如果NULL和0是等价的空指针常数，我应该使用哪个?在C常见问题解答列表中也解决了这个问题:

C programmers must understand that NULL and 0 are interchangeable in pointer contexts, and that an uncast 0 is perfectly acceptable. 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. It is only in pointer contexts that NULL and 0 are equivalent. 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. (Furthermore, ANSI allows the definition of NULL to be ((void *)0), which will not work at all 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'

如果你一定要的话。

(void*) 0是NULL， '\0'表示字符串的结束。

如果NULL和0是等价的空指针常数，我应该使用哪个?在C常见问题解答列表中也解决了这个问题:

C programmers must understand that NULL and 0 are interchangeable in pointer contexts, and that an uncast 0 is perfectly acceptable. 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. It is only in pointer contexts that NULL and 0 are equivalent. 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. (Furthermore, ANSI allows the definition of NULL to be ((void *)0), which will not work at all 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'

如果你一定要的话。

这三者在不同的上下文中定义了0的含义。

指针上下文-使用NULL，表示指针的值为0，与它是32位还是64位无关(一种情况是4字节，另一种情况是8字节的零)。 string context -表示数字0的字符的十六进制值为0x30，而NUL字符的十六进制值为0x00(用于终止字符串)。

当你查看内存时，这三个总是不同的:

NULL - 0x00000000 or 0x00000000'00000000 (32 vs 64 bit)
NUL - 0x00 or 0x0000 (ascii vs 2byte unicode)
'0' - 0x20

我希望这能解释清楚。

在ASCII表中，值为0x00的字节是称为NUL或NULL的特殊字符。在C语言中，因为你不应该在源代码中嵌入控制字符，所以这是用C字符串表示的，带有一个转义0，即\0。

但是一个真NULL不是一个值。它是一个值的缺失。对于指针来说，这意味着指针没有可指向的对象。在数据库中，它意味着字段中没有值(这与字段为空白、0或填充空格不同)。

给定的系统或数据库文件格式用来表示NULL的实际值不一定是0x00。

“NUL”不是0，而是指ASCII的NUL字符。至少，我看到它是这样使用的。空指针通常定义为0，但这取决于您正在运行的环境，以及您正在使用的操作系统或语言的规范。

在ANSI C中，空指针被指定为整数值0。所以任何不符合标准的世界都是不符合ANSI C的。