NULL in C grammar

Question

我想知道如何用C语法表征NULL，它会不变？

Answer 1

NULL is a macro object defined in some standard header <stddef.h> . On my Debian/Sid/x86-64 system with GCC 4.8 it is defined as follow (lines 394 and following of the compiler specific /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h header file)

 /* A null pointer constant.  */

 #if defined (_STDDEF_H) || defined (__need_NULL)
 #undef NULL             /* in case <stdio.h> has defined it. */
 #ifdef __GNUG__
 #define NULL __null
 #else   /* G++ */
 #ifndef __cplusplus
 #define NULL ((void *)0)
 #else   /* C++ */
 #define NULL 0
 #endif  /* C++ */
 #endif  /* G++ */
 #endif  /* NULL not defined and <stddef.h> or need NULL.  */
 #undef  __need_NULL

BTW, a pointer which has been dereferenced is known to not be the null pointer value. In other words, GCC is permitted to optimize (assuming p has been declared as int *p; )

  int x= *p;
  if (!p) do_very_complex_stuff(); 
  // or: if (p == NULL) do_very_complex_stuff(); 

as simply

  int x= *p;

at least outside of freestanding programs.

^{IIRC, that optimization made a harsh debate between Linus Torvalds and some GCC developers.}

Pedantically the NULL pointer (ie the null pointer value) is perhaps not required to be all-zero bits, but I don't know any implementation doing that today.

Answer 2

NULL is not part of C grammar. NULL is #define d in stddef.h standard header.

Answer 3

像这样的宏：

#define NULL ((void*)0)

Answer 4

C11dr §6.3.2.3 3 NULL is a null pointer constant.

"An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant..." Footnote: "The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant."

Other pointer values could also be a "null pointer constant". They would all compare equal.

Answer 5

在C中，NULL不是语法的一部分，它只是一个宏。

Answer 6

NULL不是C语法的一部分，这就是为什么你必须包含一个标题来避免编译器警告（它可能取决于你的编译器设置）