ucos系统由于构思巧妙，结构精简设计，可读性强，同时又具有实时性操作系统大部分的优点，在嵌入式产品中应用非常广泛。
之前一直都只是会用ucos却没有好好研究过它，最近项目中要用到了ucos-II所以顺便研究了一番，突然发现ucos-II的内存管理写得非常巧妙。
废话不多说，直接上代码：
先看一个内存块结构体
1 typedef struct os_mem { /* MEMORY CONTROL BLOCK */ 2 void *OSMemAddr; /* Pointer to beginning of memory partition */ 3 void *OSMemFreeList; /* Pointer to list of free memory blocks */ 4 INT32U OSMemBlkSize; /* Size (in bytes) of each block of memory */ 5 INT32U OSMemNBlks; /* Total number of blocks in this partition */ 6 INT32U OSMemNFree; /* Number of memory blocks remaining in this partition */ 7 #if OS_MEM_NAME_EN > 0u 8 INT8U *OSMemName; /* Memory partition name */ 9 #endif 10 } OS_MEM;
其中 OSMemAddr指向一块内存的起始地址；
OSMemFreeList指向一个可利用的空块的地址；
再看看内存的分配函数
1 OS_MEM *OSMemCreate (void *addr, 2 INT32U nblks, 3 INT32U blksize, 4 INT8U *perr) 5 { 6 OS_MEM *pmem; 7 INT8U *pblk; 8 void **plink; 9 INT32U loops; 10 INT32U i; 11 #if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */ 12 OS_CPU_SR cpu_sr = 0u; 13 #endif 14 15 16 17 #ifdef OS_SAFETY_CRITICAL 18 if (perr == (INT8U *)0) { 19 OS_SAFETY_CRITICAL_EXCEPTION(); 20 } 21 #endif 22 23 #ifdef OS_SAFETY_CRITICAL_IEC61508 24 if (OSSafetyCriticalStartFlag == OS_TRUE) { 25 OS_SAFETY_CRITICAL_EXCEPTION(); 26 } 27 #endif 28 29 #if OS_ARG_CHK_EN > 0u 30 if (addr == (void *)0) { /* Must pass a valid address for the memory part.*/ 31 *perr = OS_ERR_MEM_INVALID_ADDR; 32 return ((OS_MEM *)0); 33 } 34 if (((INT32U)addr & (sizeof(void *) - 1u)) != 0u){ /* Must be pointer size aligned */ 35 *perr = OS_ERR_MEM_INVALID_ADDR; 36 return ((OS_MEM *)0); 37 } 38 if (nblks < 2u) { /* Must have at least 2 blocks per partition */ 39 *perr = OS_ERR_MEM_INVALID_BLKS; 40 return ((OS_MEM *)0); 41 } 42 if (blksize < sizeof(void *)) { /* Must contain space for at least a pointer */ 43 *perr = OS_ERR_MEM_INVALID_SIZE; 44 return ((OS_MEM *)0); 45 } 46 #endif 47 OS_ENTER_CRITICAL(); 48 pmem = OSMemFreeList; /* Get next free memory partition */ 49 if (OSMemFreeList != (OS_MEM *)0) { /* See if pool of free partitions was empty */ 50 OSMemFreeList = (OS_MEM *)OSMemFreeList->OSMemFreeList; 51 } 52 OS_EXIT_CRITICAL(); 53 if (pmem == (OS_MEM *)0) { /* See if we have a memory partition */ 54 *perr = OS_ERR_MEM_INVALID_PART; 55 return ((OS_MEM *)0); 56 } 57 plink = (void **)addr; /* Create linked list of free memory blocks */ 58 pblk = (INT8U *)addr; 59 loops = nblks - 1u; 60 for (i = 0u; i < loops; i++) { 61 pblk += blksize; /* Point to the FOLLOWING block */ 62 *plink = (void *)pblk; /* Save pointer to NEXT block in CURRENT block */ 63 plink = (void **)pblk; /* Position to NEXT block */ 64 } 65 *plink = (void *)0; /* Last memory block points to NULL */ 66 pmem->OSMemAddr = addr; /* Store start address of memory partition */ 67 pmem->OSMemFreeList = addr; /* Initialize pointer to pool of free blocks */ 68 pmem->OSMemNFree = nblks; /* Store number of free blocks in MCB */ 69 pmem->OSMemNBlks = nblks; 70 pmem->OSMemBlkSize = blksize; /* Store block size of each memory blocks */ 71 *perr = OS_ERR_NONE; 72 return (pmem); 73 }
这个函数把addr指向的一块连续的内存劈成nblks个blksize大小的内存块。里面利用了一个技巧：在60-64行的这个for循环里面把每个内存块的首地址保存在它的上一个内存块的首地址空间里面。67行把pmem里面的OSMemFreeList指向第一个空块的地址(当前空块的第一个地址保存的是下一个空块的地址)。
再看看内存获取函数
1 void *OSMemGet (OS_MEM *pmem, 2 INT8U *perr) 3 { 4 void *pblk; 5 #if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */ 6 OS_CPU_SR cpu_sr = 0u; 7 #endif 8 9 10 11 #ifdef OS_SAFETY_CRITICAL 12 if (perr == (INT8U *)0) { 13 OS_SAFETY_CRITICAL_EXCEPTION(); 14 } 15 #endif 16 17 #if OS_ARG_CHK_EN > 0u 18 if (pmem == (OS_MEM *)0) { /* Must point to a valid memory partition */ 19 *perr = OS_ERR_MEM_INVALID_PMEM; 20 return ((void *)0); 21 } 22 #endif 23 OS_ENTER_CRITICAL(); 24 if (pmem->OSMemNFree > 0u) { /* See if there are any free memory blocks */ 25 pblk = pmem->OSMemFreeList; /* Yes, point to next free memory block */ 26 pmem->OSMemFreeList = *(void **)pblk; /* Adjust pointer to new free list */ 27 pmem->OSMemNFree--; /* One less memory block in this partition */ 28 OS_EXIT_CRITICAL(); 29 *perr = OS_ERR_NONE; /* No error */ 30 return (pblk); /* Return memory block to caller */ 31 } 32 OS_EXIT_CRITICAL(); 33 *perr = OS_ERR_MEM_NO_FREE_BLKS; /* No, Notify caller of empty memory partition */ 34 return ((void *)0); /* Return NULL pointer to caller */ 35 }
这个函数除了红色标出来的这几行代码其它都不重要，在这几行代码里面获取OSMemFreeList指向的内存块的地址之后把这个地址return出去使用。
那么问题来了：为什么没有遍历空块的过程？
原因就在于上面所提到的那种巧妙地方法，每个内存块的前32位保存的是下一个空块的地址，在获取到OSMemFreeList后OSMemFreeList指针应该要指向下一个空块，而下一个空块就在当前获取到的空块的第一个地址里面，所以在26行把OSMemFreeList指针指向了当前获取到的内存块的第一个地址所指向的内存块处。
最后就是内存释放函数
1 INT8U OSMemPut (OS_MEM *pmem, 2 void *pblk) 3 { 4 #if OS_CRITICAL_METHOD == 3u /* Allocate storage for CPU status register */ 5 OS_CPU_SR cpu_sr = 0u; 6 #endif 7 8 9 10 #if OS_ARG_CHK_EN > 0u 11 if (pmem == (OS_MEM *)0) { /* Must point to a valid memory partition */ 12 return (OS_ERR_MEM_INVALID_PMEM); 13 } 14 if (pblk == (void *)0) { /* Must release a valid block */ 15 return (OS_ERR_MEM_INVALID_PBLK); 16 } 17 #endif 18 OS_ENTER_CRITICAL(); 19 if (pmem->OSMemNFree >= pmem->OSMemNBlks) { /* Make sure all blocks not already returned */ 20 OS_EXIT_CRITICAL(); 21 return (OS_ERR_MEM_FULL); 22 } 23 *(void **)pblk = pmem->OSMemFreeList; /* Insert released block into free block list */ 24 pmem->OSMemFreeList = pblk; 25 pmem->OSMemNFree++; /* One more memory block in this partition */ 26 OS_EXIT_CRITICAL(); 27 return (OS_ERR_NONE); /* Notify caller that memory block was released */ 28 }
同样，最关键的代码在红色部分。首先把要释放的内存块的首地址指向OSMemFreeList指向的地址(这个地址就是下一个空块的首地址)，之后把OSMemFreeList指向刚刚释放的内存块的首地址。只用了一行代码就把一个要释放的内存块插入了空块链表，再调整OSMemFreeList指针指向刚释放的内存块，就是这么简单粗暴！！！
一句话总结：ucos-II的内存管理利用内存块的首地址保存下一个空块的首地址的方式把所有空块链接起来。至少有一个好处，那就是在获取内存块的时候少了一个对整个内存块进行遍历找出空块的过程！