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。

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

与长度前缀相比，null终止的一个优点是字符串比较的简单性，这一点我没有看到任何人提到过。考虑比较标准，它返回小于、等于或大于的有符号结果。对于长度前缀，算法必须遵循以下几行:

Compare the two lengths; record the smaller, and note if they are equal (this last step might be deferred to step 3). Scan the two character sequences, subtracting characters at matching indices (or use a dual pointer scan). Stop either when the difference is nonzero, returning the difference, or when the number of characters scanned is equal to the smaller length. When the smaller length is reached, one string is a prefix of the other. Return negative or positive value according to which is shorter, or zero if of equal length.

将其与null终止算法进行对比:

扫描两个字符序列，在匹配的索引处减去字符[注意，移动指针处理得更好]。当差值非零时停止，返回差值。注意:如果一个字符串是另一个字符串的PROPER前缀，减法中的一个字符将为NUL，即零，比较将自然地停止在那里。 如果差值为零，-only then-检查是否有字符为NUL。如果是，则返回0，否则继续到下一个字符。

以null结尾的情况更简单，并且非常容易用双指针扫描高效地实现。带长度前缀的大小写至少做同样多的工作，几乎总是更多。如果你的算法必须做大量的字符串比较[e。编译器!]，以null结尾的情况胜出。现在，这可能不那么重要了，但在过去，是的。

空终止允许基于快速指针的操作。

懒惰、寄存器节俭和可移植性考虑到任何语言的汇编核心，尤其是C语言，它比汇编高出一步(因此继承了大量汇编遗留代码)。 你会同意null字符在那些ASCII的日子里是无用的，它(可能和EOF控件字符一样好)。

让我们看看伪代码

function readString(string) // 1 parameter: 1 register or 1 stact entries
    pointer=addressOf(string) 
    while(string[pointer]!=CONTROL_CHAR) do
        read(string[pointer])
        increment pointer

共使用1个寄存器

案例2

 function readString(length,string) // 2 parameters: 2 register used or 2 stack entries
     pointer=addressOf(string) 
     while(length>0) do 
         read(string[pointer])
         increment pointer
         decrement length

共使用2个寄存器

这在当时似乎是短视的，但考虑到代码和寄存器的节俭(这在当时是PREMIUM，那时你知道，他们使用穿孔卡)。因此，更快(当处理器速度可以以kHz计)，这个“黑客”是相当不错的，可轻松移植到无寄存器处理器。

为了便于讨论，我将实现2个常见的字符串操作

stringLength(string)
     pointer=addressOf(string)
     while(string[pointer]!=CONTROL_CHAR) do
         increment pointer
     return pointer-addressOf(string)

复杂度O(n)，在大多数情况下PASCAL字符串是O(1)，因为字符串的长度是预先挂起的字符串结构(这也意味着该操作必须在更早的阶段进行)。

concatString(string1,string2)
     length1=stringLength(string1)
     length2=stringLength(string2)
     string3=allocate(string1+string2)
     pointer1=addressOf(string1)
     pointer3=addressOf(string3)
     while(string1[pointer1]!=CONTROL_CHAR) do
         string3[pointer3]=string1[pointer1]
         increment pointer3
         increment pointer1
     pointer2=addressOf(string2)
     while(string2[pointer2]!=CONTROL_CHAR) do
         string3[pointer3]=string2[pointer2]
         increment pointer3
         increment pointer1
     return string3

复杂度O(n)和预先设置字符串长度不会改变操作的复杂性，而我承认它会减少3倍的时间。

另一方面,如果你使用PASCAL字符串将不得不重新设计您的API来考虑在长度和bit-endianness注册,帕斯卡字符串的众所周知的限制255字符(0 xff)因为中存储的长度是1个字节(8位),而且你想要更长的字符串(16位- >任何)你必须考虑在一层的架构代码,这意味着在大多数情况下不相容的字符串API如果你想要更长的字符串。

例子:

One file was written with your prepended string api on an 8 bit computer and then would have to be read on say a 32 bit computer, what would the lazy program do considers that your 4bytes are the length of the string then allocate that lot of memory then attempt to read that many bytes. Another case would be PPC 32 byte string read(little endian) onto a x86 (big endian), of course if you don't know that one is written by the other there would be trouble. 1 byte length (0x00000001) would become 16777216 (0x0100000) that is 16 MB for reading a 1 byte string. Of course you would say that people should agree on one standard but even 16bit unicode got little and big endianness.

当然，C也有它的问题，但它不会受到这里提出的问题的影响。

围绕C语言的许多设计决策都源于这样一个事实:在最初实现C语言时，参数传递的代价有些昂贵。如果在两者之间作选择。

void add_element_to_next(arr, offset)
  char[] arr;
  int offset;
{
  arr[offset] += arr[offset+1];
}

char array[40];

void test()
{
  for (i=0; i<39; i++)
    add_element_to_next(array, i);
}

与

void add_element_to_next(ptr)
  char *p;
{
  p[0]+=p[1];
}

char array[40];

void test()
{
  int i;
  for (i=0; i<39; i++)
    add_element_to_next(arr+i);
}

后者会稍微便宜一点(因此是首选)，因为它只需要传递一个参数而不是两个。如果被调用的方法不需要知道数组的基址，也不需要知道其中的索引，那么将这两个值组合在一起传递一个指针比分别传递值要便宜。

While there are many reasonable ways in which C could have encoded string lengths, the approaches that had been invented up to that time would have all required functions that should be able to work with part of a string to accept the base address of the string and the desired index as two separate parameters. Using zero-byte termination made it possible to avoid that requirement. Although other approaches would be better with today's machines (modern compilers often pass parameters in registers, and memcpy can be optimized in ways strcpy()-equivalents cannot) enough production code uses zero-byte terminated strings that it's hard to change to anything else.

PS——为了在某些操作上稍微降低速度，以及在较长的字符串上稍微增加一点额外开销，可以让处理字符串的方法直接接受指向字符串的指针、经过边界检查的字符串缓冲区或标识另一个字符串的子字符串的数据结构。像“strcat”这样的函数看起来像[现代语法]

void strcat(unsigned char *dest, unsigned char *src)
{
  struct STRING_INFO d,s;
  str_size_t copy_length;

  get_string_info(&d, dest);
  get_string_info(&s, src);
  if (d.si_buff_size > d.si_length) // Destination is resizable buffer
  {
    copy_length = d.si_buff_size - d.si_length;
    if (s.src_length < copy_length)
      copy_length = s.src_length;
    memcpy(d.buff + d.si_length, s.buff, copy_length);
    d.si_length += copy_length;
    update_string_length(&d);
  }
}

比K&R strcat方法大一点，但它支持边界检查，而K&R方法不支持。此外，与当前的方法不同，它可以轻松地连接任意子字符串，例如。

/* Concatenate 10th through 24th characters from src to dest */

void catpart(unsigned char *dest, unsigned char *src)
{
  struct SUBSTRING_INFO *inf;
  src = temp_substring(&inf, src, 10, 24);
  strcat(dest, src);
}

注意，由temp_substring返回的字符串的生命周期将受到s和src的生命周期的限制，后者更短(这就是为什么该方法需要传入inf——如果它是本地的，它将在方法返回时死亡)。

In terms of memory cost, strings and buffers up to 64 bytes would have one byte of overhead (same as zero-terminated strings); longer strings would have slightly more (whether one allowed amounts of overhead between two bytes and the maximum required would be a time/space tradeoff). A special value of the length/mode byte would be used to indicate that a string function was given a structure containing a flag byte, a pointer, and a buffer length (which could then index arbitrarily into any other string).

当然，K&R并没有实现任何这样的东西，但这很可能是因为他们不想在字符串处理上花费太多精力——即使在今天，许多语言在这方面似乎都相当薄弱。