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_4.c：
heap_4.c的实现和heap_2.c有一点类似，但是实现了空闲内存合并的功能，减少了内存碎片。
heap_4.c对申请内存的大小有限制，这里作者没有深入探究，后面在看下。
/*
* A sample implementation of pvPortMalloc() and vPortFree() that combines
* (coalescences) adjacent memory blocks as they are freed, and in so doing
* limits memory fragmentation.
*
* See heap_1.c, heap_2.c and heap_3.c for alternative implementations, and the
* memory management pages of http://www.FreeRTOS.org for more information.
*/
#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
/* Block sizes must not get too small. */
#define heapMINIMUM_BLOCK_SIZE ( ( size_t ) ( xHeapStructSize * 2 ) )
/*
1个byte有8个bits
*/
/* Assumes 8bit bytes! */
#define heapBITS_PER_BYTE ( ( size_t ) 8 )
/*
内部用来模拟堆的也是一个大号的数组，这里可以通过外部的内存来指定这个大号的数组。
*/
/* Allocate the memory for the heap. */
#if( configAPPLICATION_ALLOCATED_HEAP == 1 )
/* The application writer has already defined the array used for the RTOS
heap - probably so it can be placed in a special segment or address. */
extern uint8_t ucHeap[ configTOTAL_HEAP_SIZE ];
#else
static uint8_t ucHeap[ configTOTAL_HEAP_SIZE ];
#endif /* configAPPLICATION_ALLOCATED_HEAP */
/* 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. */
/* 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;
//空闲内存管理结构体，通过它来管理释放回来的内存
/*-----------------------------------------------------------*/
/*
* Inserts a block of memory that is being freed into the correct position in
* the list of free memory blocks. The block being freed will be merged with
* the block in front it and/or the block behind it if the memory blocks are
* adjacent to each other.
*/
static void prvInsertBlockIntoFreeList( BlockLink_t *pxBlockToInsert );
/*
* Called automatically to setup the required heap structures the first time
* pvPortMalloc() is called.
*/
static void prvHeapInit( void );
/*-----------------------------------------------------------*/
/* 考虑到字节对齐后BlockLink_t的大小 */
/* 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_MASK ) - ( size_t ) 1 ) ) & ~( ( size_t ) portBYTE_ALIGNMENT_MASK ) );
/* 单向链表，注意这里的链表不是以内存大小为依据排序的，而是以内存位置为依据排序的，这样是为了方便空闲内存的合并 */
/* Create a couple of list links to mark the start and end of the list. */
static BlockLink_t xStart, *pxEnd = NULL;
/* Keeps track of the number of free bytes remaining, but says nothing about
fragmentation. */
static size_t xFreeBytesRemaining = 0U;
static size_t xMinimumEverFreeBytesRemaining = 0U;
/* 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. */
static size_t xBlockAllocatedBit = 0;
/*-----------------------------------------------------------*/
void *pvPortMalloc( size_t xWantedSize )
{
BlockLink_t *pxBlock, *pxPreviousBlock, *pxNewBlockLink;
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( pxEnd == NULL )//如果是第一次执行该函数，那么先调用下初始化函数
{
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. */
/*这里xWantedSize的大小有要求，需要最高位为0。因为后面BlockLink_t结构体中的xBlockSize的最高位需要使用。
*/
if( ( xWantedSize & xBlockAllocatedBit ) == 0 )
{
/* The wanted size is increased so it can contain a BlockLink_t
structure in addition to the requested amount of bytes. */
if( xWantedSize > 0 )
{
xWantedSize += xHeapStructSize;/* 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 */
/* Ensure that blocks are always aligned to the required number
of bytes. */
if( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) != 0x00 )
{ /* 保证字节对齐 */
/* Byte alignment required. */
xWantedSize += ( portBYTE_ALIGNMENT - ( xWantedSize & portBYTE_ALIGNMENT_MASK ) );
configASSERT( ( xWantedSize & portBYTE_ALIGNMENT_MASK ) == 0 );
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
/* 人为扩充后的大小小于空闲内存 */
if( ( xWantedSize > 0 ) && ( xWantedSize <= xFreeBytesRemaining ) )
{
/* Traverse the list from the start (lowest address) 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，就标记该内存为pxBlock
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. */
/* 如果pxBlock不是pxEnd,说明上面的while已经找到了合适的内存 */
if( pxBlock != pxEnd )
{
/* 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 pointed to - jumping over the
BlockLink_t structure at its start. */
pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + xHeapStructSize );
//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;
/* If the block is larger than required it can be split into
two. */
//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( ( 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( ( ( ( uint32_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();
}
//更新剩余内存总大小
xFreeBytesRemaining -= pxBlock->xBlockSize;
//更新最小内存统计值
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. */
//注意这里的xBlockSize的最高位被设置为1，标记了该内存是pvPortMalloc返回的。
pxBlock->xBlockSize |= xBlockAllocatedBit;
pxBlock->pxNextFreeBlock = NULL;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
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();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
#endif
configASSERT( ( ( ( uint32_t ) pvReturn ) & 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;//这里向前偏移，重新找回BlockLink_t
/* 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 );
/* 如果xBlockSize的最高位为1，说明该块内存是pvPortMalloc申请的，那么通过pxBlock->xBlockSize |= xBlockAllocatedBit;
可以知道该块内存的真实大小为pxLink->xBlockSize &= ~xBlockAllocatedBit，即最高位改回0后的值。
*/
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 ) );
}
( void ) xTaskResumeAll();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else//如果xBlockSize的最高位不为1，那么说明该块内存不是通过pvPortMalloc申请的，那么直接忽略处理
{
mtCOVERAGE_TEST_MARKER();
}
}
}
/*-----------------------------------------------------------*/
size_t xPortGetFreeHeapSize( void )
{
return xFreeBytesRemaining;
}
/*-----------------------------------------------------------*/
size_t xPortGetMinimumEverFreeHeapSize( void )
{
return xMinimumEverFreeBytesRemaining;
}
/*-----------------------------------------------------------*/
void vPortInitialiseBlocks( void )
{
/* This just exists to keep the linker quiet. */
}
/*-----------------------------------------------------------*/
static void prvHeapInit( void )
{
BlockLink_t *pxFirstFreeBlock;
uint8_t *pucAlignedHeap;
uint32_t ulAddress;
size_t xTotalHeapSize = configTOTAL_HEAP_SIZE;
/*根据内存对齐要求获取模拟堆的起始地址（ucHeap可能不符合内存对齐要求，对ucHeap进行内存对齐后赋值给pucAlignedHeap）*/
/* Ensure the heap starts on a correctly aligned boundary. */
ulAddress = ( uint32_t ) ucHeap;
if( ( ulAddress & portBYTE_ALIGNMENT_MASK ) != 0 )
{
ulAddress += ( portBYTE_ALIGNMENT - 1 );
ulAddress &= ~( ( uint32_t ) portBYTE_ALIGNMENT_MASK );
xTotalHeapSize -= ulAddress - ( uint32_t ) ucHeap;
/* xTotalHeapSize表示模拟堆的总内存大小 */
}
pucAlignedHeap = ( uint8_t * ) ulAddress;
/* 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. */
/*
和heap_2.类似，将当前模拟堆的所有内存构造成一个BlockLink_t，插入空闲单向链表中。
不同的是链表的尾部BlockLink_t不是静态的，而是放在了模拟堆空余内存的最后。
*/
/* 初始化链表头部 */
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. */
/*
计算ulAddress，值为模拟堆内存尾部向前偏移一个BlockLink_t的大小，偏移出来的这个BlockLink_t就是pxEnd
*/
ulAddress = ( ( uint32_t ) pucAlignedHeap ) + xTotalHeapSize;
ulAddress -= xHeapStructSize;
ulAddress &= ~( ( uint32_t ) portBYTE_ALIGNMENT_MASK );
pxEnd = ( void * ) ulAddress;
pxEnd->xBlockSize = 0;
pxEnd->pxNextFreeBlock = NULL;
/* 剩余的所有内存构造成一个BlockLink_t，插入队列中 */
/* 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 = ulAddress - ( uint32_t ) pxFirstFreeBlock;
pxFirstFreeBlock->pxNextFreeBlock = pxEnd;
/* 更新统计变量 */
/* Only one block exists - and it covers the entire usable heap space. */
/*
xMinimumEverFreeBytesRemaining是一个统计值，存储了当前模拟堆运行时的最小剩余内存容量。
xFreeBytesRemaining：当前模拟堆的剩余内存容量。
*/
xMinimumEverFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
xFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
/* Work out the position of the top bit in a size_t variable. */
/*
这个xBlockAllocatedBit比较特殊，这里被设置为最高位为1其余为0的一个size_t大小的值，
这样任意一个size_t大小的值和xBlockAllocatedBit互相&,如果该值最高位为1，那么结果为1，否则结果为0
*/
xBlockAllocatedBit = ( ( size_t ) 1 ) << ( ( sizeof( size_t ) * heapBITS_PER_BYTE ) - 1 );
}
/*-----------------------------------------------------------*/
/*
将一个BlockLink_t的指针插入到单向链表中，注意这里会合并空闲内存
*/
static void prvInsertBlockIntoFreeList( BlockLink_t *pxBlockToInsert )
{
BlockLink_t *pxIterator;
uint8_t *puc;
/* 首先找到和pxBlockToInsert【前面】相邻的空闲内存 */
/* 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;
/*
循环结束后，如果找到的内存和pxBlockToInsert相邻，并且在pxBlockToInsert前面，那么就把pxBlockToInsert合并到该内存中
*/
if( ( puc + pxIterator->xBlockSize ) == ( uint8_t * ) pxBlockToInsert )//如果一个内存的尾部恰好是pxBlockToInsert头部
{
pxIterator->xBlockSize += pxBlockToInsert->xBlockSize;//将pxBlockToInsert的大小合入该内存中
pxBlockToInsert = pxIterator;
}
else
{
//for循环没有找到
mtCOVERAGE_TEST_MARKER();
}
/* 判断pxBlockToInsert是否和后面的空闲内存相邻 */
/* 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 )
{ /* 将后面的内存合入pxBlockToInsert，并用pxBlockToInsert代替该内存在链表中的位置 */
/* Form one big block from the two blocks. */
pxBlockToInsert->xBlockSize += pxIterator->pxNextFreeBlock->xBlockSize;
pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock->pxNextFreeBlock;
}
else
{
pxBlockToInsert->pxNextFreeBlock = pxEnd;
}
}
else
{ /* 后面不相邻，那么只能加入链表了 */
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. */
/* 判断下前面是否已经合并了，如果合并了，就不用在更新链表了 */
if( pxIterator != pxBlockToInsert )
{
pxIterator->pxNextFreeBlock = pxBlockToInsert;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
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