C中为NULL [英] NULL in C
问题描述
C程序员在写作时被指责编写平台相关代码
int * x = 0;
他们必须写
int * x = NULL;
因为有些平台不代表NULL，因为所有位都为零。
C ++程序员似乎没有这个问题，只要他们想要
，他们就可以写0。为什么会这样？
C programmers get accused of writing platform dependant code when they write
int *x = 0;
They have to write
int *x = NULL;
because some platforms don''t represent NULL as all bits zero.
C++ programmers don''t seem to have this problem, they can write 0 whenever
they want. Why is that?
推荐答案
Serve La写道：
Serve La wrote:
C程序员在编写时被指责编写平台相关代码
int * x = 0;
他们必须写入
int * x = NULL;
因为有些平台不代表NULL，因为所有位都为零。
C programmers get accused of writing platform dependant code when they write
int *x = 0;
They have to write
int *x = NULL;
because some platforms don''t represent NULL as all bits zero.
0是一个空指针常量，当在指针上下文中使用时，无论实际位模式如何，都保证产生空指针
。
Jirka
0 is a null pointer constant and is guaranteed to yield a null pointer
when used in a pointer context, regardless of the actual bit pattern.
Jirka
" Serve La" < i@bleat.nospam.com>在消息中写道
news：bm ********** @ news1.tilbu1.nb.home.nl ...
"Serve La" <i@bleat.nospam.com> wrote in message
news:bm**********@news1.tilbu1.nb.home.nl...
C程序员被指控编写平台相关代码时写入
int * x = 0;
他们必须编写
int * x = NULL;
因为某些平台不代表NULL零点。
C ++程序员似乎没有这个问题，他们可以随时写出0。为什么？
C programmers get accused of writing platform dependant code when they write
int *x = 0;
They have to write
int *x = NULL;
because some platforms don''t represent NULL as all bits zero.
C++ programmers don''t seem to have this problem, they can write 0 whenever
they want. Why is that?
实际上，NULL应该是0.符合C编译器
识别值0（当用作指针时） ）是一个
特殊值，表示NULL。即使实现
内部使用其他二进制值来表示NULL
指针，值0也是源代码使用的值。这个
包括赋值，比较，解除引用和转换
在整数和指针类型之间。编译器将
从源代码中的0转换为其内部表示
的NULL指针。
这就是为什么你可以写：
如果（x）
{
// x是非空的
}
与以下内容完全相同：
if（NULL！= x）
{
// x是非空的
}
也完全相同：
如果（0！= x）
{
// x是非空的
}
C编译器知道0正在指针类型
表达式中使用并做正确的事情。
-
------------------------------------------ ----
Jeffrey D. Smith
Farsight Systems Corporation
24 BURLINGTON DRIVE
LONGMONT，CO 80501 -6906
303-774-9381
http：/ /www.farsight-systems.com
z / Debug调试在IBM z / OS上运行的Systems / C程序！
Actually, NULL is supposed to be 0. Conforming C compilers
recognize the value 0 (when used as a pointer) to be a
special value that means NULL. Even when the implementation
internally uses some other binary value to mean a NULL
pointer, the value 0 is what is used the source code. This
includes assignment, comparison, dereferencing, and casting
between integer and pointer types. The compiler translates
from 0 in the source code into its internal representation
of a NULL pointer.
This is why you can write:
if(x)
{
// x is non-NULL
}
which is exactly the same as:
if(NULL != x)
{
// x is non-NULL
}
which is also exactly the same as:
if(0 != x)
{
// x is non-NULL
}
The C compiler knows that 0 is being used in a pointer-type
expression and does the right thing.
--
----------------------------------------------
Jeffrey D. Smith
Farsight Systems Corporation
24 BURLINGTON DRIVE
LONGMONT, CO 80501-6906
303-774-9381
http://www.farsight-systems.com
z/Debug debugs your Systems/C programs running on IBM z/OS!
2003年10月18日星期六14:53:03 GMT
" xarax" < xa的*** @ email.com>写道：
On Sat, 18 Oct 2003 14:53:03 GMT
"xarax" <xa***@email.com> wrote:
" Serve La" < i@bleat.nospam.com>在消息中写道
新闻：bm ********** @ news1.tilbu1.nb.home.nl ...
"Serve La" <i@bleat.nospam.com> wrote in message
news:bm**********@news1.tilbu1.nb.home.nl...
C程序员被指责编写平台相关代码当他们写int * x = 0;
他们必须写
int * x = NULL;
因为有些平台不代表NULL，因为所有位都为零。 c ++程序员似乎没有这个问题，他们可以随时写0
。为什么会这样？
实际上，NULL应该是0。
C programmers get accused of writing platform dependant code when
they write int *x = 0;
They have to write
int *x = NULL;
because some platforms don''t represent NULL as all bits zero.
C++ programmers don''t seem to have this problem, they can write 0
whenever they want. Why is that?
Actually, NULL is supposed to be 0.
实际上它不需要是0.（void *）0也是一个有效的定义
的NULL。
符合C编译器
认识值0（当用作指针时）是一个
特殊值，意味着空值。
只有一个常量表达式评估为0.我不认为整数
包含值0是由标准保证的，因为它将是一个
如果空指针不是全部为零，则实现很痛。
即使实现内部使用其他二进制值来表示NULL
指针，值为0是源代码使用的。这包括整数和指针类型之间的赋值，比较，解引用和转换。编译器将
从源代码中的0转换为NULL指针的内部表示。
Actually it is not required to be 0. (void*)0 is also a valid definition
of NULL.
Conforming C compilers
recognize the value 0 (when used as a pointer) to be a
special value that means NULL.
Only a constant expression evaluation to 0. I don''t think an integer
containing a value 0 is guaranteed by the standard because it would be a
pain to implement if the null pointer was not all bits zero.
Even when the implementation
internally uses some other binary value to mean a NULL
pointer, the value 0 is what is used the source code. This
includes assignment, comparison, dereferencing, and casting
between integer and pointer types. The compiler translates
from 0 in the source code into its internal representation
of a NULL pointer.
< snip>
其余的看起来是正确的。
-
马克戈登
支付成为一个极客&高级软件开发人员
虽然我的电子邮件地址是垃圾邮件，但它是真实的，我读了它。
<snip>
the rest looked about right.
--
Mark Gordon
Paid to be a Geek & a Senior Software Developer
Although my email address says spamtrap, it is real and I read it.
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