为什么我可以释放内存两次，但是在不同情况下却不能？

Question

This is the code I have currently.

int main() {
    double *t, *k;
    Item *a = calloc(1, sizeof(Item));
    a->w = malloc(sizeof(double));
    t = a->w;
    k = t;
    free(a->w);
    free(a);
    free(k);
    return 0;
}

In this example, I was able to compile and run without getting any explicit errors. However if I free the pointer, k, before the structure, a, is freed, then I receive a double free/heap corruption error.

free(a->w);
free(k);
free(a);

Additionally, I receive the same error if I do:

free(k);
free(a->w);
free(a);

Why is it that I can free the pointer, k after the structure has been freed without any explicit errors, but I cannot free the memory k is pointing twice until the structure a is freed?

Answer 1

This is an example of Undefined behavior . This means anything can happen, such as appearing to work, crashing immediately, crashing at some point later, or strange unexpected behavior, or the often mentioned nasal demons .

From the man page :

free() frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behaviour occurs . If ptr is NULL, no operation is performed.

So triggering a double-free error may occur but is not required.