Chapter 9 - 内存管理
Created by : Mr Dk.
2019 / 11 / 20 22:57
Nanjing, Jiangsu, China
9.1 概述
为什么不直接使用 ANSI C 提供的 malloc() 和 free() 呢？
- 反复调用这两个函数，会导致内存碎片
- 执行时间不确定，不适合实时 OS 使用
μC/OS-II 的实现方式：将大块的连续内存（用户提供，可以是静态分配的数组或初始动态分配的内存）进行分区，将每个分区分成整数个大小相同的内存块，将这些内存块用链表串起来，由 μC/OS-II 的内存分配和释放函数来使用和释放。解决了内存碎片的问题。比如，将一块 320B 的内存组织成 10 个 32B 内存块的链表，这样每次从链表中可以分配 32B 的内存，还可以有多个不同内存块大小的分区。
内存管理函数可以在编译前被剪裁。
内存控制块 (Memory Control Blocks, MCB) 是用于实现内存管理的数据结构，每个 内存分区 都有其自己的 MCB。
typedef struct os_mem { /* MEMORY CONTROL BLOCK */
void *OSMemAddr; /* Pointer to beginning of memory partition */
void *OSMemFreeList; /* Pointer to list of free memory blocks */
INT32U OSMemBlkSize; /* Size (in bytes) of each block of memory */
INT32U OSMemNBlks; /* Total number of blocks in this partition */
INT32U OSMemNFree; /* Number of memory blocks remaining in this partition */
#if OS_MEM_NAME_EN > 0u
INT8U *OSMemName; /* Memory partition name */
#endif
} OS_MEM;
OSMemAddr指向内存分区首地址OSMemFreeList用于组织系统中所有的空闲 MCBOSMemBlkSize表示内存分区中内存块的大小OSMemNBlks表示内存分区中内存块的数量OSMemNFree表示内存分区中空闲内存块数量
可以在编译前定义 OS_MAX_MEM_PART，决定了系统中的最大分区数。系统初始化时会根据这个值，建立空闲 MCB 链表。
9.2 建立内存分区 - OSMemCreate() 函数
建立并初始化一块内存分区。需要给出的参数：
- 内存分区的起始地址 - 内存分区可以用静态数组或初始化前使用
malloc()建立 - 内存块的数量
- 每个内存块的大小
- 指向错误代码变量的指针
原理：
- 从空闲 MCB 中取一个 MCB
- 根据参数，将每个内存块串成一个空闲内存块单向链表
- 每个空闲内存块的开始处存一个指针，指向下一个空闲内存块
/*
*********************************************************************************************************
* CREATE A MEMORY PARTITION
*
* Description : Create a fixed-sized memory partition that will be managed by uC/OS-II.
*
* Arguments : addr is the starting address of the memory partition
*
* nblks is the number of memory blocks to create from the partition.
*
* blksize is the size (in bytes) of each block in the memory partition.
*
* perr is a pointer to a variable containing an error message which will be set by
* this function to either:
*
* OS_ERR_NONE if the memory partition has been created correctly.
* OS_ERR_MEM_INVALID_ADDR if you are specifying an invalid address for the memory
* storage of the partition or, the block does not align
* on a pointer boundary
* OS_ERR_MEM_INVALID_PART no free partitions available
* OS_ERR_MEM_INVALID_BLKS user specified an invalid number of blocks (must be >= 2)
* OS_ERR_MEM_INVALID_SIZE user specified an invalid block size
* - must be greater than the size of a pointer
* - must be able to hold an integral number of pointers
* Returns : != (OS_MEM *)0 is the partition was created
* == (OS_MEM *)0 if the partition was not created because of invalid arguments or, no
* free partition is available.
*********************************************************************************************************
*/
OS_MEM *OSMemCreate (void *addr,
INT32U nblks,
INT32U blksize,
INT8U *perr)
{
OS_MEM *pmem;
INT8U *pblk;
void **plink;
INT32U loops;
INT32U i;
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#ifdef OS_SAFETY_CRITICAL
if (perr == (INT8U *)0) {
OS_SAFETY_CRITICAL_EXCEPTION();
}
#endif
#ifdef OS_SAFETY_CRITICAL_IEC61508
if (OSSafetyCriticalStartFlag == OS_TRUE) {
OS_SAFETY_CRITICAL_EXCEPTION();
}
#endif
#if OS_ARG_CHK_EN > 0u
if (addr == (void *)0) { /* Must pass a valid address for the memory part.*/
*perr = OS_ERR_MEM_INVALID_ADDR;
return ((OS_MEM *)0);
}
if (((INT32U)addr & (sizeof(void *) - 1u)) != 0u){ /* Must be pointer size aligned */
*perr = OS_ERR_MEM_INVALID_ADDR;
return ((OS_MEM *)0);
}
if (nblks < 2u) { /* Must have at least 2 blocks per partition */
*perr = OS_ERR_MEM_INVALID_BLKS;
return ((OS_MEM *)0);
}
if (blksize < sizeof(void *)) { /* Must contain space for at least a pointer */
*perr = OS_ERR_MEM_INVALID_SIZE;
return ((OS_MEM *)0);
}
#endif
OS_ENTER_CRITICAL();
pmem = OSMemFreeList; /* Get next free memory partition */
if (OSMemFreeList != (OS_MEM *)0) { /* See if pool of free partitions was empty */
OSMemFreeList = (OS_MEM *)OSMemFreeList->OSMemFreeList;
}
OS_EXIT_CRITICAL();
if (pmem == (OS_MEM *)0) { /* See if we have a memory partition */
*perr = OS_ERR_MEM_INVALID_PART;
return ((OS_MEM *)0);
}
plink = (void **)addr; /* Create linked list of free memory blocks */
pblk = (INT8U *)addr;
loops = nblks - 1u;
for (i = 0u; i < loops; i++) {
pblk += blksize; /* Point to the FOLLOWING block */
*plink = (void *)pblk; /* Save pointer to NEXT block in CURRENT block */
plink = (void **)pblk; /* Position to NEXT block */
}
*plink = (void *)0; /* Last memory block points to NULL */
pmem->OSMemAddr = addr; /* Store start address of memory partition */
pmem->OSMemFreeList = addr; /* Initialize pointer to pool of free blocks */
pmem->OSMemNFree = nblks; /* Store number of free blocks in MCB */
pmem->OSMemNBlks = nblks;
pmem->OSMemBlkSize = blksize; /* Store block size of each memory blocks */
*perr = OS_ERR_NONE;
return (pmem);
}
9.3 获取一个内存块 - OSMemGet() 函数
从已经建立的内存分区中申请获取一个内存块。
原理：
- 从内存分区中抽取第一个空闲的内存块
- 调整
OSMemFreeList指针，使其指向下一个空闲内存块 - 维护空闲内存块的数量
用户必须知道内存分区中内存块的大小，如果暂时没有内存块可用，函数不会等待，而是立刻返回 NULL。
/*
*********************************************************************************************************
* GET A MEMORY BLOCK
*
* Description : Get a memory block from a partition
*
* Arguments : pmem is a pointer to the memory partition control block
*
* perr is a pointer to a variable containing an error message which will be set by this
* function to either:
*
* OS_ERR_NONE if the memory partition has been created correctly.
* OS_ERR_MEM_NO_FREE_BLKS if there are no more free memory blocks to allocate to caller
* OS_ERR_MEM_INVALID_PMEM if you passed a NULL pointer for 'pmem'
*
* Returns : A pointer to a memory block if no error is detected
* A pointer to NULL if an error is detected
*********************************************************************************************************
*/
void *OSMemGet (OS_MEM *pmem,
INT8U *perr)
{
void *pblk;
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#ifdef OS_SAFETY_CRITICAL
if (perr == (INT8U *)0) {
OS_SAFETY_CRITICAL_EXCEPTION();
}
#endif
#if OS_ARG_CHK_EN > 0u
if (pmem == (OS_MEM *)0) { /* Must point to a valid memory partition */
*perr = OS_ERR_MEM_INVALID_PMEM;
return ((void *)0);
}
#endif
OS_ENTER_CRITICAL();
if (pmem->OSMemNFree > 0u) { /* See if there are any free memory blocks */
pblk = pmem->OSMemFreeList; /* Yes, point to next free memory block */
pmem->OSMemFreeList = *(void **)pblk; /* Adjust pointer to new free list */
pmem->OSMemNFree--; /* One less memory block in this partition */
OS_EXIT_CRITICAL();
*perr = OS_ERR_NONE; /* No error */
return (pblk); /* Return memory block to caller */
}
OS_EXIT_CRITICAL();
*perr = OS_ERR_MEM_NO_FREE_BLKS; /* No, Notify caller of empty memory partition */
return ((void *)0); /* Return NULL pointer to caller */
}
9.4 释放一个内存块 - OSMemPut() 函数
释放一个内存块。
原理：
- 检查 MCB 查看内存分区是否已满，如果已满，说明分配和释放时出现了错误
- 如果未满，将内存块插入到空闲内存块链表的最前面
释放内存块时，必须放回原先申请的内存分区中，不能错放。
/*
*********************************************************************************************************
* RELEASE A MEMORY BLOCK
*
* Description : Returns a memory block to a partition
*
* Arguments : pmem is a pointer to the memory partition control block
*
* pblk is a pointer to the memory block being released.
*
* Returns : OS_ERR_NONE if the memory block was inserted into the partition
* OS_ERR_MEM_FULL if you are returning a memory block to an already FULL memory
* partition (You freed more blocks than you allocated!)
* OS_ERR_MEM_INVALID_PMEM if you passed a NULL pointer for 'pmem'
* OS_ERR_MEM_INVALID_PBLK if you passed a NULL pointer for the block to release.
*********************************************************************************************************
*/
INT8U OSMemPut (OS_MEM *pmem,
void *pblk)
{
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#if OS_ARG_CHK_EN > 0u
if (pmem == (OS_MEM *)0) { /* Must point to a valid memory partition */
return (OS_ERR_MEM_INVALID_PMEM);
}
if (pblk == (void *)0) { /* Must release a valid block */
return (OS_ERR_MEM_INVALID_PBLK);
}
#endif
OS_ENTER_CRITICAL();
if (pmem->OSMemNFree >= pmem->OSMemNBlks) { /* Make sure all blocks not already returned */
OS_EXIT_CRITICAL();
return (OS_ERR_MEM_FULL);
}
*(void **)pblk = pmem->OSMemFreeList; /* Insert released block into free block list */
pmem->OSMemFreeList = pblk;
pmem->OSMemNFree++; /* One more memory block in this partition */
OS_EXIT_CRITICAL();
return (OS_ERR_NONE); /* Notify caller that memory block was released */
}
9.5 查询内存分区的状态 - OSMemQuery() 函数
查询指定内存分区的有关信息，将信息复制到 OS_MEM_DATA 数据结构中。
typedef struct os_mem_data {
void *OSAddr; /* Pointer to the beginning address of the memory partition */
void *OSFreeList; /* Pointer to the beginning of the free list of memory blocks */
INT32U OSBlkSize; /* Size (in bytes) of each memory block */
INT32U OSNBlks; /* Total number of blocks in the partition */
INT32U OSNFree; /* Number of memory blocks free */
INT32U OSNUsed; /* Number of memory blocks used */
} OS_MEM_DATA;
原理：
- 直接将 MCB 中的变量复制到
OS_MEM_DATA中 - 最后一个变量的值 - 正在使用的内存块数量
OSNUsed- 直接通过计算得出
/*
*********************************************************************************************************
* QUERY MEMORY PARTITION
*
* Description : This function is used to determine the number of free memory blocks and the number of
* used memory blocks from a memory partition.
*
* Arguments : pmem is a pointer to the memory partition control block
*
* p_mem_data is a pointer to a structure that will contain information about the memory
* partition.
*
* Returns : OS_ERR_NONE if no errors were found.
* OS_ERR_MEM_INVALID_PMEM if you passed a NULL pointer for 'pmem'
* OS_ERR_MEM_INVALID_PDATA if you passed a NULL pointer to the data recipient.
*********************************************************************************************************
*/
#if OS_MEM_QUERY_EN > 0u
INT8U OSMemQuery (OS_MEM *pmem,
OS_MEM_DATA *p_mem_data)
{
#if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */
OS_CPU_SR cpu_sr = 0u;
#endif
#if OS_ARG_CHK_EN > 0u
if (pmem == (OS_MEM *)0) { /* Must point to a valid memory partition */
return (OS_ERR_MEM_INVALID_PMEM);
}
if (p_mem_data == (OS_MEM_DATA *)0) { /* Must release a valid storage area for the data */
return (OS_ERR_MEM_INVALID_PDATA);
}
#endif
OS_ENTER_CRITICAL();
p_mem_data->OSAddr = pmem->OSMemAddr;
p_mem_data->OSFreeList = pmem->OSMemFreeList;
p_mem_data->OSBlkSize = pmem->OSMemBlkSize;
p_mem_data->OSNBlks = pmem->OSMemNBlks;
p_mem_data->OSNFree = pmem->OSMemNFree;
OS_EXIT_CRITICAL();
p_mem_data->OSNUsed = p_mem_data->OSNBlks - p_mem_data->OSNFree;
return (OS_ERR_NONE);
}
#endif /* OS_MEM_QUERY_EN */