* 再次检测一下大小合格*/
        if( heapBLOCK_SIZE_IS_VALID( xWantedSize ) != 0 )
        {
            //有足够的内存
            if( ( xWantedSize > 0 ) && ( xWantedSize <= xFreeBytesRemaining ) )
            {
                /* 开始遍历*/
                pxPreviousBlock = &xStart;
                pxBlock = xStart.pxNextFreeBlock;

                while( ( pxBlock->xBlockSize < xWantedSize ) && ( pxBlock->pxNextFreeBlock != NULL ) )
                {
                    pxPreviousBlock = pxBlock;
                    pxBlock = pxBlock->pxNextFreeBlock;
                }

                /* 么找到合适的位置*/
                if( pxBlock != pxEnd )
                {
                    /* 记录一下返回的值 */
                    pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + xHeapStructSize );

                    /* 先取出来这一个控制块 */
                    pxPreviousBlock->pxNextFreeBlock = pxBlock->pxNextFreeBlock;

                    /* 再看看这一个可不可以使用了 */
                    if( ( pxBlock->xBlockSize - xWantedSize ) > heapMINIMUM_BLOCK_SIZE )
                    {
                        /* 在下一个的起始位置放一个控制块 */
                        pxNewBlockLink = ( void * ) ( ( ( uint8_t * ) pxBlock ) + xWantedSize );
                        configASSERT( ( ( ( size_t ) pxNewBlockLink ) & portBYTE_ALIGNMENT_MASK ) == 0 );

                        /* 记录一下信息 */
                        pxNewBlockLink->xBlockSize = pxBlock->xBlockSize - xWantedSize;
                        pxBlock->xBlockSize = xWantedSize;

                        /* 放进链表 */
                        prvInsertBlockIntoFreeList( pxNewBlockLink );
                    }
                    else
                    {
                        mtCOVERAGE_TEST_MARKER();
                    }
				//记录一下数据
                    xFreeBytesRemaining -= pxBlock->xBlockSize;

                    if( xFreeBytesRemaining < xMinimumEverFreeBytesRemaining )
                    {
                        xMinimumEverFreeBytesRemaining = xFreeBytesRemaining;
                    }

                    /* 这一个不在链表里面了, 设置一下标志位 */
                    heapALLOCATE_BLOCK( pxBlock );
                    pxBlock->pxNextFreeBlock = NULL;
                    xNumberOfSuccessfulAllocations++;
                }
            }
        }

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

	configASSERT( ( ( ( size_t ) pvReturn ) & ( size_t ) portBYTE_ALIGNMENT_MASK ) == 0 );
    return pvReturn;
}

static void prvHeapInit( void ) /* PRIVILEGED_FUNCTION */
{
    BlockLink_t * pxFirstFreeBlock;
    uint8_t * pucAlignedHeap;
    portPOINTER_SIZE_TYPE uxAddress;
    size_t xTotalHeapSize = configTOTAL_HEAP_SIZE;

    /* 记录一下可以使用的地址对齐的位置 */
    uxAddress = ( portPOINTER_SIZE_TYPE ) ucHeap;

    if( ( uxAddress & portBYTE_ALIGNMENT_MASK ) != 0 )
    {
        uxAddress += ( portBYTE_ALIGNMENT - 1 );
        uxAddress &= ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK );
        xTotalHeapSize -= uxAddress - ( portPOINTER_SIZE_TYPE ) ucHeap;
    }

    pucAlignedHeap = ( uint8_t * ) uxAddress;

    /* 在链表的起始记录一下可以使用的地址 */
    xStart.pxNextFreeBlock = ( void * ) pucAlignedHeap;
    xStart.xBlockSize = ( size_t ) 0;

    /* 计算一下可以使用的地址的结尾 */
    uxAddress = ( ( portPOINTER_SIZE_TYPE ) pucAlignedHeap ) + xTotalHeapSize;
    uxAddress -= xHeapStructSize;
    uxAddress &= ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK );
    pxEnd = ( BlockLink_t * ) uxAddress;
    pxEnd->xBlockSize = 0;
    pxEnd->pxNextFreeBlock = NULL;

    /* 初始化一个可以使用的列表项, 记录整个数组 */
    pxFirstFreeBlock = ( BlockLink_t * ) pucAlignedHeap;
    pxFirstFreeBlock->xBlockSize = ( size_t ) ( uxAddress - ( portPOINTER_SIZE_TYPE ) pxFirstFreeBlock );
    pxFirstFreeBlock->pxNextFreeBlock = pxEnd;

    /* 记录一下最小的内存的大小以及可以使用的大小 */
    xMinimumEverFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
    xFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
}

static void prvInsertBlockIntoFreeList( BlockLink_t * pxBlockToInsert ) /* PRIVILEGED_FUNCTION */
{
    BlockLink_t * pxIterator;
    uint8_t * puc;

    /* 按照地址进行插入, 记录一下插入位置的内存前面的哪一块的指针 */
    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;
	//看看可不可以和前面的进行合并
    if( ( puc + pxIterator->xBlockSize ) == ( uint8_t * ) pxBlockToInsert )
    {
        //可以合并的话, 直接记录这一块前面的那一块的地址
        pxIterator->xBlockSize += pxBlockToInsert->xBlockSize;
        pxBlockToInsert = pxIterator;
    }


    /* 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 )
    {
        //可以和后面的那一块的内存进行合并
        if( pxIterator->pxNextFreeBlock != pxEnd )
        {
            /* 把这一块的大小扩大以及连接后面的那一块 */
            pxBlockToInsert->xBlockSize += pxIterator->pxNextFreeBlock->xBlockSize;
            pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock->pxNextFreeBlock;
        }
        else
        {
            pxBlockToInsert->pxNextFreeBlock = pxEnd;
        }
    }
    else
    {
        //链接后面的一块
        pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock;
    }

    /* 前面一块没有连接 */
    if( pxIterator != pxBlockToInsert )
    {
        pxIterator->pxNextFreeBlock = pxBlockToInsert;
    }
    else
    {
        mtCOVERAGE_TEST_MARKER();
    }
}

void vPortFree( void * pv )
{
    uint8_t * puc = ( uint8_t * ) pv;
    BlockLink_t * pxLink;

    if( pv != NULL )
    {
        /* 获取控制块的位置 */
        puc -= xHeapStructSize;

        /* This casting is to keep the compiler from issuing warnings. */
        pxLink = ( void * ) puc;

        if( heapBLOCK_IS_ALLOCATED( pxLink ) != 0 )
        {
            if( pxLink->pxNextFreeBlock == NULL )
            {
                /* 检测值一个是不是有效的一块内存 */
                heapFREE_BLOCK( pxLink );

                vTaskSuspendAll();
                {
                    /* 添加这一块到对的位置 */
                    xFreeBytesRemaining += pxLink->xBlockSize;
                    prvInsertBlockIntoFreeList( ( ( BlockLink_t * ) pxLink ) );
                    xNumberOfSuccessfulFrees++;
                }
                ( void ) xTaskResumeAll();
            }
        }
    }
}

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

    pxHeapRegion = &( pxHeapRegions[ xDefinedRegions ] );
	//这里面还有新的数据
    while( pxHeapRegion->xSizeInBytes > 0 )
    {
        xTotalRegionSize = pxHeapRegion->xSizeInBytes;

        /* 计算一下这一个地址可以使用的对齐的位置 */
        xAddress = ( portPOINTER_SIZE_TYPE ) pxHeapRegion->pucStartAddress;

        if( ( xAddress & portBYTE_ALIGNMENT_MASK ) != 0 )
        {
            xAddress += ( portBYTE_ALIGNMENT - 1 );
            xAddress &= ~portBYTE_ALIGNMENT_MASK;

            /* 调整一下大小 */
            xTotalRegionSize -= ( size_t ) ( xAddress - ( portPOINTER_SIZE_TYPE ) pxHeapRegion->pucStartAddress );
        }

        xAlignedHeap = xAddress;

        
        if( xDefinedRegions == 0 )
        {
            /* 这是第一块 */
            xStart.pxNextFreeBlock = ( BlockLink_t * ) xAlignedHeap;
            xStart.xBlockSize = ( size_t ) 0;
        }

        /* 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 &= ~( ( size_t ) 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 = ( size_t ) ( xAddress - ( portPOINTER_SIZE_TYPE ) 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 );
}