全部博文（132）
分类： C/C++
2008-03-12 18:11:44
> > NULL, 0, \0 and nul?
This item was added on: 2003/03/13
NULL is a macro defined in several standard headers, 0 is an integer constant, '\0' is a character constant, and nul is the name of the character constant. All of these are *not* interchangeable:
NULL is to be used for pointers only since it may be defined as ((void *)0), this would cause problems with anything but pointers.
0 can be used anywhere, it is the generic symbol for each type's zero value and the compiler will sort things out.
'\0' should be used only in a character context.
nul is not defined in C or C++, it shouldn't be used unless you define it yourself in a suitable manner, like:
#define nul '\0'
Credit: Prelude
在需要其它類型的 0 的時候, 即便它可能工作也不能使用 NULL, 因為這樣做發出了錯誤的格式信息。(而且, ANSI 允許把 NULL 定義為 ((void *)0), 這在非指針的 上下文中完全無效。特別是, 不能在需要 ASCII 空字符 (NUL) 的地方用 NULL。如果有必要, 提供你自己的定義
#define NUL '\0'
用过
C++的Delphi的程序员都知道,C++中的NULL在Delphi中为nil,意思是空指针
在C++中 为 (void*)0 在Dlephi中 为 Pointer(0)
但是，这不是绝对，因为Handle
在C++中Handle,是一个指针
在Delphi中Handle,被理解为一个整数了,于是一切差别就从这里开始
先看一个函数吧!
CreateWindow
在C++的申明为：
HWND
CreateWindow(
LPCTSTR lpClassName, // pointer to
registered class name
LPCTSTR lpWindowName, //
pointer to window name
DWORD dwStyle, // window
style
int x, // horizontal position
of window
int y, // vertical position
of window
int nWidth, // window
width
int nHeight, // window
height
HWND hWndParent, // handle to
parent or owner window
HMENU hMenu, // handle to menu or
child-window identifier
HANDLE hInstance, // handle to
application instance
LPVOID lpParam // pointer to
window-creation data
);
让我们来看看有几个指针(7个，返回值，前两个参数，后四个参数)
让我们再来看看在Delphi中的申明：
function
CreateWindow(
lpClassName: PChar; // pointer
to registered class name
lpWindowName: PChar; // pointer
to window name
dwStyle:Integer; // window
style
x: Integer; // horizontal
position of window
y: Integer; // vertical
position of window
nWidth: Integer; // window
width
nHeight: Integer; // window
height
hWndParent: Integer; // handle to
parent or owner window
hMenu: Integer; // handle to menu
or child-window identifier
hInstance: Integer; // handle to
application instance
lpParam: Pointer // pointer to
window-creation data
): Integer;
有几个指针呢？（有三个，前两个和最后一个参数）
如果在调用此函数时，夸张一点，将所有指针（以C++）全用NULL代替的话，在Delphi中，函数调用应是这样的：
hWnd:= CreateWindow( ’’,’’, ws_Child,
0,0,100,100,0,0,0,nil); //此函数只是例子
可见如果NULL指的是Handle的话，在Delphi中，它等价与0；其他吗？等价于nil（PChar暂且搁下）(见第二篇)
以上是我的一些看法，愿大家指正
The language definition states that for each pointer type, there
is a special value -- the "null pointer" -- which is
distinguishable from all other pointer values and which is not
the address of any object or function. That is, the address-of
operator & will never yield a null pointer, nor will a
successful call to malloc. (malloc returns a null pointer when
it fails, and this is a typical use of null pointers: as a
"special" pointer value with some other meaning, usually "not
allocated" or "not pointing anywhere yet.")
A null pointer is conceptually different from an uninitialized pointer. A null pointer is known not to point to any object; an uninitialized pointer might point anywhere. See also questions , , and .
As mentioned in the definition above, there is a null pointer for each pointer type, and the internal values of null pointers for different types may be different. Although programmers need not know the internal values, the compiler must always be informed which type of null pointer is required, so it can make the distinction if necessary (see below).
References: Sec. 5.4 pp. 97-8; Sec. 5.4 p. 102; Sec. 5.3 p. 91; Sec. 3.2.2.3 p. 38.
According to the language definition, a constant 0 in a pointer context is converted into a null pointer at compile time. That is, in an initialization, assignment, or comparison when one side is a variable or expression of pointer type, the compiler can tell that a constant 0 on the other side requests a null pointer, and generate the correctly-typed null pointer value. Therefore, the following fragments are perfectly legal:
char *p = 0;
if(p != 0)
However, an argument being passed to a function is not necessarily recognizable as a pointer context, and the compiler may not be able to tell that an unadorned 0 "means" a null pointer. takes a variable-length, null-pointer-terminated list of character pointer arguments. To generate a null pointer in a function call context, an explicit cast is typically required, to force the 0 to be in a pointer context:
execl("/bin/sh", "sh", "-c", "ls", (char *)0);
If the (char *) cast were omitted, the compiler would not know
to pass a null pointer, and would pass an integer 0 instead.
(Note that many Unix manuals get this example wrong.)
When function prototypes are in scope, argument passing becomes an "assignment context," and most casts may safely be omitted, since the prototype tells the compiler that a pointer is required, and of which type, enabling it to correctly convert unadorned 0's. Function prototypes cannot provide the types for variable arguments in variable-length argument lists, however, so explicit casts are still required for those arguments. It is safest always to cast null pointer function arguments, to guard against varargs functions or those without prototypes, to allow interim use of non- compilers, and to demonstrate that you know what you are doing. (Incidentally, it's also a simpler rule to remember.)
Summary:
Unadorned 0 okay:
References: Sec. A7.7 p. 190, Sec. A7.14 p. 192; Sec. A7.10 p. 207, Sec. A7.17 p. 209; Sec. 4.6.3 p. 72; Sec. 3.2.2.3 .
NULL and how is it #defined?
As a matter of style, many people prefer not to have unadorned
0's scattered throughout their programs. For this reason, the
preprocessor macro NULL is #defined (by (void *)0, about which more
later). A programmer who wishes to make explicit the
distinction between 0 the integer and 0 the null pointer can
then use NULL whenever a null pointer is required. This is a
stylistic convention only; the preprocessor turns NULL back to 0
which is then recognized by the compiler (in pointer contexts)
as before. In particular, a cast may still be necessary before
NULL (as before 0) in a function call argument. (The table
under above applies for NULL as well as 0.)
NULL should only be used for pointers; see .
References: Sec. 5.4 pp. 97-8; Sec. 5.4 p. 102; Sec. 13.1 p. 283; Sec. 4.1.5 p. 99, Sec. 3.2.2.3 p. 38,
NULL be #defined on a machine which uses a nonzero
bit pattern as the internal representation of a null pointer?
Programmers should never need to know the internal
representation(s) of null pointers, because they are normally
taken care of by the compiler. If a machine uses a nonzero bit
pattern for null pointers, it is the compiler's responsibility
to generate it when the programmer requests, by writing "0" or
"NULL," a null pointer. Therefore, #defining NULL as 0 on a
machine for which internal null pointers are nonzero is as valid
as on any other, because the compiler must (and can) still
generate the machine's correct null pointers in response to
unadorned 0's seen in pointer contexts.
NULL were defined as follows:
#define NULL ((char *)0)
NULL
work?
Not in general. The problem is that there are machines which
use different internal representations for pointers to different
types of data. The suggested #definition would make uncast NULL
arguments to functions expecting pointers to characters to work
correctly, but pointer arguments to other types would still be
problematical, and legal constructions such as
FILE *fp =NULL;
could fail.
Nevertheless, C allows the alternate
#define NULL ((void *)0)
definition for NULL. Besides helping incorrect programs to work
(but only on machines with homogeneous pointers, thus
questionably valid assistance) this definition may catch
programs which use NULL incorrectly (e.g. when the ASCII NUL
character was really intended; see ).
References:
#define Nullptr(type) (type *)0
This trick, though popular in some circles, does not buy much. It is not needed in assignments and comparisons; see . It does not even save keystrokes. Its use suggests to the reader that the author is shaky on the subject of null pointers, and requires the reader to check the #definition of the macro, its invocations, and all other pointer usages much more carefully. See also .
if(p)" to test for non-
null pointers valid? What if the internal representation for
null pointers is nonzero?
When C requires the boolean value of an expression (in the if,
while, for, and do statements, and with the &&, ||, !, and ?:
operators), a false value is produced when the expression
compares equal to zero, and a true value otherwise. That is,
whenever one writes
if(expr)
where "expr" is any expression at all, the compiler essentially acts as if it had been written as
if(expr != 0)
Substituting the trivial pointer expression "p" for "expr," we have
if(p) is equivalent to if(p != 0)
and this is a comparison context, so the compiler can tell that the (implicit) 0 is a null pointer, and use the correct value. There is no trickery involved here; compilers do work this way, and generate identical code for both statements. The internal representation of a pointer does not matter.
The boolean negation operator, !, can be described as follows:
!expr is essentially equivalent to expr?0:1
It is left as an exercise for the reader to show that
if(!p) is equivalent to if(p == 0)
"Abbreviations" such as if(p), though perfectly legal, are
considered by some to be bad style.
See also .
References: Sec. A7.4.7 p. 204; Sec. 5.3 p. 91; Secs. 3.3.3.3, 3.3.9, 3.3.13, 3.3.14, 3.3.15, 3.6.4.1, and 3.6.5 .
NULL" and "0" are equivalent, which should I use?
Many programmers believe that "NULL" should be used in all
pointer contexts, as a reminder that the value is to be thought
of as a pointer. Others feel that the confusion surrounding
"NULL" and "0" is only compounded by hiding "0" behind a
#definition, and prefer to use unadorned "0" instead. There is
no one right answer. C programmers must understand that "NULL"
and "0" are interchangeable and that an uncast "0" is perfectly
acceptable in initialization, assignment, and comparison
contexts. Any usage of "NULL" (as opposed to "0") should be
considered a gentle reminder that a pointer is involved;
programmers should not depend on it (either for their own
understanding or the compiler's) for distinguishing pointer 0's
from integer 0's.
NULL should not be used when another kind of 0 is required,
even though it might work, because doing so sends the wrong
stylistic message. ( allows the #definition of NULL to be
(void *)0, which will not work in non-pointer contexts.) In
particular, do not use NULL when the ASCII null character (NUL)
is desired. Provide your own definition
#define NUL '\0'
if you must.
References: Sec. 5.4 p. 102.
NULL (rather than 0) in case
the value of NULL changes, perhaps on a machine with nonzero
null pointers?
No. Although symbolic constants are often used in place of
numbers because the numbers might change, this is not the
reason that NULL is used in place of 0. Once again, the
language guarantees that source-code 0's (in pointer contexts)
generate null pointers. NULL is used only as a stylistic
convention.
NULL is guaranteed to be 0, but the null pointer
is not?
When the term "null" or "NULL" is casually used, one of several
things may be meant:
NULL macro, which is #defined to be "0" or
"(void *)0". Finally, as red herrings, we have...
'\0') character, but not a null pointer, which
brings us full circle...
NULL" for sense 4.
C programmers traditionally like to know more than they need to
about the underlying machine implementation. The fact that null
pointers are represented both in source code, and internally to
most machines, as zero invites unwarranted assumptions. The use
of a preprocessor macro (NULL) suggests that the value might
change later, or on some weird machine. The construct
"if(p == 0)" is easily misread as calling for conversion of p to
an integral type, rather than 0 to a pointer type, before the
comparison. Finally, the distinction between the several uses
of the term "null" (listed above) is often overlooked.
One good way to wade out of the confusion is to imagine that C had a keyword (perhaps "nil", like Pascal) with which null pointers were requested. The compiler could either turn "nil" into the correct type of null pointer, when it could determine the type from the source code, or complain when it could not. Now, in fact, in C the keyword for a null pointer is not "nil" but "0", which works almost as well, except that an uncast "0" in a non-pointer context generates an integer zero instead of an error message, and if that uncast 0 was supposed to be a null pointer, the code may not work.
Follow these two simple rules:
NULL".
NULL" is an argument in a
function call, cast it to the pointer type expected by
the function being called.
The rest of the discussion has to do with other people's misunderstandings, or with the internal representation of null pointers (which you shouldn't need to know), or with C refinements. Understand questions , , and , and consider and , and you'll do fine.
If for no other reason, doing so would be ill-advised because it would unnecessarily constrain implementations which would otherwise naturally represent null pointers by special, nonzero bit patterns, particularly when those values would trigger automatic hardware traps for invalid accesses.
Besides, what would this requirement really accomplish? Proper understanding of null pointers does not require knowledge of the internal representation, whether zero or nonzero. Assuming that null pointers are internally zero does not make any code easier to write (except for a certain ill-advised usage of calloc; see ). Known-zero internal pointers would not obviate casts in function calls, because the size of the pointer might still be different from that of an int. (If "nil" were used to request null pointers rather than "0," as mentioned in , the urge to assume an internal zero representation would not even arise.)
The Prime 50 series used segment 07777, offset 0 for the null
pointer, at least for PL/I. Later models used segment 0, offset
0 for null pointers in C, necessitating new instructions such as
TCNP (Test C Null Pointer), evidently as a sop to all the extant
poorly-written C code which made incorrect assumptions. Older,
word-addressed Prime machines were also notorious for requiring
larger byte pointers (char *'s) than word pointers (int *'s).
The Eclipse MV series from Data General has three
architecturally supported pointer formats (word, byte, and bit
pointers), two of which are used by C compilers: byte pointers
for char * and void *, and word pointers for everything else.
Some Honeywell-Bull mainframes use the bit pattern 06000 for (internal) null pointers.
The CDC Cyber 180 Series has 48-bit pointers consisting of a ring, segment, and offset. Most users (in ring 11) have null pointers of 0xB00000000000.
The Symbolics Lisp Machine, a tagged architecture, does not even
have conventional numeric pointers; it uses the pair
Depending on the "memory model" in use, 80*86 processors (PC's) may use 16 bit data pointers and 32 bit function pointers, or vice versa.
The old HP 3000 series computers use a different addressing scheme for byte addresses than for word addresses; void and char pointers therefore have a different representation than an int (structure, etc.) pointer to the same address would have.
This message, which occurs only under MS-DOS (see, therefore, ) means that you've written, via an uninitialized and/or null pointer, to location zero.
A debugger will usually let you set a data breakpoint on location 0. Alternately, you could write a bit of code to copy 20 or so bytes from location 0 into another buffer, and periodically check that it hasn't changed.