第一章 FreeRTOS 整体架构
第二章 FreeRTOS 编程规范
第三章 FreeRTOS 内存管理
第四章 FreeRTOS 任务管理
第五章 FreeRTOS 消息队列
第六章 FreeRTOS 软件定时器
第七章 FreeRTOS 信号量
第八章 FreeRTOS 互斥量
第九章 FreeRTOS 任务通知
第十章 FreeRTOS 事件组
本章主要介绍FreeRTOS的内存管理。简要说明内存管理的作用，FreeRTOS自带的五个heap文件。详细解析 heap4.c的实现。
需要具备一定的链表知识。
嵌入式软件开发中，有一定代码规模的软件产品，会自己进行内存管理。针对操作系统而言，内存管理也是其核心功能。基本思路是将一段已知的内存块给到对应的软件产品，作为它的内存池。内存管理将软件产品中需要操作内存的，都限制在这个内存池中。这样能在复杂的系统中，便于内存的规划和问题定位。
FreeRTOS支持静态内存和动态内存，其选择取决于应用场景。
TaskHandle_t xTaskCreateStatic( TaskFunction_t pxTaskCode,
const char * const pcName, /*lint !e971 Unqualified char types are allowed for strings and single characters only. */
const uint32_t ulStackDepth,
void * const pvParameters,
UBaseType_t uxPriority,
StackType_t * const puxStackBuffer,
StaticTask_t * const pxTaskBuffer ) PRIVILEGED_FUNCTION;
puxStackBuffer 即为创建任务时，给到当前任务的静态内存。
/*
- Map to the memory management routines required for the port.
*/
void * pvPortMalloc( size_t xSize ) ;
void vPortFree( void * pv ) ;
void vPortInitialiseBlocks( void ) ;
size_t xPortGetFreeHeapSize( void ) ;
如果FreeRTOS是动态的创建对象，FreeRTOS认为直接使用 C标准库的 malloc 和 free 存在如下问题：
FreeRTOS提供了5个 heap文件，简要说明如下：
默认大家已经掌握单向链表的知识。链表中的一个节点就代表了一个一定大小的内存块。
核心是通过一个单向链表，将 ucHeap 这块内存池进行切割（申请内存时）和碎片合并（释放内存时）。申请内存时从链表中找到合适的节点以后从链表中删除，释放内存时将对应的节点重新插入到链表中。
/* Define the linked list structure. This is used to link free blocks in order
* of their memory address. */
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;
/* Allocate the memory for the heap. */
static uint8_t ucHeap[ configTOTAL_HEAP_SIZE ];
/* The size of the structure placed at the beginning of each allocated memory
* block must by correctly byte aligned. */
static const size_t xHeapStructSize = ( sizeof( BlockLink_t ) + ( ( size_t ) ( portBYTE_ALIGNMENT - 1 ) ) ) & ~( ( size_t ) portBYTE_ALIGNMENT_MASK );
/* The size of the structure placed at the beginning of each allocated memory
* block must by correctly byte aligned. */
static const size_t xHeapStructSize = ( sizeof( BlockLink_t ) + ( ( size_t )
/* Create a couple of list links to mark the start and end of the list. */
PRIVILEGED_DATA static BlockLink_t xStart, * pxEnd = NULL;
/* Keeps track of the number of calls to allocate and free memory as well as the
* number of free bytes remaining, but says nothing about fragmentation. */
PRIVILEGED_DATA static size_t xFreeBytesRemaining = 0U;
PRIVILEGED_DATA static size_t xMinimumEverFreeBytesRemaining = 0U;
PRIVILEGED_DATA static size_t xNumberOfSuccessfulAllocations = 0;
PRIVILEGED_DATA static size_t xNumberOfSuccessfulFrees = 0;
/* Gets set to the top bit of an size_t type. When this bit in the xBlockSize
* member of an BlockLink_t structure is set then the block belongs to the
* application. When the bit is free the block is still part of the free heap
* space. */
PRIVILEGED_DATA static size_t xBlockAllocatedBit = 0;
static void prvHeapInit( void ) /* PRIVILEGED_FUNCTION */
{
BlockLink_t * pxFirstFreeBlock;
uint8_t * pucAlignedHeap;
size_t uxAddress;
size_t xTotalHeapSize = configTOTAL_HEAP_SIZE;
/* Ensure the heap starts on a correctly aligned boundary. */
uxAddress = ( size_t ) ucHeap;
if( ( uxAddress & portBYTE_ALIGNMENT_MASK ) != 0 )
{
uxAddress += ( portBYTE_ALIGNMENT - 1 );
uxAddress &= ~( ( size_t ) portBYTE_ALIGNMENT_MASK );
xTotalHeapSize -= uxAddress - ( size_t ) ucHeap;
}
pucAlignedHeap = ( uint8_t * ) uxAddress;
/* 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;
/* pxEnd is used to mark the end of the list of free blocks and is inserted
* at the end of the heap space. */
uxAddress = ( ( size_t ) pucAlignedHeap ) + xTotalHeapSize;
uxAddress -= xHeapStructSize;
uxAddress &= ~( ( size_t ) portBYTE_ALIGNMENT_MASK );
pxEnd = ( void * ) uxAddress;
pxEnd->xBlockSize = 0;
pxEnd->pxNextFreeBlock = NULL;
/* To start with there is a single free block that is sized to take up the
* entire heap space, minus the space taken by pxEnd. */
pxFirstFreeBlock = ( void * ) pucAlignedHeap;
pxFirstFreeBlock->xBlockSize = uxAddress - ( size_t ) pxFirstFreeBlock;
pxFirstFreeBlock->pxNextFreeBlock = pxEnd;
/* Only one block exists - and it covers the entire usable heap space. */
xMinimumEverFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
xFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
/* Work out the position of the top bit in a size_t variable. */
xBlockAllocatedBit = ( ( size_t ) 1 ) << ( ( sizeof( size_t ) * heapBITS_PER_BYTE ) - 1 );
}
void * pvPortMalloc( size_t xWantedSize )
{
BlockLink_t * pxBlock, * pxPreviousBlock, * pxNewBlockLink;
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( pxEnd == NULL )
{
prvHeapInit();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
/* Check the requested block size is not so large that the top bit is
* set. The top bit of the block size member of the BlockLink_t structure
* is used to determine who owns the block - the application or the
* kernel, so it must be free. */
if( ( xWantedSize & xBlockAllocatedBit ) == 0 )
{
/* The wanted size must be increased so it can contain a BlockLink_t
* structure in addition to the requested amount of bytes. */
if( ( xWantedSize > 0 ) &&
( ( xWantedSize + xHeapStructSize ) > xWantedSize ) ) /* Overflow check */
{
xWantedSize += xHeapStructSize;
/* Ensure that blocks are always aligned. */
if( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) != 0x00 )
{
/* Byte alignment required. Check for overflow. */
if( ( xWantedSize + ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) ) )
> xWantedSize )
{
xWantedSize += ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) );
configASSERT( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) == 0 );
}
else
{
xWantedSize = 0;
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
xWantedSize = 0;
}
if( ( xWantedSize > 0 ) && ( xWantedSize <= xFreeBytesRemaining ) )
{
/* Traverse the list from the start (lowest address) 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 the end marker was reached then a block of adequate size
* was not found. */
if( pxBlock != pxEnd )
{
/* Return the memory space pointed to - jumping over the
* BlockLink_t structure at its start. */
pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + xHeapStructSize );
/* 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 );
configASSERT( ( ( ( size_t ) pxNewBlockLink ) & portBYTE_ALIGNMENT_MASK ) == 0 );
/* 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 );
}
else
{
mtCOVERAGE_TEST_MARKER();
}
xFreeBytesRemaining -= pxBlock->xBlockSize;
if( xFreeBytesRemaining < xMinimumEverFreeBytesRemaining )
{
xMinimumEverFreeBytesRemaining = xFreeBytesRemaining;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
/* The block is being returned - it is allocated and owned
* by the application and has no "next" block. */
pxBlock->xBlockSize |= xBlockAllocatedBit;
pxBlock->pxNextFreeBlock = NULL;
xNumberOfSuccessfulAllocations++;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
traceMALLOC( pvReturn, xWantedSize );
}
( void ) xTaskResumeAll();
#if ( configUSE_MALLOC_FAILED_HOOK == 1 )
{
if( pvReturn == NULL )
{
extern void vApplicationMallocFailedHook( void );
vApplicationMallocFailedHook();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif /* if ( configUSE_MALLOC_FAILED_HOOK == 1 ) */
configASSERT( ( ( ( size_t ) pvReturn ) & ( size_t ) portBYTE_ALIGNMENT_MASK ) == 0 );
return pvReturn;
}
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 ) );
xNumberOfSuccessfulFrees++;
}
( void ) xTaskResumeAll();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
}
static void prvInsertBlockIntoFreeList( BlockLink_t * pxBlockToInsert ) /* PRIVILEGED_FUNCTION */
{
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();
}
}