C free()如何工作? [英] How does C free() work?

问题描述

可能重复:
malloc()和free()如何工作

Possible Duplicate:
How malloc() and free() work

#include <stdio.h>
#include <stdlib.h>

int * alloc()
{
    int *p = (int *)calloc(5,4);
    printf("%d\n",p);
    return p;
}

int main()
{
 int *p = alloc();

 free(p);
 printf("%d\n",p);
 p[0] = 1;
 p[1] = 2;
 printf("%d %d\n",p[0],p[1]);
}

对于代码段，我首先分配5个整数.然后我释放内存. 当我打印f p时，为什么p sill具有与首先分配的内存地址相同的值? 而且我还可以为p [0]和p [1]赋值.这是否意味着free()什么也不做?有一次，我 分配内存，虽然我已经释放了它，但以后可以使用.

As to the code segment, I allocate 5 ints,first. And then I free the memory. When I printf p, why does p sill have a value same to the memory address allocated first? And I also can assign value to p[0] and p[1]. Does this mean free() do nothing? Once I allocate memory, I can use later though I have freed it.

推荐答案

free释放该地址的内存 .它不会更改p变量本身.但是，在那之后用p进行任何操作都是未定义的行为.如果您在释放后立即使用它，它似乎可以工作，但是它仍然是完全错误的，并且可能导致崩溃甚至更糟.

free releases the memory at that address. It doesn't change the p variable itself. However, doing anything with p after that point is undefined behavior. It may seem to work if you use it immediately after freeing, but it's still completely wrong, and could cause a crash or worse.

free是特定于实现的.但是，在大多数实现中，它将写入堆中的簿记数据，以注意该内存现在可用.例如，它可能将特定的块标记为未使用，或将其与相邻的空闲块组合.

free is implementation-specific. However, on most implementations, it will write to book-keeping data in the heap to note that the memory is now available. For instance, it might mark a particular chunk as unused, or combine the chunk with an adjacent free one.

请注意，对于指针使用%d也是未定义的.

Note that using %d for a pointer is also undefined.

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