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ARM, FreeRTOS, 操作系统

FreeRTOS代码阅读笔记:heap_2.c

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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