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。)
不一定是基本原理，而是长度编码的对应物
Certain forms of dynamic length encoding are superior to static length encoding as far as memory is concerned, it all depends on usage. Just look at UTF-8 for proof. It's essentially an extensible character array for encoding a single character. This uses a single bit for each extended byte. NUL termination uses 8 bits. Length-prefix I think can be reasonably termed infinite length as well by using 64 bits. How often you hit the case of your extra bits is the deciding factor. Only 1 extremely large string? Who cares if you're using 8 or 64 bits? Many small strings (Ie Strings of English words)? Then your prefix costs are a large percentage.
Length-prefixed strings allowing time savings is not a real thing. Whether your supplied data is required to have length provided, you're counting at compile time, or you're truly being provided dynamic data that you must encode as a string. These sizes are computed at some point in the algorithm. A separate variable to store the size of a null terminated string can be provided. Which makes the comparison on time-savings moot. One just has an extra NUL at the end... but if the length encode doesn't include that NUL then there's literally no difference between the two. There's no algorithmic change required at all. Just a pre-pass you have to manually design yourself instead of having a compiler/runtime do it for you. C is mostly about doing things manually.
Length-prefix being optional is a selling point. I don't always need that extra info for an algorithm so being required to do it for a every string makes my precompute+compute time never able to drop below O(n). (Ie hardware random number generator 1-128. I can pull from an "infinite string". Let's say it only generates characters so fast. So our string length changes all the time. But my usage of the data probably doesn't care how many random bytes I have. It just wants the next available unused byte as soon as it can get it after a request. I could be waiting on the device. But I could also have a buffer of characters pre-read. A length comparison is a needless waste of computation. A null check is more efficient.)
Length-prefix is a good guard against buffer overflow? So is sane usage of library functions and implementation. What if I pass in malformed data? My buffer is 2 bytes long but I tell the function it's 7! Ex: If gets() was intended to be used on known data it could've had an internal buffer check that tested compiled buffers and malloc() calls and still follow spec. If it was meant to be used as a pipe for unknown STDIN to arrive at unknown buffer then clearly one can't know abut the buffer size which means a length arg is pointless, you need something else here like a canary check. For that matter, you can't length-prefix some streams and inputs, you just can't. Which means the length check has to be built into the algorithm and not a magic part of the typing system. TL;DR NUL-terminated never had to be unsafe, it just ended up that way via misuse.
counter-counter point: NUL-termination is annoying on binary. You either need to do length-prefix here or transform NUL bytes in some way: escape-codes, range remapping, etc... which of course means more-memory-usage/reduced-information/more-operations-per-byte. Length-prefix mostly wins the war here. The only upside to a transform is that no additional functions have to be written to cover the length-prefix strings. Which means on your more optimized sub-O(n) routines you can have them automatically act as their O(n) equivalents without adding more code. Downside is, of course, time/memory/compression waste when used on NUL heavy strings. Depending on how much of your library you end up duplicating to operate on binary data, it may make sense to work solely with length-prefix strings. That said one could also do the same with length-prefix strings... -1 length could mean NUL-terminated and you could use NUL-terminated strings inside length-terminated.
Concat: "O(n+m) vs O(m)" I'm assuming your referring to m as the total length of the string after concatenating because they both have to have that number of operations minimum (you can't just tack-on to string 1, what if you have to realloc?). And I'm assuming n is a mythical amount of operations you no longer have to do because of a pre-compute. If so, then the answer is simple: pre-compute. If you're insisting you'll always have enough memory to not need to realloc and that's the basis of the big-O notation then the answer is even more simple: do binary search on allocated memory for end of string 1, clearly there's a large swatch of infinite zeros after string 1 for us to not worry about realloc. There, easily got n to log(n) and I barely tried. Which if you recall log(n) is essentially only ever as large as 64 on a real computer, which is essentially like saying O(64+m), which is essentially O(m). (And yes that logic has been used in run-time analysis of real data structures in-use today. It's not bullshit off the top of my head.)
Concat()/Len() again: Memoize results. Easy. Turns all computes into pre-computes if possible/necessary. This is an algorithmic decision. It's not an enforced constraint of the language.
String suffix passing is easier/possible with NUL termination. Depending on how length-prefix is implemented it can be destructive on original string and can sometimes not even be possible. Requiring a copy and pass O(n) instead of O(1).
Argument-passing/de-referencing is less for NUL-terminated versus length-prefix. Obviously because you're passing less information. If you don't need length, then this saves a lot of footprint and allows optimizations.
You can cheat. It's really just a pointer. Who says you have to read it as a string? What if you want to read it as a single character or a float? What if you want to do the opposite and read a float as a string? If you're careful you can do this with NUL-termination. You can't do this with length-prefix, it's a data type distinctly different from a pointer typically. You'd most likely have to build a string byte-by-byte and get the length. Of course if you wanted something like an entire float (probably has a NUL inside it) you'd have to read byte-by-byte anyway, but the details are left to you to decide.
TL;DR您使用二进制数据吗?如果不是，那么null终止允许更多的算法自由。如果是，那么代码数量vs速度/内存/压缩是你的主要关注点。两种方法的混合或记忆可能是最好的。
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。