数据在realloc之后写入

Question

我是一个允许您向游戏添加问题的功能。我使用realloc来增加内存，这样我就可以存储更多问题。

脚本：

struct Question* AddQuestions(int* amountQuest){
    struct Question* questionsLocation = NULL;
    int startQuestion = 0;//How many question from before.
    int amount;
    if (*amountQuest > 0){
        startQuestion = *amountQuest;
    }

    system("cls");
    printf("How many statement do you want to add? ");
    scanf_s("%d", &amount);
    printf("\n");

    *amountQuest = amount + startQuestion;//Total amount of questions
    //Realloc or allocate new block if null
    questionsLocation = (struct Question*)realloc(questionsLocation,*amountQuest * sizeof(struct Question));

    if (questionsLocation != NULL){ // Check if allocating worked
        //Add questions
        for (int i = startQuestion; i < *amountQuest; i++){//Starts att startQuestion so I don't override the old question.
            printf("Statement %d: ", i + 1);
            fflush(stdin);
            fgets(questionsLocation[i].question, 200, stdin);

            printf("Dificulty: ");
            scanf_s("%d", &questionsLocation[i].difficult);

            printf("Right answer [1 = true / 0 = false] : ");
            scanf_s("%d", &questionsLocation[i].rightAnswer);

            printf("\n");
        }

        return questionsLocation;
    }
    else{
        printf("Problem occurred, try again");
        fflush(stdin);
        getchar();
        return NULL;
    }
}

当我添加新问题时，旧的获取覆盖。 图片：当我添加第一个问题时没有问题

图片：当我添加更多问题时，我遇到了问题。

Answer 1

您的代码不会尝试保留早期的问题。它始终使用realloc()指针调用NULL，从而产生全新的分配：

struct Question* questionsLocation = NULL;
/* ...there are no assignments to questionsLocation here... */
questionsLocation = (struct Question*)realloc(questionsLocation,
                                      *amountQuest * sizeof(struct Question));

要保留数据，您需要保留从realloc()获得的指针，并在下次重新分配内存时使用它。

执行此操作的一种方法是更改​​函数以questionsLocation作为参数：

struct Question* AddQuestions(int* amountQuest, struct Question* questionsLocation) {
    ...
    questionsLocation = (struct Question*)realloc(questionsLocation, ...);
    ...
    return questionsLocation;
}

然后像这样使用它：

int amountQuest = 0;
struct Question* questionsLocation = NULL;
...
questionsLocation = AddQuestion(&amountQuest, questionsLocation);

（为清楚起见，我没有包含任何错误检查。你可能想添加一些。）

Answer 2

您需要将questionsLocation和startQuestion声明为static，这样他们才能将其值从一次调用保留到下一次。

static struct Question* questionsLocation = NULL;
static int startQuestion = 0;//How many question from before.

但是，更好的方法是让您的函数将这些值作为参数接收。

struct Question* AddQuestions(int* amountQuest, structQuestion*questionsLocation, int startQuestion){

Answer 3

realloc（）的正确方法是

尺寸应该能够容纳您之前的问题以及新问题。对于Eg：

n = num_of_old_questions + num_of_new_questions;

struct Question *temp = realloc(questionsLocation, sizeof(struct Question) * n); 

if(temp != NULL)
{
  questionsLocation = temp;
}

如果realloc()失败，那么您也将丢失原始指针数据，因此最好使用temp进行重新分配，并在将其指定给原始指针之前检查返回值