c - realloc 在 vi​​sual studio 中失败，但可以使用 gcc (ntdll.dll)

Question

我使用的是 Visual Studio 2017，我在 C 中有这段代码。当我在 VS 中运行时它失败了，但是当我用 gcc 编译时它运行完美。错误是这样的:

Exception thrown at 0x779BE643 (ntdll.dll) in GestaoFinanceira.exe: 0xC0000005: Access violation reading location 0x00000009.

这是我的代码。

我正在使用这个结构类型:

struct budget {
    struct tm time;
    float valor;
};

struct geral {
    struct budget *b;
    int nelem_budget;
};

在 main 中，我调用了一个函数，该函数将从文件中读取并保存值是结构 g->b[]作为 g->nelem_budget 中的元素数

int main() {
    struct geral *g = (struct geral *)malloc(sizeof(struct geral));

    readFileBudget(g);
    // just to check if the read was ok
    showBudget(g->b, g->nelem_budget);

    addBudget(g);
    // to check again if the item added is ok
    showBudget(g->b, g->nelem_budget);

    system("Pause");
    return 0;
}

这是错误出现的时间:

void addBudget(struct geral *g) {
    int month, year;
    float value;

    // in the future, this values are to be an user input
    month = 5;
    year = 2015;
    value = 5000;

    printf("Month: %d, Year: %d, Value: %f\n", month, year, value);

    g->b[g->nelem_budget].time.tm_mon = month;
    g->b[g->nelem_budget].time.tm_year = year;
    g->b[g->nelem_budget].valor = value;

    g->nelem_budget++;

    struct budget *tmp = NULL;
    tmp = g->b;
    g->b = realloc(g->b, g->nelem_budget * sizeof(struct budget));
    if (g->b == NULL) {          //reallocated pointer ptr1
        free(tmp);
        printf("error-addBudget->realloc");
        exit(EXIT_FAILURE);
    }

    // just for debug
    printf("month: %d, year: %d, value: %f\n", g->b[g->nelem_budget - 1].time.tm_mon, g->b[g->nelem_budget - 1].time.tm_year, g->b[g->nelem_budget - 1].valor);
}

我在 g->b 末尾有一个空值数组，这就是我首先添加值，然后调用 realloc 的原因.

Answer 1

您需要先重新分配，然后再添加新元素，而不是相反。

我不喜欢这个

struct budget *tmp = NULL;
    tmp = g->b;
    g->b = realloc(g->b, g->nelem_budget * sizeof(struct budget));
    if (g->b == NULL) {          //reallocated pointer ptr1
        free(tmp);
        printf("error-addBudget->realloc");
        exit(EXIT_FAILURE);
    }

有几个原因:

realloc 应该这样调用:

struct budget *tmp;

tmp = realloc(g->b, (g->nelem_budget + 1) * sizeof *tmp);

if(tmp == NULL)
{
    // error handling
    return 0; // return error code
}

g->b = tmp;
g->nelem_budget++;

不要在main 以外的函数中执行exit。而不是关闭程序， 使 addBudget 成功返回 1，失败返回 0。 addBudget 的调用者 应该决定在失败时做什么，而不是 addBudget。例如，如果你 把这些函数放在一个库中，你的程序会在出错时自动结束， 这有时是您不希望发生的事情。

所以，函数应该是这样的:

int addBudget(struct geral *g) {
    if(g == NULL)
        return 0;

    int month, year;
    float value;

    // in the future, this values are to be an user input
    month = 5;
    year = 2015;
    value = 5000;

    // reallocate first

    struct budget *tmp = NULL;
    tmp = realloc(g->b, (g->nelem_budget + 1) * sizeof *tmp);

    if(tmp == NULL)
    {
        fprintf(stderr, "error-addBudget->realloc\n");
        return 0;
    }

    g->b = tmp;

    // add new budget after reallocation
    printf("Month: %d, Year: %d, Value: %f\n", month, year, value);

    g->b[g->nelem_budget].time.tm_mon = month;
    g->b[g->nelem_budget].time.tm_year = year;
    g->b[g->nelem_budget].valor = value;

    g->nelem_budget++;

    // just for debug
    printf("month: %d, year: %d, value: %f\n", g->b[g->nelem_budget - 1].time.tm_mon, g->b[g->nelem_budget - 1].time.tm_year, g->b[g->nelem_budget - 1].valor);

    return 1;
}

然后在 main 中:

int main() {
    // using calloc to initialize
    // everything with 0
    struct geral *g = calloc(1, sizeof *g);

    if(g == NULL)
    {
        fprintf(stderr, "Not engough memory\n");
        return 1;
    }

    // you should also check of readFileBudget
    // fails
    readFileBudget(g);

    // just to check if the read was ok
    showBudget(g->b, g->nelem_budget);

    if(addBudget(g) == 0)
    {
        free(g->b);
        free(g);
        return 1;
    }

    // to check again if the item added is ok
    showBudget(g->b, g->nelem_budget);


    // don't forget to free the memory
    free(g->b);
    free(g);

    system("Pause");
    return 0;
}

另见:do not cast malloc

并且错误信息应该打印到stderr，而不是stdout。