复制到指针内存？ [英] copy to pointer memory ?

问题描述

嗨

我有以下代码。


int * p;

p = malloc（10）;

int * j;

j = p;

免费（p）;


这是合法吗？ j还会指向前面提到的相同的内存吗？

我可以使用j访问该内存吗？


我在某个地方读过，虽然免费是

，但内存仍然可用。

所以在我的情况下j应该仍然指向那个

记忆吧？


MC

Hi
I have the following piece of code.

int *p;
p = malloc(10);
int *j;
j = p;
free(p);

Is this legal ? will j still point to the same
memory that p pointed to earlier ?
Can I access that memory using j ?

I read somewhere that even though free is
called, the memory is still available.
so in my case j should still point to that
memory right ?

MC

推荐答案

" MC" <毫安********* @ yahoo.com>写在

< bj ********** @ zcars0v6.ca.nortel.com>：

"MC" <ma*********@yahoo.com> wrote in
<bj**********@zcars0v6.ca.nortel.com>:

嗨
我有以下代码。

int * p;
p = malloc（10）;
int * j;
j = p;
免费（p）;

这是合法的吗？
是的。

还会指向与之前指出的相同的内存吗？
是的，p仍然指向那里。

我可以使用j访问该内存吗？
不可以。你不能使用它，因为你已经

免费（）了。

我在某个地方看到即使免费也是
调用，内存仍然可用。
这无法保证 - 它调用未定义的行为。

如果你试图访问内存，/任何/可能发生。

所以在我的案例j应该仍然指向内存对吗？
对，但见上文。
MC

Hi
I have the following piece of code.

int *p;
p = malloc(10);
int *j;
j = p;
free(p);

Is this legal ? Yes.
will j still point to the same
memory that p pointed to earlier ? Yes, and p still points there, too.
Can I access that memory using j ? No. You are not allowed to use it, since you have already
free()''d it.

I read somewhere that even though free is
called, the memory is still available. This is not guaranteed by any means - it invokes undefined behaviour.
If you try to acces the memory, /anything/ can happen.
so in my case j should still point to that
memory right ? Right, but see above.
MC

Irrwahn

-

今天没有信号。

Irrwahn
--
No sig today.

2003年9月3日星期三，MC写道：

On Wed, 3 Sep 2003, MC wrote:

嗨
我有以下代码。

int * p;
p = malloc（10）;
int * j;
j = p;
free（p）; 我可以用j访问那个内存吗？

我读到的地方虽然免费是
打电话，记忆仍然可用。
所以在我的情况下，j仍然应该指向那个
记忆吧？

MC

Hi
I have the following piece of code.

int *p;
p = malloc(10);
int *j;
j = p;
free(p);

Is this legal ? will j still point to the same
memory that p pointed to earlier ?
Can I access that memory using j ?

I read somewhere that even though free is
called, the memory is still available.
so in my case j should still point to that
memory right ?

MC

[来自常见问题]

--------------------------------- -----------------------------------

7.20：你不能你释放后使用动态分配的内存，

你可以吗？


答：不会。某些早期的malloc（）文档说明了

释放内存的内容未受干扰，但是这个错误的建议保证从来都不是通用的，而且不需要

C标准。


很少有程序员会使用故意释放内存的内容

，但很容易这么做。考虑

以下（正确）代码，用于释放单链表：


struct list * listp，* nextp;

for（listp = base; listp！= NULL; listp = nextp）{

nextp = listp-> next;

free（listp）;

}


并注意如果更明显的循环迭代将会发生什么

表达式listp = listp-> next使用了，没有临时

nextp指针。


参考文献：K& R2 Sec。 7.8.5 p。 167; ISO秒7.10.3;理由

秒4.10.3.2; H& S Sec。 16.2 p。 387; CT& P Sec。 7.10 p。 95.


7.21：为什么调用free（）后指针无效？

使用（赋值，比较）指针是多么不安全价值之后

它被释放了吗？


答：当你调用free（）时，传递的内存是指

指针被释放，但调用者指针

的值可能保持不变，因为C'的值传递语义

表示调用函数永远不会永久地改变它们的参数的价值

。 （另见问题4.8。）


严格来说，已释放的指针值无效，* b $ b无效，*任何*使用它，甚至如果没有被解除引用，理论上可以导致麻烦，虽然作为一个质量的实施问题，大多数实现可能都不会出现

out他们的方式为无害的指针无害使用产生例外。


参考文献：ISO Sec。 7.10.3;基本原理3.2.2.3。

====================================== ============ =================

希望这会有所帮助，


-dj

[ from the FAQ ]
--------------------------------------------------------------------
7.20: You can''t use dynamically-allocated memory after you free it,
can you?

A: No. Some early documentation for malloc() stated that the
contents of freed memory were "left undisturbed," but this ill-
advised guarantee was never universal and is not required by the
C Standard.

Few programmers would use the contents of freed memory
deliberately, but it is easy to do so accidentally. Consider
the following (correct) code for freeing a singly-linked list:

struct list *listp, *nextp;
for(listp = base; listp != NULL; listp = nextp) {
nextp = listp->next;
free(listp);
}

and notice what would happen if the more-obvious loop iteration
expression listp = listp->next were used, without the temporary
nextp pointer.

References: K&R2 Sec. 7.8.5 p. 167; ISO Sec. 7.10.3; Rationale
Sec. 4.10.3.2; H&S Sec. 16.2 p. 387; CT&P Sec. 7.10 p. 95.

7.21: Why isn''t a pointer null after calling free()?
How unsafe is it to use (assign, compare) a pointer value after
it''s been freed?

A: When you call free(), the memory pointed to by the passed
pointer is freed, but the value of the pointer in the caller
probably remains unchanged, because C''s pass-by-value semantics
mean that called functions never permanently change the values
of their arguments. (See also question 4.8.)

A pointer value which has been freed is, strictly speaking,
invalid, and *any* use of it, even if is not dereferenced, can
theoretically lead to trouble, though as a quality of
implementation issue, most implementations will probably not go
out of their way to generate exceptions for innocuous uses of
invalid pointers.

References: ISO Sec. 7.10.3; Rationale Sec. 3.2.2.3.
================================================== =================
Hope this helps,

-dj

MC写道：

您好
我有以下代码。

int * p;
p = malloc（10）;
int * j;
j = p;
免费（p）;

这合法吗？


在C99中，是的。在C89中，no：可执行语句不能在声明之前使用
。但是一个简单的重新排列使得它在C89中也是合法的：


int * p;

int * j;

p = malloc（10）;

j = p;

free（p）;

将j指向p先前指出的记忆是什么？


号。更准确地说，符合

计划不可能找到，这意味着任何试图找出

都会调用未定义的行为。

我可以使用j访问该内存吗？


编号房屋;这是malloc（）调用。在一张名为p的纸条上写下它的地址。制作一张

的那张纸条的复印件并称之为j。给你友好的当地纵火犯

`p''让他在指定的地址烧掉房子。

你口袋里还有'j''的事实并不代表

烧毁的房子是适合居住的。

我在某个地方看到即使被调用免费，内存仍然可用。


我在某处看到地球是扁平的。

所以在我的情况下j应该仍然指向那个
记忆吧？

Hi
I have the following piece of code.

int *p;
p = malloc(10);
int *j;
j = p;
free(p);

Is this legal ?
In C99, yes. In C89, no: executable statements cannot
precede declarations. But a simple rearrangement makes it
legal in C89, too:

int *p;
int *j;
p = malloc(10);
j = p;
free (p);
will j still point to the same
memory that p pointed to earlier ?
No. More precisely, "It''s impossible for a conforming
program to find out," meaning that any attempt to find out
invokes undefined behavior.
Can I access that memory using j ?
No. Build a house; that''s the malloc() call. Write
down its address on a slip of paper called `p''. Make a
photocopy of that slip of paper and call it `j''. Give
`p'' to your friendly local arsonist and have him burn
down the house at the indicated address. The fact that
you''ve still got `j'' in your pocket doesn''t mean the
burned-out house is habitable.
I read somewhere that even though free is
called, the memory is still available.
I read somewhere that the Earth is flat.
so in my case j should still point to that
memory right ?

这是不可能的。尝试使用以前指向的内存`j''

（它已被烧毁，还记得吗？）并且没有告诉可能发生的事情。




推荐阅读：comp.lang.c的第7节经常

问答（FAQ）列表

http://www.eskimo.com/~scs/C-faq /top.html


-
呃*********@sun.com

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