使用指向第一个成员的指针调用 free 是否有效？

Question

可以在指向结构第一个成员的指针上调用 free 吗（结构是与 malloc 相关的结构）？我知道原则上指针无论如何都指向正确的东西......

struct s {int x;};
//in main
struct s* test;
test = (struct s*) malloc(sizeof(*test));
int* test2;
test2 = &(test->x);
free(test2); //is this okay??

还有，如果int x换成struct，答案会不会改变？

更新：我为什么要写这样的代码？

struct s {int x;};
struct sx1 {struct s test; int y;}; //extending struct s
struct sx2 {struct s test; int z;}; //another
// ** some functions to keep track of the number of references to each variable of type struct s 
int release(struct s* ptr){
  //if the number of references to *ptr is 0 call free on ptr
}
int main(){
    struct sx1* test1;
    struct sx2* test2;
    test1 = (sx1*) malloc(sizeof(*sx1));
    test2 = (sx2*) malloc(sizeof(*sx2));
    //code that changes the number of references to test1 and test2, calling functions defined in **
    release(test1);
    release(test2);
}

Answer 1

根据 C11 标准，章节 §6.7.2.1

[...] There may be unnamed padding within a structure object, but not at its beginning.

这意味着结构的开头不能有任何填充。因此，第一个成员将具有与结构变量相同的地址。

free() 需要一个之前由 malloc() 或家人返回的指针。

在您的例子中，您传递的地址与 malloc() 返回的地址相同。所以，你很高兴。

Answer 2

是的，没关系。

6.7.2.1

Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

也就是说这样定义的：

struct s {int x;};
struct s* test;
test = (struct s*) malloc(sizeof(*test));
int* p = &(test->x);
free(p);

使用指向第一个成员的指针调用 free 是否有效？

Is it vali to call free with a pointer to the first member?

c

malloc

free

struct

padding