free（）问题 [英] free() question
问题描述
如果这是一个愚蠢的问题，我道歉。我只是好奇...是
有任何理由为什么free（）在给出之后没有将指针（传递给
）设置为NULL内存回到堆？我决定
为free（）编写一个包装器来做这件事，我想知道为什么free（）
本身并没有这样做。
Aaron
-
/ usr / bin / fortune说：
你怎么能在系统工作人如此拥挤？
Aaron Walker写道：
我很抱歉这是一个愚蠢的问题。我只是很好奇......是否有任何理由为什么free（）在将内存返回堆后没有将指针（传递给
它）设置为NULL？我决定为free（）编写一个包装器来做这件事，我想知道为什么free（）
本身并没有这样做。
没有愚蠢的问题。按照设计，free（）接收一个值，即要释放的分配的
地址。它不知道
的价值来自哪里。考虑..
int * arr，* tmp;
arr = malloc（N * sizeof * arr）;
tmp = arr;
... arr和tmp都保存malloc（）返回的内存地址。它现在无论我打电话都没有任何区别。
免费（arr）;
或者
free（tmp）;
...因为free（）函数接收到分配的地址而且
没有关于arr或者tmp。 free（）不能将
设置为NULL。
-
Joe Wright http://www.jw-wright.com
所有内容都应该是虽然简单，但并不简单。
---阿尔伯特爱因斯坦---
Aaron Walker< ka **** *@REMOVETHIScfl.rr.com>在消息新闻中写道：< 3P ********************* @ twister.tampabay.rr。 com> ...如果这是一个愚蠢的问题，我道歉。我只是好奇......是否有任何理由为什么free（）在将内存返回堆后没有将指针（传递给
它）设置为NULL？
这不是一个愚蠢的问题，但*是*常见问题（7.21）
如果free（）修改它的论点是这样的，它不可实现
作为C函数。
-thomas
Aaron Walker写道：
如果这是一个愚蠢的问题，我道歉。我只是很好奇......是否有任何理由为什么free（）在将内存返回堆后没有将指针（传递给
它）设置为NULL？我决定为free（）编写一个包装器来做这件事，我想知道为什么free（）
本身并没有这样做。
Aaron
有些用法不相信这会是一个特别有用的功能。
如果目标是能够按顺序测试指针确定
是否仍指向有效的可访问内存，针对NULL的测试
是不够的。考虑：
int * p1 = malloc（5 * sizeof * p1）;
int * p2 = p1;
magic_free_and_NULL （p1）;
if（p1！= NULL）/ * OK * /
{
p1 [0] = 1234;
}
if（p2！= NULL）/ * BAM！未定义。 * /
{
p2 [1] = 4321; / * BAM！未定义。 * /
}
在这个例子中，测试p2甚至都不安全，更不用说解除引用
了。 （已释放的内存指针的值是不确定的。）
另外，请考虑这个：
免费（p - 1 ）;
这里应该设置为NULL什么？
将释放的指针设置为NULL在某些情况下很方便，但不是很好
作为确定何时释放内存的一般技术。
-Kevin
-
我的电子邮件地址有效，但会定期更改。
要联系我，请使用最近发布的地址。
I apologize if this is a stupid question. I was just curious... is
there any reason why free() doesn''t set the pointer (that was passed to
it) to NULL after giving the memory back to the heap? I decided to
write a wrapper for free() to do just that, and I wondered why free()
itself didn''t do it.
Aaron
--
/usr/bin/fortune says:
How can you work when the system''s so crowded?
Aaron Walker wrote:
I apologize if this is a stupid question. I was just curious... is
there any reason why free() doesn''t set the pointer (that was passed to
it) to NULL after giving the memory back to the heap? I decided to
write a wrapper for free() to do just that, and I wondered why free()
itself didn''t do it.
There are no stupid questions. As designed, free() receives a value, the
address of the allocation to be freed. It doesn''t know where the value
came from. Consider..
int *arr, *tmp;
arr = malloc(N * sizeof *arr);
tmp = arr;
...Both arr and tmp hold the address of memory returned by malloc(). It
doesn''t make any difference now whether I call..
free(arr);
or
free(tmp);
...because the free() function receives the address of the allocation and
no information about arr or tmp at all. It is not possible for free() to
set them to NULL.
--
Joe Wright http://www.jw-wright.com
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---
Aaron Walker <ka*****@REMOVETHIScfl.rr.com> wrote in message news:<3P*********************@twister.tampabay.rr. com>...I apologize if this is a stupid question. I was just curious... is
there any reason why free() doesn''t set the pointer (that was passed to
it) to NULL after giving the memory back to the heap?
It''s not a stupid question, but it *is* a FAQ (7.21)
If free() modified its argument like this it wouldn''t be implementable
as a C function.
-thomas
Aaron Walker wrote:
I apologize if this is a stupid question. I was just curious... is
there any reason why free() doesn''t set the pointer (that was passed to
it) to NULL after giving the memory back to the heap? I decided to
write a wrapper for free() to do just that, and I wondered why free()
itself didn''t do it.
Aaron
Some of use don''t believe this would be a particularly useful feature.
If the goal is to be able to test a pointer in order to determine
whether it still points to valid accessible memory, a test against NULL
is not sufficient. Consider:
int *p1 = malloc(5 * sizeof *p1);
int *p2 = p1;
magic_free_and_NULL(p1);
if (p1 != NULL) /* OK */
{
p1[0] = 1234;
}
if (p2 != NULL) /* BAM! Undefined. */
{
p2[1] = 4321; /* BAM! Undefined. */
}
In this example, it''s not even safe to test p2, let alone dereference
it. (The value of a pointer to memory that has been freed is indeterminate.)
Also, consider this:
free(p - 1);
What should be set to NULL here?
Setting freed pointers to NULL is handy in some cases, but is not good
as a general technique for determining when memory has been freed.
-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.
这篇关于free（）问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
