空终止字符串的基本原理是什么?

尽管我很喜欢C和c++，但我还是忍不住对空结尾字符串的选择抓耳挠脑:

Length prefixed (i.e. Pascal) strings existed before C Length prefixed strings make several algorithms faster by allowing constant time length lookup. Length prefixed strings make it more difficult to cause buffer overrun errors. Even on a 32 bit machine, if you allow the string to be the size of available memory, a length prefixed string is only three bytes wider than a null terminated string. On 16 bit machines this is a single byte. On 64 bit machines, 4GB is a reasonable string length limit, but even if you want to expand it to the size of the machine word, 64 bit machines usually have ample memory making the extra seven bytes sort of a null argument. I know the original C standard was written for insanely poor machines (in terms of memory), but the efficiency argument doesn't sell me here. Pretty much every other language (i.e. Perl, Pascal, Python, Java, C#, etc) use length prefixed strings. These languages usually beat C in string manipulation benchmarks because they are more efficient with strings. C++ rectified this a bit with the std::basic_string template, but plain character arrays expecting null terminated strings are still pervasive. This is also imperfect because it requires heap allocation. Null terminated strings have to reserve a character (namely, null), which cannot exist in the string, while length prefixed strings can contain embedded nulls.

其中一些东西比C语言出现得更晚，所以C语言不知道它们是有道理的。然而，在C语言出现之前，有些语言就已经很简单了。为什么会选择空终止字符串，而不是明显更好的长度前缀?

编辑:因为有些人问了关于我上面提到的效率点的事实(他们不喜欢我已经提供的事实)，他们源于以下几点:

使用空结尾字符串的Concat需要O(n + m)时间复杂度。长度前缀通常只需要O(m)。使用空结尾字符串的长度需要O(n)时间复杂度。长度前缀为O(1)。 Length和concat是迄今为止最常见的字符串操作。在一些情况下，以空结尾的字符串可能更有效，但这种情况发生的频率要低得多。

从下面的答案，这些是一些情况下，空终止字符串更有效:

When you need to cut off the start of a string and need to pass it to some method. You can't really do this in constant time with length prefixing even if you are allowed to destroy the original string, because the length prefix probably needs to follow alignment rules. In some cases where you're just looping through the string character by character you might be able to save a CPU register. Note that this works only in the case that you haven't dynamically allocated the string (Because then you'd have to free it, necessitating using that CPU register you saved to hold the pointer you originally got from malloc and friends).

上面这些词都没有length和concat常见。

下面的答案中还有一个断言:

你需要把绳子的一端剪掉

但这个是不正确的——对于以null结尾的字符串和有长度前缀的字符串，它的时间是相同的。(以Null结尾的字符串只是在你想要的新结尾的地方插入一个Null，长度前缀只是从前缀中减去。)

当前回答

来自马的口

None of BCPL, B, or C supports character data strongly in the language; each treats strings much like vectors of integers and supplements general rules by a few conventions. In both BCPL and B a string literal denotes the address of a static area initialized with the characters of the string, packed into cells. In BCPL, the first packed byte contains the number of characters in the string; in B, there is no count and strings are terminated by a special character, which B spelled *e. This change was made partially to avoid the limitation on the length of a string caused by holding the count in an 8- or 9-bit slot, and partly because maintaining the count seemed, in our experience, less convenient than using a terminator.

Dennis M Ritchie, C语言的开发

2010-12-11 20:25:59

其他回答

C语言中没有字符串。C语言中的“string”只是一个指向char的指针。所以也许你问错问题了。

“省略字符串类型的基本原理是什么”可能更相关。对此，我要指出C不是面向对象的语言，只有基本的值类型。字符串是一个更高级别的概念，必须以某种方式组合其他类型的值来实现。C处于较低的抽象级别。

鉴于下面的狂风暴雨

我只是想指出，我并不是想说这是一个愚蠢或糟糕的问题，或者C语言表示字符串的方式是最好的选择。我试图澄清的是，如果考虑到C语言没有区分字符串作为数据类型与字节数组的机制这一事实，那么这个问题就会更简洁。考虑到今天计算机的处理和存储能力，这是最好的选择吗?可能不会。但事后诸葛总是20/20之类的。

2010-12-11 20:19:22

我认为，这是有历史原因的，我在维基百科上找到了这个:

At the time C (and the languages that it was derived from) were developed, memory was extremely limited, so using only one byte of overhead to store the length of a string was attractive. The only popular alternative at that time, usually called a "Pascal string" (though also used by early versions of BASIC), used a leading byte to store the length of the string. This allows the string to contain NUL and made finding the length need only one memory access (O(1) (constant) time). But one byte limits the length to 255. This length limitation was far more restrictive than the problems with the C string, so the C string in general won out.

2010-12-11 20:21:11

不知怎的，我把这个问题理解为C中没有编译器支持以长度为前缀的字符串。下面的例子显示，至少你可以开始你自己的C字符串库，其中字符串长度在编译时计算，使用这样的构造:

#define PREFIX_STR(s) ((prefix_str_t){ sizeof(s)-1, (s) })

typedef struct { int n; char * p; } prefix_str_t;

int main() {
    prefix_str_t string1, string2;

    string1 = PREFIX_STR("Hello!");
    string2 = PREFIX_STR("Allows \0 chars (even if printf directly doesn't)");

    printf("%d %s\n", string1.n, string1.p); /* prints: "6 Hello!" */
    printf("%d %s\n", string2.n, string2.p); /* prints: "48 Allows " */

    return 0;
}

然而，这不会带来任何问题，因为你需要小心什么时候特别释放字符串指针，什么时候它是静态分配的(字面字符数组)。

编辑:作为对这个问题更直接的回答，我的观点是，这是C既可以支持可用的字符串长度(作为编译时间常数)的方式，如果你需要它，但如果你只想使用指针和零终止，仍然没有内存开销。

当然，使用以零结尾的字符串似乎是推荐的做法，因为标准库一般不接受字符串长度作为参数，而且提取长度的代码不像char * s = "abc"那样简单，正如我的示例所示。

2010-12-12 07:25:31

即使在32位机器上，如果允许字符串的大小与可用内存相同，带前缀的长度字符串也只比以空结尾的字符串宽3个字节。

首先，对于短字符串来说，额外的3个字节可能是相当大的开销。具体来说，零长度字符串现在占用的内存是原来的4倍。我们中的一些人正在使用64位机器，因此我们要么需要8个字节来存储零长度的字符串，要么字符串格式无法处理平台支持的最长字符串。

可能还需要处理对齐问题。假设我有一个包含7个字符串的内存块，比如“solo\0second\0\0four\0five\0\0seventh”。第二个字符串从偏移量5开始。硬件可能要求32位整数以4的倍数的地址对齐，因此您必须添加填充，从而进一步增加开销。相比之下，C表示非常节省内存。(内存效率很好;例如，它有助于缓存性能。)

2012-07-23 12:45:26

假设C以Pascal的方式实现字符串，通过前缀长度:7字符长字符串与3字符字符串的数据类型相同吗?如果答案是肯定的，那么当我将前者分配给后者时，编译器应该生成什么样的代码?字符串应该被截断，还是自动调整大小?如果调整大小，该操作是否应该被锁保护以使其线程安全?不管你喜不喜欢，C语言的方法回避了所有这些问题。

2010-12-12 04:26:17

空终止字符串的基本原理是什么?

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