这段代码有什么问题？ [英] what's wrong with this code?
问题描述
你好，
我是C的新手，我正在阅读K& R，我在本书的代码中找到了
。 K& R表示，释放后释放的东西是不允许的。
并以此为例。
for（p = head; p！= NULL; p-> next）
free（p）;
它有什么问题？
好看的事情p-> next在第一次运行循环时，部分并没有真正推进指针p
，指针p被释放但是在
秒（因为它是没有提前并且已经被释放了它被释放了
秒。
现在我不记得那时会发生什么（通常它只是工作或
只是普通的崩溃，因为我很少遇到这种情况）。
希望这有帮助
TDR
cc0064263 < CA ************* @ yahoo.com>在消息中写道
新闻：DM ******************** @ comcast.com ...你好，
我是C的新手，我正在阅读K& R，我来到这本书的代码中。 K& R说，释放后释放的东西是不正确的
并以此为例。
for（p = head; p！= NULL; p-> next）
free（p）;
它出了什么问题？
在文章< DM ******************** @ comcast.com>中，cc0064263写道：你好，
我是C的新手，我正在阅读K& R，我来到这本书的代码中。 K& R说，释放后释放的东西是不正确的
并以此为例。
for（p = head; p！= NULL; p-> next）
free（p）;
它有什么问题？
至少有两个问题这段代码：p->接下来什么都不做
和p在被释放后被访问。
即使是第一个问题都没有（即代码说'p = p-> next''）
第二个仍然（可能）杀死你的pprogram。
while循环中的for循环如下所示：
p = head;
while（p！= NULL）
{
free（p）;
p = p-> next;
}
或许现在为什么在免费后访问p会更清楚？
为了做到这一点，你可以这样做：
p_type p = head;
p_type last = NULL;
while（p！= NULL）
{
last = p;
p = p-> next;
free（last）;
}
（其中p_type是某种指针类型）。在这里，你将p的旧值保存到
last，得到你想要的p值，然后*然后*释放保存的旧值。
HTH
rlc
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加入讨论这种语言的定义
ja *********** @ lists.sourceforge.net http://jail-ust.sourceforge.net
（发送邮件至 JA *********** @ lists.sourceforge .net ）
（我在第一个回复时写了一个错字）
TDR < TD ******* @ hotmail.com>在消息中写道
新闻：3f ******** @ news.tm.net.my ...
好看的东西p- >接着"部分并没有真正推进指针
p
作为第一次循环运行的结果，指针p被释放但是在
$ b循环的$ b第二次运行（因为它没有提前并且已经被释放）它
第二次被释放
。
现在我真的不记得会发生什么事情（通常它只是工作
或
因为我很少遇到这种情况而只是简单的崩溃）。
希望这有助于
TDR" cc0064263" < CA ************* @ yahoo.com>在消息中写道
新闻：DM ******************** @ comcast.com ...你好，
我是C的新手，我正在阅读K& R，我来到这本书的代码中。 K& R说，释放后释放的东西是不正确的
并以此为例。
for（p = head; p！= NULL; p-> next）
free（p）;
它有什么问题？
hello,
i am very new to C and i am reading K&R and i came
accross this code in the book. K&R says that it is
incorrect to free something after it has been freed
and gives this as an example.
for (p=head; p!= NULL; p->next)
free(p);
what''s wrong with it?
Well looks of things the "p->next" part doesn''t really advance the pointer p
as a result on the first run of the loop, the pointer p is freed but on the
second (since it was not advance and was already freed) it gets freed
second.
Now I don''t really remember what would happen then (usually it just works or
just plain crashes as I rarely met such cases).
hope this helps
TDR
"cc0064263" <ca*************@yahoo.com> wrote in message
news:DM********************@comcast.com...hello,
i am very new to C and i am reading K&R and i came
accross this code in the book. K&R says that it is
incorrect to free something after it has been freed
and gives this as an example.
for (p=head; p!= NULL; p->next)
free(p);
what''s wrong with it?
In article <DM********************@comcast.com>, cc0064263 wrote:hello,
i am very new to C and i am reading K&R and i came
accross this code in the book. K&R says that it is
incorrect to free something after it has been freed
and gives this as an example.
for (p=head; p!= NULL; p->next)
free(p);
what''s wrong with it?
There are at least two problems with this code: p->next doesn''t do anything
and p is accessed after it''s been freed.
Even if the first problem weren''t there (i.e. the code said `p = p->next'')
the second would still (possibly) kill your pprogram.
The for loop in while form looks like this:
p = head;
while (p != NULL)
{
free(p);
p = p->next;
}
perhaps it is a bit clearer now why p is accessed after the free?
To do it right, you can do it like this:
p_type p = head;
p_type last = NULL;
while (p != NULL)
{
last = p;
p = p->next;
free(last);
}
(in which p_type is some pointer type). Here, you save the old value of p to
last, get the value you want for p, and *then* free the saved, old value.
HTH
rlc
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Just: Jail Uses Silly Terms
Join the discussion on the definition of this language at
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(send mail to ja*********************@lists.sourceforge.net)
( I made a typo on the first reply)
"TDR" <td*******@hotmail.com> wrote in message
news:3f********@news.tm.net.my...
Well looks of things the "p->next" part doesn''t really advance the pointer
p
as a result on the first run of the loop, the pointer p is freed but on the
second run of the loop (since it was not advance and was already freed) it
gets freed a
second time.
Now I don''t really remember what would happen then (usually it just works
or
just plain crashes as I rarely met such cases).
hope this helps
TDR"cc0064263" <ca*************@yahoo.com> wrote in message
news:DM********************@comcast.com...hello,
i am very new to C and i am reading K&R and i came
accross this code in the book. K&R says that it is
incorrect to free something after it has been freed
and gives this as an example.
for (p=head; p!= NULL; p->next)
free(p);
what''s wrong with it?
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