Buffer overflow that overwrites local variables

Buffer overflow that overwrites local variables - c

I'm doing a buffer overflow exercise where the source code is given. The exercise allows you to change the number of argument vectors you feed into the program so you can get around the null problem making it easy.
However the exercise also mentions that it is possible to use just 1 argument vector to compromise this code. I'm curious to see how this can be done. Any ideas on how to approach this would be greatly appreciated.
The problem here is that length needs to be overwritten in order for the overflow to take place and the return address to be compromised. To my knowledge, you can't really use NULLs in the string since they are being passed in via execve arguments. So the length ends up being a very large number as you have to write some non zero number causing the entire stack to go boom, it's the same case with the return address. Am I missing something obvious? Does strlen need to be exploited. I saw some references to arithmetic overflow of signed numbers but I'm not sure if turning the local variables does anything.
The code is posted below and returns to a main function which then ends the program and runs on a little endian system with all stack protection turned off as this is an introductory exercise for infosec:
int TrickyOverflowSeq ( char *in )
{
char to_be_exploited[128];
int c;
int limit;
limit = strlen(in);
if (limit > 144)
limit = 144;
for (c = 0; c <= limit; c++)
to_be_exploited[c] = in[c];
return(0);
}

I don't know where arg comes from, but since your buffer is only 128 bytes, and you cap the max length to 144, you need only pass in a string longer than 128 bytes to cause a buffer overrun when copying in to to_be_exploited. Any malicious code would be in the input buffer from positions 129 to 144.
Whether or not that will properly set up a return to a different location depends on many factors.

However the exercise also mentions that it is possible to use just 1 argument vector to compromise this code. I'm curious to see how this can be done.
...
The problem here is that length needs to be overwritten in order for the overflow to take place and the return address to be compromised.
It seems pretty straightforward to me. That magic number 144 makes sense if sizeof(int) == 8, which it would if you are building for 64-bit.
So assuming a stack layout where to_be_exploited comes before c and limit, you can simply pass in a very long string with junk in the bytes starting at offset 136 (i.e., 128 + sizeof(int)), and then carefully crafted junk in the bytes starting with offset 144. This will overwrite limit starting with that byte, thus disabling the length check. Then the carefully crafted junk overwrites the return address.
You could put almost anything into the 8 bytes starting at offset 136 and have them make a number that is large enough to disable the security check. Just make sure you don't end up with a negative number. For example, the string "HAHAHAHA" would evaluate, as an integer, to 5206522089439316033. This number is larger than 144... actually, it's too large as you want this function to stop copying once your string is copied. So you just need to figure out how long your attack string actually is and put the correct bytes for that length into that position, and the attack will be copied in.
Note that normal string-handling functions in C use a NUL byte as a terminator, and stop copying. This function doesn't do that; it just trusts limit. So you could put any junk you want in the input string to exploit this function. However, if normal C library functions need to copy the input data, you might end up needing to avoid NUL bytes.
Of course nobody should put code this silly into production.
EDIT: I wrote the above in a hurry. Now that I have more time, I re-read your question and I think I better understand what you wanted to have explained.
You are wondering how a string can correctly clobber limit with a correct length without having strlen() chop it off short. This is impossible on a big-endian computer, but perfectly possible on a little-endian computer.
On a little-endian computer, the first byte is the least significant byte. See the Wikipedia entry:
http://en.wikipedia.org/wiki/Endianness
Any number that is not ridiculously large must have zero in its most significant bytes. On a big-endian computer that means the first several bytes will all be zero, will act like a NUL, and will cause strlen() to chop the string before the function can clobber limit. However, on a little-endian computer, the important bytes you want copied will all come before the NUL bytes.
In the early days of the Internet, it was common for big-endian computers (often bought from Sun Microsystems) to run Internet server apps. These days, commodity x86 server hardware is most common, and x86 is little-endian. In practice, anyone deploying such exploitable code as the TrickyOverflowSeq() function will get 0wned.
If you don't think this answer is thorough enough, please post a comment explaining what part you think I need to cover better and I'll update the answer.

I am aware that this is quite an old post, however I stumbled on your question because I found myself in the same situation with exactly the same questions as the ones you ask in your post and in the comments.
A few minutes later, I solved the problem. I don't know how much of it I should "spoil" here, since AFAIK this is a typical problem in many Computer Security courses. I can say however that the solution can indeed be achieved with exactly one argument... and with a couple of environment variables. Additional hint: environment variables are stored after function arguments on the stack (as in in higher addresses than the function arguments).

Related

String array in C prints whole array instead of value at pointer [duplicate]

As much as I love C and C++, I can't help but scratch my head at the choice of null terminated strings:
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.
Several of these things have come to light more recently than C, so it would make sense for C to not have known of them. However, several were plain well before C came to be. Why would null terminated strings have been chosen instead of the obviously superior length prefixing?
EDIT: Since some asked for facts (and didn't like the ones I already provided) on my efficiency point above, they stem from a few things:
Concat using null terminated strings requires O(n + m) time complexity. Length prefixing often require only O(m).
Length using null terminated strings requires O(n) time complexity. Length prefixing is O(1).
Length and concat are by far the most common string operations. There are several cases where null terminated strings can be more efficient, but these occur much less often.
From answers below, these are some cases where null terminated strings are more efficient:
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).
None of the above are nearly as common as length and concat.
There's one more asserted in the answers below:
You need to cut off the end of the string
but this one is incorrect -- it's the same amount of time for null terminated and length prefixed strings. (Null terminated strings just stick a null where you want the new end to be, length prefixers just subtract from the prefix.)

From the horse's mouth
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, Development of the C Language

C doesn't have a string as part of the language. A 'string' in C is just a pointer to char. So maybe you're asking the wrong question.
"What's the rationale for leaving out a string type" might be more relevant. To that I would point out that C is not an object oriented language and only has basic value types. A string is a higher level concept that has to be implemented by in some way combining values of other types. C is at a lower level of abstraction.
in light of the raging squall below:
I just want to point out that I'm not trying to say this is a stupid or bad question, or that the C way of representing strings is the best choice. I'm trying to clarify that the question would be more succinctly put if you take into account the fact that C has no mechanism for differentiating a string as a datatype from a byte array. Is this the best choice in light of the processing and memory power of todays computers? Probably not. But hindsight is always 20/20 and all that :)

The question is asked as a Length Prefixed Strings (LPS) vs zero terminated strings (SZ) thing, but mostly expose benefits of length prefixed strings. That may seem overwhelming, but to be honest we should also consider drawbacks of LPS and advantages of SZ.
As I understand it, the question may even be understood as a biased way to ask "what are the advantages of Zero Terminated Strings ?".
Advantages (I see) of Zero Terminated Strings:
very simple, no need to introduce new concepts in language, char
arrays/char pointers can do.
the core language just include minimal syntaxic sugar to convert
something between double quotes to a
bunch of chars (really a bunch of
bytes). In some cases it can be used
to initialize things completely
unrelated with text. For instance xpm
image file format is a valid C source
that contains image data encoded as a
string.
by the way, you can put a zero in a string literal, the compiler will
just also add another one at the end of the literal: "this\0is\0valid\0C".
Is it a string ? or four strings ? Or a bunch of bytes...
flat implementation, no hidden indirection, no hidden integer.
no hidden memory allocation involved (well, some infamous non
standard functions like strdup
perform allocation, but that's mostly
a source of problem).
no specific issue for small or large hardware (imagine the burden to
manage 32 bits prefix length on 8
bits microcontrollers, or the
restrictions of limiting string size
to less than 256 bytes, that was a problem I actually had with Turbo Pascal eons ago).
implementation of string manipulation is just a handful of
very simple library function
efficient for the main use of strings : constant text read
sequentially from a known start
(mostly messages to the user).
the terminating zero is not even mandatory, all necessary tools
to manipulate chars like a bunch of
bytes are available. When performing
array initialisation in C, you can
even avoid the NUL terminator. Just
set the right size. char a[3] =
"foo"; is valid C (not C++) and
won't put a final zero in a.
coherent with the unix point of view "everything is file", including
"files" that have no intrinsic length
like stdin, stdout. You should remember that open read and write primitives are implemented
at a very low level. They are not library calls, but system calls. And the same API is used
for binary or text files. File reading primitives get a buffer address and a size and return
the new size. And you can use strings as the buffer to write. Using another kind of string
representation would imply you can't easily use a literal string as the buffer to output, or
you would have to make it have a very strange behavior when casting it to char*. Namely
not to return the address of the string, but instead to return the actual data.
very easy to manipulate text data read from a file in-place, without useless copy of buffer,
just insert zeroes at the right places (well, not really with modern C as double quoted strings are const char arrays nowaday usually kept in non modifiable data segment).
prepending some int values of whatever size would implies alignment issues. The initial
length should be aligned, but there is no reason to do that for the characters datas (and
again, forcing alignment of strings would imply problems when treating them as a bunch of
bytes).
length is known at compile time for constant literal strings (sizeof). So why would
anyone want to store it in memory prepending it to actual data ?
in a way C is doing as (nearly) everyone else, strings are viewed as arrays of char. As array length is not managed by C, it is logical length is not managed either for strings. The only surprising thing is that 0 item added at the end, but that's just at core language level when typing a string between double quotes. Users can perfectly call string manipulation functions passing length, or even use plain memcopy instead. SZ are just a facility. In most other languages array length is managed, it's logical that is the same for strings.
in modern times anyway 1 byte character sets are not enough and you often have to deal with encoded unicode strings where the number of characters is very different of the number of bytes. It implies that users will probably want more than "just the size", but also other informations. Keeping length give use nothing (particularly no natural place to store them) regarding these other useful pieces of information.
That said, no need to complain in the rare case where standard C strings are indeed inefficient. Libs are available. If I followed that trend, I should complain that standard C does not include any regex support functions... but really everybody knows it's not a real problem as there is libraries available for that purpose. So when string manipulation efficiency is wanted, why not use a library like bstring ? Or even C++ strings ?
EDIT: I recently had a look to D strings. It is interesting enough to see that the solution choosed is neither a size prefix, nor zero termination. As in C, literal strings enclosed in double quotes are just short hand for immutable char arrays, and the language also has a string keyword meaning that (immutable char array).
But D arrays are much richer than C arrays. In the case of static arrays length is known at run-time so there is no need to store the length. Compiler has it at compile time. In the case of dynamic arrays, length is available but D documentation does not state where it is kept. For all we know, compiler could choose to keep it in some register, or in some variable stored far away from the characters data.
On normal char arrays or non literal strings there is no final zero, hence programmer has to put it itself if he wants to call some C function from D. In the particular case of literal strings, however the D compiler still put a zero at the end of each strings (to allow easy cast to C strings to make easier calling C function ?), but this zero is not part of the string (D does not count it in string size).
The only thing that disappointed me somewhat is that strings are supposed to be utf-8, but length apparently still returns a number of bytes (at least it's true on my compiler gdc) even when using multi-byte chars. It is unclear to me if it's a compiler bug or by purpose. (OK, I probably have found out what happened. To say to D compiler your source use utf-8 you have to put some stupid byte order mark at beginning. I write stupid because I know of not editor doing that, especially for UTF-8 that is supposed to be ASCII compatible).

I think, it has historical reasons and found this in wikipedia:
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.

Calavera is right, but as people don't seem to get his point, I'll provide some code examples.
First, let's consider what C is: a simple language, where all code has a pretty direct translation into machine language. All types fit into registers and on the stack, and it doesn't require an operating system or a big run-time library to run, since it were meant to write these things (a task to which is superbly well-suited, considering there isn't even a likely competitor to this day).
If C had a string type, like int or char, it would be a type which didn't fit in a register or in the stack, and would require memory allocation (with all its supporting infrastructure) to be handled in any way. All of which go against the basic tenets of C.
So, a string in C is:
char s*;
So, let's assume then that this were length-prefixed. Let's write the code to concatenate two strings:
char* concat(char* s1, char* s2)
{
/* What? What is the type of the length of the string? */
int l1 = *(int*) s1;
/* How much? How much must I skip? */
char *s1s = s1 + sizeof(int);
int l2 = *(int*) s2;
char *s2s = s2 + sizeof(int);
int l3 = l1 + l2;
char *s3 = (char*) malloc(l3 + sizeof(int));
char *s3s = s3 + sizeof(int);
memcpy(s3s, s1s, l1);
memcpy(s3s + l1, s2s, l2);
*(int*) s3 = l3;
return s3;
}
Another alternative would be using a struct to define a string:
struct {
int len; /* cannot be left implementation-defined */
char* buf;
}
At this point, all string manipulation would require two allocations to be made, which, in practice, means you'd go through a library to do any handling of it.
The funny thing is... structs like that do exist in C! They are just not used for your day-to-day displaying messages to the user handling.
So, here is the point Calavera is making: there is no string type in C. To do anything with it, you'd have to take a pointer and decode it as a pointer to two different types, and then it becomes very relevant what is the size of a string, and cannot just be left as "implementation defined".
Now, C can handle memory in anyway, and the mem functions in the library (in <string.h>, even!) provide all the tooling you need to handle memory as a pair of pointer and size. The so-called "strings" in C were created for just one purpose: showing messages in the context of writting an operating system intended for text terminals. And, for that, null termination is enough.

Obviously for performance and safety, you'll want to keep the length of a string while you're working with it rather than repeatedly performing strlen or the equivalent on it. However, storing the length in a fixed location just before the string contents is an incredibly bad design. As Jörgen pointed out in the comments on Sanjit's answer, it precludes treating the tail of a string as a string, which for example makes a lot of common operations like path_to_filename or filename_to_extension impossible without allocating new memory (and incurring the possibility of failure and error handling). And then of course there's the issue that nobody can agree how many bytes the string length field should occupy (plenty of bad "Pascal string" languages used 16-bit fields or even 24-bit fields which preclude processing of long strings).
C's design of letting the programmer choose if/where/how to store the length is much more flexible and powerful. But of course the programmer has to be smart. C punishes stupidity with programs that crash, grind to a halt, or give your enemies root.

Lazyness, register frugality and portability considering the assembly gut of any language, especially C which is one step above assembly (thus inheriting a lot of assembly legacy code).
You would agree as a null char would be useless in those ASCII days, it (and probably as good as an EOF control char ).
let's see in pseudo code
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
total 1 register use
case 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
total 2 register used
That might seem shortsighted at that time, but considering the frugality in code and register ( which were PREMIUM at that time, the time when you know, they use punch card ). Thus being faster ( when processor speed could be counted in kHz), this "Hack" was pretty darn good and portable to register-less processor with ease.
For argument sake I will implement 2 common string operation
stringLength(string)
pointer=addressOf(string)
while(string[pointer]!=CONTROL_CHAR) do
increment pointer
return pointer-addressOf(string)
complexity O(n) where in most case PASCAL string is O(1) because the length of the string is pre-pended to the string structure (that would also mean that this operation would have to be carried in an earlier stage).
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
complexity O(n) and prepending the string length wouldn't change the complexity of the operation, while I admit it would take 3 time less time.
On another hand, if you use PASCAL string you would have to redesign your API for taking in account register length and bit-endianness, PASCAL string got the well known limitation of 255 char (0xFF) beacause the length was stored in 1 byte (8bits), and it you wanted a longer string (16bits->anything) you would have to take in account the architecture in one layer of your code, that would mean in most case incompatible string APIs if you wanted longer string.
Example:
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.
Of course C would have its issues too but, would be very little affected by the issues raised here.

In many ways, C was primitive. And I loved it.
It was a step above assembly language, giving you nearly the same performance with a language that was much easier to write and maintain.
The null terminator is simple and requires no special support by the language.
Looking back, it doesn't seem that convenient. But I used assembly language back in the 80s and it seemed very convenient at the time. I just think software is continually evolving, and the platforms and tools continually get more and more sophisticated.

Assuming for a moment that C implemented strings the Pascal way, by prefixing them by length: is a 7 char long string the same DATA TYPE as a 3-char string? If the answer is yes, then what kind of code should the compiler generate when I assign the former to the latter? Should the string be truncated, or automatically resized? If resized, should that operation be protected by a lock as to make it thread safe? The C approach side stepped all these issues, like it or not :)

Somehow I understood the question to imply there's no compiler support for length-prefixed strings in C. The following example shows, at least you can start your own C string library, where string lengths are counted at compile time, with a construct like this:
#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;
}
This won't, however, come with no issues as you need to be careful when to specifically free that string pointer and when it is statically allocated (literal char array).
Edit: As a more direct answer to the question, my view is this was the way C could support both having string length available (as a compile time constant), should you need it, but still with no memory overhead if you want to use only pointers and zero termination.
Of course it seems like working with zero-terminated strings was the recommended practice, since the standard library in general doesn't take string lengths as arguments, and since extracting the length isn't as straightforward code as char * s = "abc", as my example shows.

"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."
First, extra 3 bytes may be considerable overhead for short strings. In particular, a zero-length string now takes 4 times as much memory. Some of us are using 64-bit machines, so we either need 8 bytes to store a zero-length string, or the string format can't cope with the longest strings the platform supports.
There may also be alignment issues to deal with. Suppose I have a block of memory containing 7 strings, like "solo\0second\0\0four\0five\0\0seventh". The second string starts at offset 5. The hardware may require that 32-bit integers be aligned at an address that is a multiple of 4, so you have to add padding, increasing the overhead even further. The C representation is very memory-efficient in comparison. (Memory-efficiency is good; it helps cache performance, for example.)

One point not yet mentioned: when C was designed, there were many machines where a 'char' was not eight bits (even today there are DSP platforms where it isn't). If one decides that strings are to be length-prefixed, how many 'char's worth of length prefix should one use? Using two would impose an artificial limit on string length for machines with 8-bit char and 32-bit addressing space, while wasting space on machines with 16-bit char and 16-bit addressing space.
If one wanted to allow arbitrary-length strings to be stored efficiently, and if 'char' were always 8-bits, one could--for some expense in speed and code size--define a scheme were a string prefixed by an even number N would be N/2 bytes long, a string prefixed by an odd value N and an even value M (reading backward) could be ((N-1) + M*char_max)/2, etc. and require that any buffer which claims to offer a certain amount of space to hold a string must allow enough bytes preceding that space to handle the maximum length. The fact that 'char' isn't always 8 bits, however, would complicate such a scheme, since the number of 'char' required to hold a string's length would vary depending upon the CPU architecture.

The null termination allows for fast pointer based operations.

Not a Rationale necessarily but a counterpoint to length-encoded
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 Are you using binary data? If no, then NUL-termination allows more algorithmic freedom. If yes, then code quantity vs speed/memory/compression is your main concern. A blend of the two approaches or memoization might be best.

Many design decisions surrounding C stem from the fact that when it was originally implemented, parameter passing was somewhat expensive. Given a choice between e.g.
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);
}
versus
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);
}
the latter would have been slightly cheaper (and thus preferred) since it only required passing one parameter rather than two. If the method being called didn't need to know the base address of the array nor the index within it, passing a single pointer combining the two would be cheaper than passing the values separately.
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--In exchange for a slight speed penalty on some operations, and a tiny bit of extra overhead on longer strings, it would have been possible to have methods that work with strings accept pointers directly to strings, bounds-checked string buffers, or data structures identifying substrings of another string. A function like "strcat" would have looked something like [modern syntax]
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);
}
}
A little bigger than the K&R strcat method, but it would support bounds-checking, which the K&R method doesn't. Further, unlike the current method, it would be possible to easily concatenate an arbitrary substring, e.g.
/* 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);
}
Note that the lifetime of the string returned by temp_substring would be limited by those of s and src, which ever was shorter (which is why the method requires inf to be passed in--if it was local, it would die when the method returned).
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).
Of course, K&R didn't implement any such thing, but that's most likely because they didn't want to spend much effort on string handling--an area where even today many languages seem rather anemic.

According to Joel Spolsky in this blog post,
It's because the PDP-7 microprocessor, on which UNIX and the C programming language were invented, had an ASCIZ string type. ASCIZ meant "ASCII with a Z (zero) at the end."
After seeing all the other answers here, I'm convinced that even if this is true, it's only part of the reason for C having null-terminated "strings". That post is quite illuminating as to how simple things like strings can actually be quite hard.

I don't buy the "C has no string" answer. True, C does not support built-in higher-level types but you can still represent data-structures in C and that's what a string is. The fact a string is just a pointer in C does not mean the first N bytes cannot take on special meaning as a the length.
Windows/COM developers will be very familiar with the BSTR type which is exactly like this - a length-prefixed C string where the actual character data starts not at byte 0.
So it seems that the decision to use null-termination is simply what people preferred, not a necessity of the language.

One advantage of NUL-termination over length-prefixing, which I have not seen anyone mention, is the simplicity of string comparison. Consider the comparison standard which returns a signed result for less-than, equal, or greater-than. For length-prefixing the algorithm has to be something along the following lines:
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.
Contrast this with the NUL-termination algorithm:
Scan the two character sequences, subtracting characters at matching indices [note that this is handled better with moving pointers]. Stop when the difference is nonzero, returning the difference. NOTE: If one string is a PROPER prefix of the other, one of the characters in the subtraction will be NUL, i.e zero, and the comparison will naturally stop there.
If the difference is zero, -only then- check if either character is NUL. If so, return zero, otherwise continue to next character.
The NUL-terminated case is simpler, and very easy to implement efficiently with a dual pointer scan. The length-prefixed case does at least as much work, nearly always more. If your algorithm has to do a lot of string comparisons [e.g a compiler!], the NUL-terminated case wins out. Nowadays that might not be as important, but back in the day, heck yeah.

gcc accept the codes below:
char s[4] = "abcd";
and it's ok if we treat is as an array of chars but not string. That is, we can access it with s[0], s[1], s[2], and s[3], or even with memcpy(dest, s, 4). But we'll get messy characters when we trying with puts(s), or worse with strcpy(dest, s).

I think the better question is why you think C owes you anything? C was designed to give you what you need, nothing more. You need to loose the mentality that the language must provide you with everything. Or just continue to use your higher level languages that will give you the luxary of String, Calendar, Containers; and in the case of Java you get one thing in tonnes of variety. Multiple types String, multiple types of unordered_map(s).
Too bad for you, this was not the purpose of C. C was not designed to be a bloated language that offers from a pin to an anchor. Instead you must rely on third party libraries or your own. And there is nothing easier than creating a simple struct that will contain a string and its size.
struct String
{
const char *s;
size_t len;
};
You know what the problem is with this though. It is not standard. Another language might decide to organize the len before the string. Another language might decide to use a pointer to end instead. Another might decide to use six pointers to make the String more efficient. However a null terminated string is the most standard format for a string; which you can use to interface with any language. Even Java JNI uses null terminated strings.
Lastly, it is a common saying; the right data structure for the task. If you find that need to know the size of a string more than anything else; well use a string structure that allows you to do that optimally. But don't make claims that that operation is used more than anything else for everybody. Like, why is knowing the size of a string more important than reading its contents. I find that reading the contents of a string is what I mostly do, so I use null terminated strings instead of std::string; which saves me 5 pointers on a GCC compiler. If I can even save 2 pointers that is good.

Reading the same input multiple times - C

I want to ask you if is it possible to read the same input (stdin) multiple times? I am about to get really big number, containing thousands of digits (so I am unable to store it in variable, (and also I can not use folders!). My idea is to put the digits into int array, but I don't know how big the array should be, because amount of digits in input may vary. I have to write general solution.
So my question is, how to solve it, and how to find out amount of digits (so I can initialize array), before I copy digits into array. I tried using scanf(), multiple times, or scanf() and getchar, but it is not working. See my code:
int main(){
int c;
int amountOfDigits=5;
while(scanf("%1d",&c)!=' '){//finding out number of digits with scanf
if(isdigit(c)==0){
break;
}
amountOfDigits++;
}
int digits[amountOfDigits];//now i know lenght of array, and initialize it
for(int i=0;i<amountOfDigits;i++){//putting digits into array
digits[i]=getchar();
}
for(int i=0;i<amountOfDigits;i++){//printing array
printf("%d",digits[i]);
}
printf("\n");
return 0;
}

is it possible to read the same input (stdin) multiple times?
(I am guessing you are a student beginning to learn programming, and you are using Linux; adapt my answer if not)
For your homework, you don't need to read the same input several times. In some cases, it is possible (when the standard input is a genuine file -seekable-, that is when you use some redirection in your command). In other cases (e.g. when the standard input is a pipe, e.g. with a command pipeline; or with here documents in your shell command...) it is not possible to read several times stdin (but you don't need to). In general, don't expect stdin to be seekable with fseek or rewind (it usually is not).
(I am not going to do your homework, but here are useful hints)
so I am unable to store it in variable, (and also I can not use folders!)
You could do several things:
(since you mentioned folders....) you might use some more sophisticated ways of storing data on the disk (but in your particular case, I don't recommend that ...). These ways could be some direct-accessed file (ugly), or some indexed file à la gdbm, or some database à la sqlite or even some RDBMS server like PostGreSQL.
In your case, you don't need any of these; I'm mentioning it since you mentioned "folders" and you meant "directories"!
you really should use some heap allocated memory, so read about C dynamic memory allocation and read carefully the documentation of each standard memory management functions like malloc, realloc, free. Your program should probably use all these three functions (don't forget that malloc & realloc could fail).
Read this and that answers. Both are surprisingly relevant.
You probably should keep somehow:
a pointer to heap allocated int-s (actually, you could use char-s)
the allocated size of that pointer
the used length of that thing, that is the actual number of useful digits.
You certainly don't want to grow your array by repeated realloc at each loop (that is inefficient). In practice, you would adapt some growing scheme like newsize = 3*oldsize/2 + 10 to avoid reallocating memory at each step (of your input loop).
you should thank your teacher for a so useful exercise, but you should not expect StackOverflow to do your homework!
Be also aware of arbitrary-precision arithmetic (called bignums or bigints). It is actually hard to code efficiently, so in real-life you would use some library like GMPlib.

Design and usage of the memfrob function

From the man page of memfrob:
void *memfrob(void *s, size_t n);
The memfrob() function
encrypts the first n bytes of the memory area s by exclusive-ORing
each character with the number 42. The effect can be reversed by using
memfrob() on the encrypted memory area.
Note that this function is not a proper encryption routine as the XOR
constant is fixed, and is only suitable for hiding strings.
I have the following questions regarding the memfrob function:
Why is the XORing done with number 42?
Is there any reason why XOR constant is fixed and why the designers of memfrob did not leave choice of the constant to the user?
In what sense is it suitable for hiding strings? Since it can be reversed and therefore shouldn't be used in applications where encryption is important, what it is used for on practice?

The purpose of memfrob() (if you want to call that a purpose) is to hide strings so you don't see them when you run strings. This might be useful when your binary contains plaintext passwords and you want to stop nosey people from finding out what these passwords are. Of course, it's not hard to crack XOR-by-42, but it's better than nothing.
The number to XOR with can be arbitrary but it needs to stay constant over successive releases of the glibc so code that relies on the number being 42 doesn't break.
Some people consider memfrob() to be a joke function, but I am not sure if this is really the case. None the less, you should not use it because it isn't a standard function (and thus not available on non-glibc platforms) and because obfuscation is not a substitute for proper security.
The joke of it is that it is the meaning of Life. It's similar to rot-13 in that it's a most trivial encryption and running it again restores the string. Rot-13 doesn't do anything to spaces in the plaintext while memfrob has the odd result of swapping spaces and line feeds:
space = 100000 (32), 42 = 101010, 32^42 = 001010 (10 = LF, and 10^42 back to 32)
While these obfuscate they are poor encryption because they can be recognized just by looking at the result: lots of r's and n's then it's rot13; lots of CRs, \ and ^ then memfrob.

Using memfrob() to encrypt / decrypt your password in memory is not good at all. As mentioned above this function gets processed in the memory of the system. When one uses a simple debugger like gdb for example and putting a breakpoint on the memfrob() function the values can easily be read.

Partially Prefixed Null Terminated Strings [duplicate]

From the horse's mouth
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, Development of the C Language

C doesn't have a string as part of the language. A 'string' in C is just a pointer to char. So maybe you're asking the wrong question.
"What's the rationale for leaving out a string type" might be more relevant. To that I would point out that C is not an object oriented language and only has basic value types. A string is a higher level concept that has to be implemented by in some way combining values of other types. C is at a lower level of abstraction.
in light of the raging squall below:
I just want to point out that I'm not trying to say this is a stupid or bad question, or that the C way of representing strings is the best choice. I'm trying to clarify that the question would be more succinctly put if you take into account the fact that C has no mechanism for differentiating a string as a datatype from a byte array. Is this the best choice in light of the processing and memory power of todays computers? Probably not. But hindsight is always 20/20 and all that :)

The question is asked as a Length Prefixed Strings (LPS) vs zero terminated strings (SZ) thing, but mostly expose benefits of length prefixed strings. That may seem overwhelming, but to be honest we should also consider drawbacks of LPS and advantages of SZ.
As I understand it, the question may even be understood as a biased way to ask "what are the advantages of Zero Terminated Strings ?".
Advantages (I see) of Zero Terminated Strings:
very simple, no need to introduce new concepts in language, char
arrays/char pointers can do.
the core language just include minimal syntaxic sugar to convert
something between double quotes to a
bunch of chars (really a bunch of
bytes). In some cases it can be used
to initialize things completely
unrelated with text. For instance xpm
image file format is a valid C source
that contains image data encoded as a
string.
by the way, you can put a zero in a string literal, the compiler will
just also add another one at the end of the literal: "this\0is\0valid\0C".
Is it a string ? or four strings ? Or a bunch of bytes...
flat implementation, no hidden indirection, no hidden integer.
no hidden memory allocation involved (well, some infamous non
standard functions like strdup
perform allocation, but that's mostly
a source of problem).
no specific issue for small or large hardware (imagine the burden to
manage 32 bits prefix length on 8
bits microcontrollers, or the
restrictions of limiting string size
to less than 256 bytes, that was a problem I actually had with Turbo Pascal eons ago).
implementation of string manipulation is just a handful of
very simple library function
efficient for the main use of strings : constant text read
sequentially from a known start
(mostly messages to the user).
the terminating zero is not even mandatory, all necessary tools
to manipulate chars like a bunch of
bytes are available. When performing
array initialisation in C, you can
even avoid the NUL terminator. Just
set the right size. char a[3] =
"foo"; is valid C (not C++) and
won't put a final zero in a.
coherent with the unix point of view "everything is file", including
"files" that have no intrinsic length
like stdin, stdout. You should remember that open read and write primitives are implemented
at a very low level. They are not library calls, but system calls. And the same API is used
for binary or text files. File reading primitives get a buffer address and a size and return
the new size. And you can use strings as the buffer to write. Using another kind of string
representation would imply you can't easily use a literal string as the buffer to output, or
you would have to make it have a very strange behavior when casting it to char*. Namely
not to return the address of the string, but instead to return the actual data.
very easy to manipulate text data read from a file in-place, without useless copy of buffer,
just insert zeroes at the right places (well, not really with modern C as double quoted strings are const char arrays nowaday usually kept in non modifiable data segment).
prepending some int values of whatever size would implies alignment issues. The initial
length should be aligned, but there is no reason to do that for the characters datas (and
again, forcing alignment of strings would imply problems when treating them as a bunch of
bytes).
length is known at compile time for constant literal strings (sizeof). So why would
anyone want to store it in memory prepending it to actual data ?
in a way C is doing as (nearly) everyone else, strings are viewed as arrays of char. As array length is not managed by C, it is logical length is not managed either for strings. The only surprising thing is that 0 item added at the end, but that's just at core language level when typing a string between double quotes. Users can perfectly call string manipulation functions passing length, or even use plain memcopy instead. SZ are just a facility. In most other languages array length is managed, it's logical that is the same for strings.
in modern times anyway 1 byte character sets are not enough and you often have to deal with encoded unicode strings where the number of characters is very different of the number of bytes. It implies that users will probably want more than "just the size", but also other informations. Keeping length give use nothing (particularly no natural place to store them) regarding these other useful pieces of information.
That said, no need to complain in the rare case where standard C strings are indeed inefficient. Libs are available. If I followed that trend, I should complain that standard C does not include any regex support functions... but really everybody knows it's not a real problem as there is libraries available for that purpose. So when string manipulation efficiency is wanted, why not use a library like bstring ? Or even C++ strings ?
EDIT: I recently had a look to D strings. It is interesting enough to see that the solution choosed is neither a size prefix, nor zero termination. As in C, literal strings enclosed in double quotes are just short hand for immutable char arrays, and the language also has a string keyword meaning that (immutable char array).
But D arrays are much richer than C arrays. In the case of static arrays length is known at run-time so there is no need to store the length. Compiler has it at compile time. In the case of dynamic arrays, length is available but D documentation does not state where it is kept. For all we know, compiler could choose to keep it in some register, or in some variable stored far away from the characters data.
On normal char arrays or non literal strings there is no final zero, hence programmer has to put it itself if he wants to call some C function from D. In the particular case of literal strings, however the D compiler still put a zero at the end of each strings (to allow easy cast to C strings to make easier calling C function ?), but this zero is not part of the string (D does not count it in string size).
The only thing that disappointed me somewhat is that strings are supposed to be utf-8, but length apparently still returns a number of bytes (at least it's true on my compiler gdc) even when using multi-byte chars. It is unclear to me if it's a compiler bug or by purpose. (OK, I probably have found out what happened. To say to D compiler your source use utf-8 you have to put some stupid byte order mark at beginning. I write stupid because I know of not editor doing that, especially for UTF-8 that is supposed to be ASCII compatible).

I think, it has historical reasons and found this in wikipedia:
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.

Calavera is right, but as people don't seem to get his point, I'll provide some code examples.
First, let's consider what C is: a simple language, where all code has a pretty direct translation into machine language. All types fit into registers and on the stack, and it doesn't require an operating system or a big run-time library to run, since it were meant to write these things (a task to which is superbly well-suited, considering there isn't even a likely competitor to this day).
If C had a string type, like int or char, it would be a type which didn't fit in a register or in the stack, and would require memory allocation (with all its supporting infrastructure) to be handled in any way. All of which go against the basic tenets of C.
So, a string in C is:
char s*;
So, let's assume then that this were length-prefixed. Let's write the code to concatenate two strings:
char* concat(char* s1, char* s2)
{
/* What? What is the type of the length of the string? */
int l1 = *(int*) s1;
/* How much? How much must I skip? */
char *s1s = s1 + sizeof(int);
int l2 = *(int*) s2;
char *s2s = s2 + sizeof(int);
int l3 = l1 + l2;
char *s3 = (char*) malloc(l3 + sizeof(int));
char *s3s = s3 + sizeof(int);
memcpy(s3s, s1s, l1);
memcpy(s3s + l1, s2s, l2);
*(int*) s3 = l3;
return s3;
}
Another alternative would be using a struct to define a string:
struct {
int len; /* cannot be left implementation-defined */
char* buf;
}
At this point, all string manipulation would require two allocations to be made, which, in practice, means you'd go through a library to do any handling of it.
The funny thing is... structs like that do exist in C! They are just not used for your day-to-day displaying messages to the user handling.
So, here is the point Calavera is making: there is no string type in C. To do anything with it, you'd have to take a pointer and decode it as a pointer to two different types, and then it becomes very relevant what is the size of a string, and cannot just be left as "implementation defined".
Now, C can handle memory in anyway, and the mem functions in the library (in <string.h>, even!) provide all the tooling you need to handle memory as a pair of pointer and size. The so-called "strings" in C were created for just one purpose: showing messages in the context of writting an operating system intended for text terminals. And, for that, null termination is enough.

Obviously for performance and safety, you'll want to keep the length of a string while you're working with it rather than repeatedly performing strlen or the equivalent on it. However, storing the length in a fixed location just before the string contents is an incredibly bad design. As Jörgen pointed out in the comments on Sanjit's answer, it precludes treating the tail of a string as a string, which for example makes a lot of common operations like path_to_filename or filename_to_extension impossible without allocating new memory (and incurring the possibility of failure and error handling). And then of course there's the issue that nobody can agree how many bytes the string length field should occupy (plenty of bad "Pascal string" languages used 16-bit fields or even 24-bit fields which preclude processing of long strings).
C's design of letting the programmer choose if/where/how to store the length is much more flexible and powerful. But of course the programmer has to be smart. C punishes stupidity with programs that crash, grind to a halt, or give your enemies root.

Lazyness, register frugality and portability considering the assembly gut of any language, especially C which is one step above assembly (thus inheriting a lot of assembly legacy code).
You would agree as a null char would be useless in those ASCII days, it (and probably as good as an EOF control char ).
let's see in pseudo code
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
total 1 register use
case 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
total 2 register used
That might seem shortsighted at that time, but considering the frugality in code and register ( which were PREMIUM at that time, the time when you know, they use punch card ). Thus being faster ( when processor speed could be counted in kHz), this "Hack" was pretty darn good and portable to register-less processor with ease.
For argument sake I will implement 2 common string operation
stringLength(string)
pointer=addressOf(string)
while(string[pointer]!=CONTROL_CHAR) do
increment pointer
return pointer-addressOf(string)
complexity O(n) where in most case PASCAL string is O(1) because the length of the string is pre-pended to the string structure (that would also mean that this operation would have to be carried in an earlier stage).
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
complexity O(n) and prepending the string length wouldn't change the complexity of the operation, while I admit it would take 3 time less time.
On another hand, if you use PASCAL string you would have to redesign your API for taking in account register length and bit-endianness, PASCAL string got the well known limitation of 255 char (0xFF) beacause the length was stored in 1 byte (8bits), and it you wanted a longer string (16bits->anything) you would have to take in account the architecture in one layer of your code, that would mean in most case incompatible string APIs if you wanted longer string.
Example:
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.
Of course C would have its issues too but, would be very little affected by the issues raised here.

In many ways, C was primitive. And I loved it.
It was a step above assembly language, giving you nearly the same performance with a language that was much easier to write and maintain.
The null terminator is simple and requires no special support by the language.
Looking back, it doesn't seem that convenient. But I used assembly language back in the 80s and it seemed very convenient at the time. I just think software is continually evolving, and the platforms and tools continually get more and more sophisticated.

Assuming for a moment that C implemented strings the Pascal way, by prefixing them by length: is a 7 char long string the same DATA TYPE as a 3-char string? If the answer is yes, then what kind of code should the compiler generate when I assign the former to the latter? Should the string be truncated, or automatically resized? If resized, should that operation be protected by a lock as to make it thread safe? The C approach side stepped all these issues, like it or not :)

Somehow I understood the question to imply there's no compiler support for length-prefixed strings in C. The following example shows, at least you can start your own C string library, where string lengths are counted at compile time, with a construct like this:
#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;
}
This won't, however, come with no issues as you need to be careful when to specifically free that string pointer and when it is statically allocated (literal char array).
Edit: As a more direct answer to the question, my view is this was the way C could support both having string length available (as a compile time constant), should you need it, but still with no memory overhead if you want to use only pointers and zero termination.
Of course it seems like working with zero-terminated strings was the recommended practice, since the standard library in general doesn't take string lengths as arguments, and since extracting the length isn't as straightforward code as char * s = "abc", as my example shows.

"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."
First, extra 3 bytes may be considerable overhead for short strings. In particular, a zero-length string now takes 4 times as much memory. Some of us are using 64-bit machines, so we either need 8 bytes to store a zero-length string, or the string format can't cope with the longest strings the platform supports.
There may also be alignment issues to deal with. Suppose I have a block of memory containing 7 strings, like "solo\0second\0\0four\0five\0\0seventh". The second string starts at offset 5. The hardware may require that 32-bit integers be aligned at an address that is a multiple of 4, so you have to add padding, increasing the overhead even further. The C representation is very memory-efficient in comparison. (Memory-efficiency is good; it helps cache performance, for example.)

One point not yet mentioned: when C was designed, there were many machines where a 'char' was not eight bits (even today there are DSP platforms where it isn't). If one decides that strings are to be length-prefixed, how many 'char's worth of length prefix should one use? Using two would impose an artificial limit on string length for machines with 8-bit char and 32-bit addressing space, while wasting space on machines with 16-bit char and 16-bit addressing space.
If one wanted to allow arbitrary-length strings to be stored efficiently, and if 'char' were always 8-bits, one could--for some expense in speed and code size--define a scheme were a string prefixed by an even number N would be N/2 bytes long, a string prefixed by an odd value N and an even value M (reading backward) could be ((N-1) + M*char_max)/2, etc. and require that any buffer which claims to offer a certain amount of space to hold a string must allow enough bytes preceding that space to handle the maximum length. The fact that 'char' isn't always 8 bits, however, would complicate such a scheme, since the number of 'char' required to hold a string's length would vary depending upon the CPU architecture.

The null termination allows for fast pointer based operations.

Many design decisions surrounding C stem from the fact that when it was originally implemented, parameter passing was somewhat expensive. Given a choice between e.g.
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);
}
versus
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);
}
the latter would have been slightly cheaper (and thus preferred) since it only required passing one parameter rather than two. If the method being called didn't need to know the base address of the array nor the index within it, passing a single pointer combining the two would be cheaper than passing the values separately.
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--In exchange for a slight speed penalty on some operations, and a tiny bit of extra overhead on longer strings, it would have been possible to have methods that work with strings accept pointers directly to strings, bounds-checked string buffers, or data structures identifying substrings of another string. A function like "strcat" would have looked something like [modern syntax]
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);
}
}
A little bigger than the K&R strcat method, but it would support bounds-checking, which the K&R method doesn't. Further, unlike the current method, it would be possible to easily concatenate an arbitrary substring, e.g.
/* 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);
}
Note that the lifetime of the string returned by temp_substring would be limited by those of s and src, which ever was shorter (which is why the method requires inf to be passed in--if it was local, it would die when the method returned).
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).
Of course, K&R didn't implement any such thing, but that's most likely because they didn't want to spend much effort on string handling--an area where even today many languages seem rather anemic.

According to Joel Spolsky in this blog post,
It's because the PDP-7 microprocessor, on which UNIX and the C programming language were invented, had an ASCIZ string type. ASCIZ meant "ASCII with a Z (zero) at the end."
After seeing all the other answers here, I'm convinced that even if this is true, it's only part of the reason for C having null-terminated "strings". That post is quite illuminating as to how simple things like strings can actually be quite hard.

I don't buy the "C has no string" answer. True, C does not support built-in higher-level types but you can still represent data-structures in C and that's what a string is. The fact a string is just a pointer in C does not mean the first N bytes cannot take on special meaning as a the length.
Windows/COM developers will be very familiar with the BSTR type which is exactly like this - a length-prefixed C string where the actual character data starts not at byte 0.
So it seems that the decision to use null-termination is simply what people preferred, not a necessity of the language.

gcc accept the codes below:
char s[4] = "abcd";
and it's ok if we treat is as an array of chars but not string. That is, we can access it with s[0], s[1], s[2], and s[3], or even with memcpy(dest, s, 4). But we'll get messy characters when we trying with puts(s), or worse with strcpy(dest, s).

I think the better question is why you think C owes you anything? C was designed to give you what you need, nothing more. You need to loose the mentality that the language must provide you with everything. Or just continue to use your higher level languages that will give you the luxary of String, Calendar, Containers; and in the case of Java you get one thing in tonnes of variety. Multiple types String, multiple types of unordered_map(s).
Too bad for you, this was not the purpose of C. C was not designed to be a bloated language that offers from a pin to an anchor. Instead you must rely on third party libraries or your own. And there is nothing easier than creating a simple struct that will contain a string and its size.
struct String
{
const char *s;
size_t len;
};
You know what the problem is with this though. It is not standard. Another language might decide to organize the len before the string. Another language might decide to use a pointer to end instead. Another might decide to use six pointers to make the String more efficient. However a null terminated string is the most standard format for a string; which you can use to interface with any language. Even Java JNI uses null terminated strings.
Lastly, it is a common saying; the right data structure for the task. If you find that need to know the size of a string more than anything else; well use a string structure that allows you to do that optimally. But don't make claims that that operation is used more than anything else for everybody. Like, why is knowing the size of a string more important than reading its contents. I find that reading the contents of a string is what I mostly do, so I use null terminated strings instead of std::string; which saves me 5 pointers on a GCC compiler. If I can even save 2 pointers that is good.

sprintf corrupting arrays in IAR microcontroller

I am currently learning embedded programming, and thus working on an IAR-platform using a TI microcontroller with ARM architecture. Since I am not at all familiar with the technicalities related to this kind of programming, or C programming in general, I would like to ask a basic question:
I have the following simple code snippet:
int i;
for(i = 0; i < NUM_SAMPLES; i++)
{
sinTable[i] = sinf(2*i*dT*PI);
}
for(i = 0; i < NUM_SAMPLES; i++)
{
char out[32];
sprintf(out,"sin: %.7f, %.7f;", i*dT, sinTable[i]);
putString(out);
delay(DELAY_100US);
}
Where sinTable[] is a global variable of size NUM_SAMPLES, putString(*char) is a function which writes to an RS232-port, and delay(float) is a simple delay-function.
My problem is that once the sprintf(...) is called, it corrupts sinTable, giving some very peculiar results when plotting the table on the receiver end of the COM-signal.
I don't expect that I run out of memory, as the MC has 64KB SRAM.
Does anyone have any thoughts?

Ensure that your stack pointer is on a 64-bit boundary when main is reached.
The symptom your are seeing is typical of a stack aligned on an odd 32-bit boundary. Everything seems to work properly until a double is used as a variadac argument. This breaks when the code expects such arguments to be on 8-byte boundaries.

Upon further review:
I suspect that Michael Burr's response regarding stack utilization was on the right track. Selecting a smaller printf library might be sufficient, but if you can increase your stack size, that seems safer. Note that the IAR C/C++ Development Guide includes info on linker stack usage analysis.
Original:
When I upgraded from IAR 6.1 (licensed) to 6.4 (kickstart), I ran into a similar problem - vsnprintf was writing "all over RAM", even though the return value indicated the number of characters written was well within the target bounds. The "solution" was to avoid the printf library that has multibyte support.
Project Options > General > Library Options > printf small w/o multi-byte
might want to also uncheck
Project Options > C/C++ Compiler / Language 2 / enable multibyte
I tried to report this to IAR, but since my support contract is expired ...
Unfortunately, a similar problem is back with IAR 7.3.4, and the multibyte "fix" does NOT seem to be sufficient. Happens with both sprintf() and snprintf(), although the out-of-bounds corruption is not identical between those 2.

You seem pretty confident that your result string is only 31 characters long. The only way to corrupt another variable with your sprintf statement is that your result string is longer than 32 bytes (31 chars and a nul-byte), therefore overwriting other parts of memory. Make your numbers smaller or make your temporary buffer larger.

Thank you to anyone who have suggested a solution to this problem. I eventually ended up writing a conversion method that gives a hex-representation of the string and transmitted that instead, omitting the sprintf(...) completely.
It is very crude, but suits my needs.

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