一． 常见错误与预防
1. 分配后忘记释放内存
void func(void)
{
p = malloc(len);
do_something(p);
return; /*错误！退出程序时没有释放内存*/
}
预防： 编写代码时malloc()和free()保证成对出现，避免忘记资源回收。
int func(void)
{
p = malloc(len);
if (condition)
return -1; /*错误！退出程序时没有释放内存*/
free(p);
return 0;
}
预防： 一旦使用动态内存分配，请仔细检查程序的退出分支是否已经释放该动态内存。
2. 释放内存调用错误指针
void func(void)
{
p = malloc(len);
val = *p++; /*错误！动态内存句柄不可移动*/
free(p);
}
预防： 千万不要修改动态内存句柄！可以另外赋值给其他指针变量，再对该动态内存进行访问操作。
3. 分配内存不够导致溢出
void func(void)
{
len = strlen(str);
p = malloc(len);
strcpy(p, str); /*错误！str的’’写到动态内存外*/
}
预防： 分配内存前仔细思考长度是否足够，千万注意字符串拷贝占用内存比字符串长度大1。
二． 自动查错机制
尽管在开发过程中坚守原则和谨慎编程甚至严格测试，然而内存泄露的错误还是难以杜绝，如何让系统自动查出内存泄露的错误呢？
一种比较好的方法是建立日志块，即每次分配内存时记录该内存块的指针和大小，释放时再去除该日志块，如果有内存泄露就会有对应的日志块记录这些内存没有释放，这样就可以提醒程序员进行查错。
有了上述日志块操作函数，再来实现动态内存分配与释放函数就很容易了。只有当处于DEBUG版本和打开内存调试DMEM_DBG时才进行日志登录，否则MallocExt()和FreeExt()函数与malloc()和free()是等价的，这样保证了系统处于发布版本时的性能。
（代码已经过严格测试，但这不是盈利的商业代码，即没有版权。但如果因代码错误带来的任何损失作者具有免责权利）
代码部分：
首先定义日志块结构体：
/* Log of dynamic memory usage */
typedef struct _dmem_log
{
struct _dmem_log *p_stNext; /* Point to next log */
const void *p_vDMem; /* Point to allocated memory by this pointer */
INT32S iSize; /* Size of the allocated memory */
} DMEM_LOG;
然后为该结构体开辟内存：
static DMEM_LOG *s_pstFreeLog; /* Point to free log pool by this pointer */
static INT8U s_byNumUsedLog;
static DMEM_LOG *s_pstHeadLog; /* Point to used log chain by this pointer */
/* Pool of dynamic memory log */
#define NUM_DMEM_LOG 20
static DMEM_LOG s_astDMemLog[NUM_DMEM_LOG];
下面是内存日志块的操作函数：初始化、插入日志和移除日志：
/********************************************************** * Initialize DMem Log
* Description : Initialize log of dynamic memory
* Arguments : void
* Returns : void
* Notes :
**********************************************************/
static void InitDMemLog(void)
{
INT16S nCnt;
/* Initialize pool of log */
for (nCnt = 0; nCnt < NUM_DMEM_LOG; ++nCnt)
{
/* Point to next one */
s_astDMemLog[nCnt].p_stNext = &s_astDMemLog[nCnt + 1];
}
s_astDMemLog[NUM_DMEM_LOG – 1].p_stNext = NULL;
s_pstFreeLog = &s_astDMemLog[0]; /* Point to the 1th log */
return;
}
/********************************************************** * Log DMem
* Description : Join an allocated memory into log pool
* Arguments : const void *p_vAddr point to address of this allocated memory by this pointer
* INT32S iSize size of this allocated memory
* Returns : void
* Notes :
**********************************************************/
static void LogDMem(const void *p_vAddr, INT32S iSize)
{
ASSERT(p_vAddr && iSize > 0);
DMEM_LOG *p_stLog;
#if OS_CRITICAL_METHOD == 3
OS_CPU_SR cpu_sr;
#endif
/* Get a log from free pool */
OS_ENTER_CRITICAL(); /* Avoid race condition on s_pstFreeLog */
if (!s_pstFreeLog)
{
OS_EXIT_CRITICAL();
PRINTF(“Allocate DMemLog failed.
“);
return;
}
p_stLog = s_pstFreeLog;
s_pstFreeLog = s_pstFreeLog->p_stNext;
OS_EXIT_CRITICAL();
/* Don’t need to protect this log that is free one currently */
p_stLog->p_vDMem = p_vAddr;
p_stLog->iSize = iSize;
/* Put this log into used chain */
OS_ENTER_CRITICAL(); /* Avoid race condition */
p_stLog->p_stNext = s_pstHeadLog;
s_pstHeadLog = p_stLog;
++s_byNumUsedLog;
OS_EXIT_CRITICAL();
return;
}
/********************************************************** * Unlog DMem
* Description : Remove an allocated memory from log pool
* Arguments : const void *p_vAddr point to address of this allocated memory by this pointer
* Returns : void
* Notes :
**********************************************************/
static void UnlogDMem(const void *p_vAddr)
{
ASSERT(p_vAddr);
DMEM_LOG *p_stLog, *p_stPrev;
#if OS_CRITICAL_METHOD == 3
OS_CPU_SR cpu_sr;
#endif
/* Search the log */
OS_ENTER_CRITICAL(); /*Avoid race condition */
p_stLog = p_stPrev = s_pstHeadLog;
while (p_stLog)
{
if (p_vAddr == p_stLog->p_vDMem)
{
break; /* Have found */
}
p_stPrev = p_stLog;
p_stLog = p_stLog->p_stNext; /* Move to next one */
}
if (!p_stLog)
{
OS_EXIT_CRITICAL();
PRINTF(“Search Log failed.
“);
return;
}
/* Remove from used pool */
if (p_stLog == s_pstHeadLog)
{
s_pstHeadLog = s_pstHeadLog->p_stNext;
}
else
{
p_stPrev->p_stNext = p_stLog->p_stNext;
}
–s_byNumUsedLog;
OS_EXIT_CRITICAL();
/* Don’t need to protect this log that is free one currently */
p_stLog->p_vDMem = NULL;
p_stLog->iSize = 0;
/* Add into free pool */
OS_ENTER_CRITICAL(); /* Avoid race condition */
p_stLog->p_stNext = s_pstFreeLog;
s_pstFreeLog = p_stLog;
OS_EXIT_CRITICAL();
return;
}
带日志记录功能的内存分配MallocExt()和内存释放FreeExt()函数：
/*********************************************************
* Malloc Extension
* Description : Malloc a block of memory and log it if need
* Arguments : INT32S iSize size of desired allocate memory
* Returns: void *NULL= failed, otherwise=pointer of allocated memory
* Notes :
**********************************************************/
void *MallocExt(INT32S iSize)
{
ASSERT(iSize > 0);
void *p_vAddr;
p_vAddr = malloc(iSize);
if (!p_vAddr)
{
PRINTF(“malloc failed at %s line %d.
“, __FILE__, __LINE__);
}
else
{
#if (DMEM_DBG && DBG_VER)
memset(p_vAddr, 0xA3, iSize); /* Fill gargage for debug */
LogDMem(p_vAddr, iSize); /* Log memory for debug */
#endif
}
return p_vAddr;
}
/**********************************************************
* Free Extension
* Description : Free a block of memory and unlog it if need
* Arguments : void * p_vMem point to the memory by this pointer
* Returns : void
* Notes :
**********************************************************/
void FreeExt(void *p_vMem)
{
ASSERT(p_vMem);
free(p_vMem);
#if (DMEM_DBG && DBG_VER)
UnlogDMem(p_vMem); /* Remove memory from log */
#endif
return;
}
编辑：黄飞