C 指针、空闲内存

Question

我有这些结构:

typedef struct memory_slice {
    size_t startAdd; //Start of slice
    size_t dim; //Dim in bytes
    void *slice; //Chunk of memory
    struct memory_sclice *next; //Next element in list
} ms;

typedef struct heap_s {
    struct memory_slice *empty; //List of ptr to free memory
    struct memory_slice *used;
    heap_policy_t pol;
} heap_t;

我创建了两个函数作为构造函数和析构函数:

heap_t *heapCreate (size_t heapSize) {
    heap_t *p = malloc(sizeof(heap_t));

    if (heapSize != 0) {
        p->empty = malloc(sizeof(ms)); //Only one chunk
        p->empty->slice = malloc (sizeof(heapSize));
        p->empty->dim = heapSize;
    } else {
        p->empty = malloc(sizeof(ms));
        size_t ap = 1024;
        p->empty->slice = malloc (sizeof(ap));
        p->empty->dim = ap;
    }

    p->empty->startAdd = 0;
    p->empty->next = NULL;
    p->used = NULL;
    p->pol = first_fit;

    return p;
}

//Destructor of struct
void heapDestroy (heap_t *h) {
    //Free all slices
    struct memory_sclice *app;

    while (h->empty != NULL) {
        free(h->empty->slice);
        app = h->empty;
        h->empty = h->empty->next;
        free(app);
    }

    while (h->used != NULL) {
        free(h->used->slice);
        app = h->used;
        h->used = h->used->next;
        //free(app->slice);
        free(app);
    }

    free(h); //Delete main structure
}

这段代码可以工作，但我不明白我无法像这样“free(app->slice)”那样释放内存:

while (h->empty != NULL) {
        app = h->empty;
        free(app->slice);
        h->empty = h->empty->next;
        free(app);
    }

有人可以告诉我使用“free(app->slice)”的问题出在哪里吗？？

Answer 1

提供的代码有几个对struct memory_cslice的引用；这些需要更改为 struct memory_slice 。除非添加如下内容，否则代码不会编译:

typedef enum { first_fit, best_fit, worst_fit } heap_policy_t;

您拥有并使用过valgrind吗？您可以使用吗？

有两个对 malloc() 的有问题的调用(在实际代码中不相邻):

p->empty->slice = malloc(sizeof(heapSize));
...
p->empty->slice = malloc(sizeof(ap));

在每种情况下，sizeof() 的参数都是 size_t 类型，因此实际上这两个 block 分配了相同的空间量，并且很可能4 字节或 8 字节。通常，将 sizeof() 与 malloc() 一起使用是正确的；这是不正确的情况之一。如果您编写以下内容，则可以简化代码:

heap_t *heapCreate(size_t heapSize)
{
    heap_t *p = malloc(sizeof(heap_t));

    if (heapSize == 0)
        heapSize = 1024;
    p->empty = malloc(sizeof(ms));
    p->empty->slice = malloc(heapSize);
    p->empty->dim = heapSize;
    p->empty->startAdd = 0;
    p->empty->next = NULL;
    p->used = NULL;
    p->pol = first_fit;
    return p;
}

当然，您应该处理 malloc() 失败的情况，因此应该改进为:

heap_t *heapCreate(size_t heapSize)
{
    heap_t *p = malloc(sizeof(heap_t));
    if (p != 0)
    {
        if (heapSize == 0)
            heapSize = 1024;
        if ((p->empty = malloc(sizeof(ms))) == 0)
        {
            free(p);
            return 0;
        }
        if ((p->empty->slice = malloc(heapSize)) == 0)
        {
            free(p->empty);
            free(p);
            return 0;
        }
        p->empty->dim = heapSize;
        p->empty->startAdd = 0;
        p->empty->next = NULL;
        p->used = NULL;
        p->pol = first_fit;
    }
    return p;
}