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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