c - 如何为结构分配堆内存？

Question

我有以下代码

struct USER{

   int human_id_number;

   char first_name_letter;

   int minutes_since_sneezing;

} *administrator;

现在我要分配堆内存

这是我的尝试

administrator *newStruct = (administor*)malloc(sizeof(administrator));

不确定这是否正确...

Answer 1

struct USER {
   int human_id_number;
   char first_name_letter;
   int minutes_since_sneezing;
} *administrator;

这不仅仅是一个结构声明，它也是一个变量声明......它是一样的:

struct USER {
   int human_id_number;
   char first_name_letter;
   int minutes_since_sneezing;
};

struct USER *administrator;

因此，当您随后使用 sizeof(administrator) 时，您将得到“指针的大小”...这很可能不是你想要什么。

你可能想做更像这样的事情:

struct USER {
   int human_id_number;
   char first_name_letter;
   int minutes_since_sneezing;
};

int main(void) {
    struct USER *administrator;

    administrator = malloc(sizeof(*administrator));
    /* - or - */
    administrator = malloc(sizeof(struct USER));

    /* check that some memory was actually allocated */
    if (administrator == NULL) {
        fprintf(stderr, "Error: malloc() returned NULL...\n");
        return 1;
    }

    /* ... */

    /* don't forget to free! */
    free(administrator)

    return 0;
}

sizeof(*administrator) 和 sizeof(struct USER) 都会给你“USER 结构的大小”，因此，malloc() 的结果将是指向足够内存以保存结构数据的指针。