Question

我有一点问题 - 出于某种原因，当我尝试将内存重新分配给指向结构的指针的指针时，为一个函数中的额外结构增加一个内存块的数量，Intellisense告诉我“表达式”必须是可修改的左值“。事实是，如果我尝试分配给input本身，一切都很好 - 问题在于我尝试重新分配到input + 1。

正在使用的结构：

struct Entry_t
{
    char *word;
    Year **years;
    char **syn;
    Meaning **mean;
    Entry *next;
};

struct Mean_t
{
    char *word;
    Entry **enword;
    Meaning *next;
};

功能：

/*
| Function: IsInMean
| Action: Checks whether the meaning already exists
| Input: Master meaning struct, the string
| Returns: The address of the necessary place if yes, NULL otherwise
*/
Meaning *IsInMean(Meaning *MEANS, char *str)
{
    if(MEANS)
    {
        if(strcmp(MEANS->word,str) == 0)
            return MEANS;
        else
            return IsInMean(MEANS + 1,str);
    }
    else
        return MEANS;
}

/*
| Function: FindMeanPlace
| Action: Checks where to wedge the meaning.
| Input: The master Meanings dictionary, the string
| Returns: The address of the place where to wedge, NULL if all are bigger.
*/
Meaning *FindMeanPlace(Meaning *MEANS, char *str)
{
    int cmp;
    if((cmp = strcmp(MEANS->word,str)) > 0)
        return NULL;
    else if(cmp < 0 && strcmp(MEANS->next->word,str) > 0)
        return MEANS;
    else
        return FindMeanPlace(MEANS + 1,str);
}

/*
| Function: NewMean
| Action: Creates and initializes a new meaning struct
| and places in it a new entry
| Input: Standard input, new entry, string with the meaning
| Returns: The address of the new meaning struct
*/
Meaning *NewMean(STANDARD_INPUT,Entry *new_e, char *str)
{
    Meaning *temp = (Meaning*)malloc(sizeof(Meaning));
    InitMean(temp);
    temp->word = (char*)calloc(strlen(str) + 1,sizeof(char));
    strcpy(temp->word,str);
    *(temp->enword) = new_e;
    return temp;
}

/*
| Function: SetMean
| Action: Sets the meanings field of an entry
| Input: Standard input, address of the new entry and
| the appropriate sub-string
| Returns: nada
*/
void SetMean(STANDARD_INPUT, Entry *new_e, char *str)
{
    char *cutout, delim[] = ",_";
    char **temp = NULL;
    int len = 0, cmp, index;
    Entry **entemp, *etemp;
    Meaning *input, *mtemp;
    cutout = strtok(str,delim);
    while(cutout)
    {
        temp = (char**)realloc(temp,(len + 1)*sizeof(char*));
        if(!temp)
            Pexit(STANDARD_C);
        *temp = (char*)calloc(strlen(cutout) + 1,sizeof(char));
        if(!(*temp))
            Pexit(STANDARD_C);
        strcpy(*temp,cutout);
        temp++;
        len++;
        cutout = strtok(NULL,delim);
    }
    QsortCust(STANDARD_C,temp,len + 1);
    while(temp)
    {
        index = 0;
        if(input = IsInMean(MEANS,*temp))
        {
            entemp = input->enword;
            if(strcmp(entemp[0]->word,new_e->word) > 0)
                //entemp + 1 = (Entry**)realloc(entemp,sizeof(entemp) + sizeof(Entry*));  "expression must be modifiable lvalue"
            while(entemp + index)
            {
                if((cmp = strcmp((entemp[index])->word,new_e->word)) < 0 && strcmp((entemp[index + 1])->word,new_e->word) > 0)
                {
                    //(entemp + index + 1) = (Entry**)realloc(entemp + index,sizeof(entemp + index) + sizeof(Entry*));  "expression must be modifiable lvalue"
                    //if(!(entemp + index + 1))
                    //  Pexit(STANDARD_C);
                }
                else if(cmp <0)
                {
                    index++;
                }
            }
        }
        else
        {
            input = FindMeanPlace(MEANS,*temp);
            mtemp  = input->next;
            input->next = NewMean(STANDARD_C,new_e,*temp);
        }
    }
}

STANDARD_INPUT被定义为主词典（第一个条目*），年度词典（第一年*）和含义词典（第一个含义*）。 STANDARD_C被定义为标准输入的变量名称。

Answer 1

如果你想做什么（根据评论）是放大涂鸦矢量entemp本身，但是将新空间添加到开头而不是结尾，你不能这样做一次操作。 realloc必须始终传递从malloc（或calloc或之前调用realloc）收到的取消偏移指针，并始终最后添加空间。您必须使用memmove自行滑动所有内容。例如：

nelements += 1;
entemp = realloc(entemp, nelements * sizeof(Entry *));
assert(entemp);
memmove(entemp, entemp+1, (nelements - 1) * sizeof(Entry *));

另外，正如我刚刚注意到的那样：sizeof(entemp) 不返回entemp指向的内存块的分配大小。它返回指针本身的大小。无法检索C堆上的块的分配大小;你必须自己跟踪它们。这就是我的nelements变量正在做的事情。

密切相关的强制性切线评论：不要强制转换realloc的返回值，就像不投放malloc或calloc的返回值一样。这不仅仅是一种风格的东西：当你不需要时可以隐藏错误！

不太相关但仍然是强制性的样式挑剔：NAMES_IN_ALL_CAPS，按照长期惯例，为常量保留。不要将它们用于变量名称。而扩展到整个变量/参数声明的魔术宏是Right Out。

函数中的指针“必须是可修改的左值”错误

1 个答案: