嵌入式C语言中动态内存使用常见错误与预防

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。
  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