what should I use when I want to copy src_str to dst_arr and why?
char dst_arr[10];
char *src_str = "hello";
PS: my head is spinning faster than the disk of my computer after reading a lot of things on how good or bad is strncpy and strlcpy.
Note: I know strlcpy is not available everywhere. That is not the concern here.
strncpy is never the right answer when your destination string is zero-terminated. strncpy is a function intended to be used with non-terminated fixed-width strings. More precisely, its purpose is to convert a zero-terminated string to a non-terminated fixed-width string (by copying). In other words, strncpy is not meaningfully applicable here.
The real choice you have here is between strlcpy and plain strcpy.
When you want to perform "safe" (i.e. potentially truncated) copying to dst_arr, the proper function to use is strlcpy.
As for dst_ptr... There's no such thing as "copy to dst_ptr". You can copy to memory pointed by dst_ptr, but first you have to make sure it points somewhere and allocate that memory. There are many different ways to do it.
For example, you can just make dst_ptr to point to dst_arr, in which case the answer is the same as in the previous case - strlcpy.
Or you can allocate the memory using malloc. If the amount of memory you allocated is guaranteed to be enough for the string (i.e. at least strlen(src_str) + 1 bytes is allocated), then you can use the plain strcpy or even memcpy to copy the string. There's no need and no reason to use strlcpy in this case , although some people might prefer using it, since it somehow gives them the feeling of extra safety.
If you intentionally allocate less memory (i.e. you want your string to get truncated), then strlcpy becomes the right function to use.
strlcpy() is safer than strncpy() so you might as well use it.
Systems that don't have it will often have a s_strncpy() that does the same thing.
Note : you can't copy anything to dst_ptr until it points to something
I did not know of strlcpy. I just found here that:
The strlcpy() and strlcat() functions copy and concatenate strings
respectively. They are designed to be safer, more consistent, and
less error prone replacements for strncpy(3) and strncat(3).
So strlcpy seams safer.
Edit: A full discussion is available here.
Edit2:
I realize that what I wrote above does not answer the "in your case" part of your question. If you understand the limitations of strncpy, I guess you can use it and write good code around it to avoid its pitfalls; but if your are not sure about your understanding of its limits, use strlcpy.
My understanding of the limitations of strncpy and strlcpy is that you can do something very bad with strncpy (buffer overflow), and the worst you can do with strlcpy is to loose one char in the process.
You should always the standard function, which in this case is the C11 strcpy_s() function. Not strncpy(), as this is unsafe not guaranteeing zero termination. And not the OpenBSD-only strlcpy(), as it is also unsafe, and OpenBSD always comes up with it's own inventions, which usually don't make it into any standard.
See
http://en.cppreference.com/w/c/string/byte/strcpy
The function strcpy_s is similar to the BSD function strlcpy, except that
strlcpy truncates the source string to fit in the destination (which is a security risk)
strlcpy does not perform all the runtime checks that strcpy_s does
strlcpy does not make failures obvious by setting the destination to a null string or calling a handler if the call fails.
Although strcpy_s prohibits truncation due to potential security risks, it's possible to truncate a string using bounds-checked strncpy_s instead.
If your C library doesn't have strcpy_s, use the safec lib.
https://rurban.github.io/safeclib/doc/safec-3.1/df/d8e/strcpy__s_8c.html
First of all, your dst_ptr has no space allocated and you haven't set it to point at the others, so assigning anything to that would probably cause a segmentation fault.
Strncpy should work perfectly fine - just do:
strncpy(dst_arr, src_str, sizeof(dst_arr));
and you know you wont overflow dst_arr. If you use a bigger src_str you might have to put your own null-terminator at the end of dst_arr, but in this case your source is < your dest, so it will be padded with nulls anyway.
This works everywhere and its safe, so I wouldn't look at anything else unless its intellectual curiousity.
Also note that it would be good to use a non-magic number for the 10 so you know the size of that matches the size of the strncpy :)
you should not use strncpy and not strlcpy for this. Better you use
*dst_arr=0; strncat(dst_arr,src_arr,(sizeof dst_arr)-1);
or without an initialization
sprintf(dst_arr,"%.*s",(sizeof dst_arr)-1,src_arr);
dst_arr here must be an array NOT a pointer.
Related
C11 & C++14 standards have dropped gets() function that is inherently insecure & leads to security problems because it doesn't performs bounds checking results in buffer overflow. Then why C11 standard doesn't drop strcat() & strcpy() functions? strcat() function doesn't check to see whether second string will fit in the 1st array. strcpy() function also contains no provision for checking boundary of target array. What if the source array has more characters than destination array can hold? Most probably program will crash at runtime.
So, wouldn't it be nice if these two unsafe functions completely removed from the language? Why they are still exist? What is the reason? Wouldn't it is fine to have only functions like strncat(),strncpy()? If I am not wrong Microsoft C & C++ compiler provides safe versions of these functions strcpy_s(),strcat_s(). Then why they aren't officially implemented by other C compilers to provide safety?
gets() is inherently unsafe, because in general it can overflow the target if too much data is received on stdin. This:
char s[MANY];
gets(s);
will cause undefined behavior if more than MANY characters are entered, and there is typically nothing the program can do to prevent it.
strcpy() and strcat() can be used completely safely, since they can overflow the target only if the source string is too long to be contained in the target array. The source string is contained in an array object that is under the control of the program itself, not of any external input. For example, this:
char s[100];
strcpy(s, "hello");
strcat(s, ", ");
strcat(s, "world");
cannot possibly overflow unless the program itself is modified.
strncat() can be used as a safer version of strcat() -- as long as you specify the third argument correctly. One problem with strncat() is that it only gives you one way of handling the case where there's not enough room in the target array: it silently truncates the string. Sometimes that might be what you want, but sometimes you might want to detect the overflow and do something about it.
As for strncpy(), it is not simply a safer version of strcpy(). It's not inherently dangerous, but if you're not very careful you can easily leave the target array without a terminating '\0' null character, leading to undefined behavior next time you pass it to a function expecting a pointer to a string. As it happens, I've written about this.
strcpy and strcat aren't similar to gets. The problem of gets is, it's used to read from input, so it's out of the programmer's control whether there will be buffer overflow.
C99 Rational explains strncpy as:
Rationale for International Standard — Programming Languages — C §7.21.2.4 The strncpy function
strncpy was initially introduced into the C library to deal with fixed-length name fields in structures such as directory entries. Such fields are not used in the same way as strings: the trailing null is unnecessary for a maximum-length field, and setting trailing bytes for shorter 5 names to null assures efficient field-wise comparisons. strncpy is not by origin a “bounded strcpy,” and the Committee preferred to recognize existing practice rather than alter the function to better suit it to such use.
Myth 1: strcpy() is unsafe and how it works comes as a great surprise to a veteran C programmer.
Myth 2: strncpy() is safe.
Myth 3: strncpy() is a safer version of strcpy().
Myth 4: Microsoft is some kind of authority of the use of the C language and know what they are talking about.
strcat() and strcpy() are perfectly safe functions.
Also note that strncpy was never intended to be a safe version of strcpy. It is used for an obscure, obsolete string format used in an ancient version of Unix. strncpy is actually very unsafe (one of many blog post about it here), unlike strcpy, since very few programmers seem to be able to use the former without producing fatal bugs (no null termination).
A better question is why the inherently unsafe strncpy() wasn't removed from the language. Is anyone working with obscure Unix strings from the 1970s much?
When removing a function completely, one of the major things the standards have to mainly consider is how much of code it could break and how many people (programmers, library writers, compiler vendors, etc) would be annoyed (or would oppose) with the change.
gets() was deprecated from LSB (Linux Standard Base). POSIX-2008 made it obsolete and gets() has been historically known to be a seriously bad function and has always been strongly discouraged to use in any code. Pretty much every C programmer knew it's seriously dangerous to use gets(). So the chances of its removal breaking any production code is very very little, it not, non-existing. So it was easy to remove gets() from C11 for the committee.
But it's not the case with strcpy, strcat, etc. They can be used safely and it's still being used by many programmers in new code. While they can be subject to be buffer overflow, it's mostly programmer's control while gets() isn't.
There can be argument made to use snprintf in place of strcpy and strcat. But it would seem pointless in simple cases like:
char buf[256];
strcpy(buf, "hello");
(if buf was a pointer, then the allocate size need to tracked for use in snprintf)
because as a programmer, I know, the above is perfectly safe. More importantly a lot of legacy code would break. Basically, there's no such strong arguments can be made to remove strcpy, etc functions as they can be used safely.
What you are talking about is scenarios which will lead to undefined behavior.
Let's say
char a[3] = "string";
for(i=0;i<5;i++)
printf("%c\n",a[i]);
You have array out of bound access and the standard hasn't removed this because it is you who is assigning the value and it is under your control.
Same with strcpy() and strcat() .
So standard can't remove all scenarios leading to UB.
Whereas gets() we know is not under the programmers control and it is taking data from some stream and you never know what the input might be and there is a high probability you might end up with buffer overflow so it has been removed and a safer function fgets() has been added.
I've seen some posters stating that strdup is evil. Is there a consensus on this? I've used it without any guilty feelings and can see no reason why it is worse than using malloc/memcpy.
The only thing I can think might earn strdup a reputation is that callers might misuse it (eg. not realise they have to free the memory returned; try to strcat to the end of a strdup'ed string). But then malloc'ed strings are not free from the possibility of misuse either.
Thanks for the replies and apologies to those who consider the question unhelpful (votes to close). In summary of the replies, it seems that there is no general feeling that strdup is evil per se, but a general consensus that it can, like many other parts of C, be used improperly or unsafely.
There is no 'correct' answer really, but for the sake of accepting one, I accepted #nneoneo's answer - it could equally have been #R..'s answer.
Two reasons I can think of:
It's not strictly ANSI C, but rather POSIX. Consequently, some compilers (e.g. MSVC) discourage use (MSVC prefers _strdup), and technically the C standard could define its own strdup with different semantics since str is a reserved prefix. So, there are some potential portability concerns with its use.
It hides its memory allocation. Most other str functions don't allocate memory, so users might be misled (as you say) into believing the returned string doesn't need to be freed.
But, aside from these points, I think that careful use of strdup is justified, as it can reduce code duplication and provides a nice implementation for common idioms (such as strdup("constant string") to get a mutable, returnable copy of a literal string).
My answer is rather supporting strdup and it is no worse than any other function in C.
POSIX is a standard and strdup is not too difficult to implement if portability becomes an issue.
Whether to free the memory allocated by strdup shouldn't be an issue if anyone taken a little time to read the man page and understand how strdup works. If one doesn't understand how a function works, it's very likely the person is going to mess up something, this is applicable to any function, not just strdup.
In C, memory & most other things are managed by the programmer, so strdup is no worse than forgetting to free malloc'ed memory, failing to null terminate a string, using incorrect format string in scanf (and invoking undefined behaviour), accessing dangling pointer etc.
(I really wanted to post this as a comment, but couldn't add in a single comment. Hence, posted it as an answer).
I haven't really heard strdup described as evil, but some possible reasons some people dislike it:
It's not standard C (but is in POSIX). However I find this reason silly because it's nearly a one-line function to add on systems that lack it.
Blindly duplicating strings all over the place rather than using them in-place when possible wastes time and memory and introduces failure cases into code that might otherwise be failure-free.
When you do need a copy of a string, it's likely you actually need more space to modify or build on it, and strdup does not give you that.
I think the majority of the concern about strdup comes from security concerns regarding buffer over runs, and improperly formatted strings. If a non-null terminated string is passed to strdup it can allocated an undefined length string. I don't know if this can be specifically leveraged into an attack but in general it is good secure coding practice to only use string functions which take a maximum length instead of relying on the null character alone.
Many people obviously don't, but I personally find strdup evil for several reasons,
the main one being it hides the allocation. The other str* functions and most other standard functions require no free afterwards, so strdup looks innocuous enough and you can forget to clean up after it. dmckee suggested to just add it to your mental list of functions that need cleaning up after, but why? I don't see a big advantage over reducing two medium-length lines to one short one.
It allocates memory on the heap always, and with C99's (is it 99?) VLAs, you have yet another reason to just use strcpy (you don't even need malloc). You can't always do this, but when you can, you should.
It's not part of the ISO standard (but it is part of the POSIX standard, thanks Wiz), but that's really a small point as R.. mentioned that it can be added easily. If you write portable programs, I'm not sure how you'd tell if it was already defined or not though...
These are of course a few of my own reasons, no one else's. To answer your question, there is no consensus that I'm aware of.
If you're writing programs just for yourself and you find strdup no problem, then there's much less reason not to use it than if you are writing a program to be read by many people of many skill levels and ages.
My reason for disliking strdup, which hasn't been mentioned, is that it is resource allocation without a natural pair. Let's try a silly game: I say malloc, you say free. I say open you say close. I say create you say destroy. I say strdup you say ....?
Actually, the answer to strdup is free of course, and the function would have been better named malloc_and_strcpy to make that clear. But many C programmers don't think of it that way and forgets that strdup requires its opposite or "ending" free to deallocate.
In my experience, it is very common to find memory leaks in code which calls strdup. It's an odd function which combines strlen, malloc and strcpy.
Why is strdup considered to be evil
Conflicts with Future language directions.
Reliance on errno state.
Easier to make your own strdup() that is not quite like the POISX one nor the future C2x one.
With C2x on the way with certain inclusion of strdup(), using strdup() before that has these problems.
The C2x proposed strdup() does not mention errno whereas POSIX does. Code that relies on setting errno to ENOMEM or EINVAL can have trouble in the future.
The C2x proposed char *strdup(const char *s1) uses a const char * as the parameter. User coded versions of strdup() too often use char *s1, incurring a difference that can break select code that counts on the char * signature. I.E. function pointers.
User code that did roll their own strdup() were not following C's Future language directions with its "Function names that begin with str, mem, or wcs and a lowercase letter may be added to the
declarations in the <string.h> header" and so may incur library conflicts with the new strdup() and user's strdup().
If user code wants strdup() code before C2x, consider naming it something different like my_strdup() and use a const char * parameter. Minimize or avoid any reliance on the state of errno after the call returns NULL.
My my_strdup() effort - warts and all.
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Possible Duplicate:
Why should you use strncpy instead of strcpy?
I'm reading a book about computers/cryptographic etc. And often the writer use thing such as strncpy(dest, src, strlen(src)); instead of strcpy(dest, src); it makes no much sense for me.. Well, I'm a professional programmer.
The question is: It make really a real difference? real applications use something like this?
The author is almost certainly abusing the strn* functions. Unfortunately, there is almost never a good reason to use strn* functions, since they don't actually do what you want.
Let's take a look at strcpy and strncpy:
char *strcpy(char *dest, const char *src);
char *strncpy(char *dest, const char *src, size_t n);
The strcpy function copies src into dest, including a trailing \0. It is only rarely useful:
Unless you know the source string fits in the destination buffer, you have a buffer overflow. Buffer overflows are security hazards, they can crash your program, and cause it to behave incorrectly.
If you do know the size of the source string and destination buffer, you might as well use memcpy.
By comparison, the strncpy copies at most n bytes of src into dest and then pads the rest with \0. It is only rarely useful:
Unless you know the source string is smaller than the destination buffer, you cannot be certain that the resulting buffer will be nul-terminated. This can cause errors elsewhere in the program, if you assume the result is nul-terminated.
If you do know the source string is smaller, again, you might as well use memcpy.
You can simply terminate the string after you call strncpy, but are you sure that you want to silently truncate the result? I'd imagine most of the time, you'd rather have an error message.
Why do these functions exist?
The strcpy function is occasionally handy, but it is mostly a relic of an era where people did not care very much about validating input. Feeding a string that is too large to a program would crash it, and the advice was "don't do that".
The strncpy function is useful when you want to transmit data in fixed-size fields, and you don't want to put garbage in the remainder of the field. This is mostly a relic of an era where people used fixed-size fields.
So you will rarely see strcat or strncpy in modern C software.
A worse problem
However, your example combines the worst of both worlds. Let's examine this piece of source code:
strncpy(dest, src, strlen(src));
This copies src into dest, without a \0 terminator and without bounds checking. It combines the worst aspect of strcpy (no bounds checking) with the worst aspect of strncpy (no terminator). If you see code like this, run away.
How to work with strings
Good C code typically uses one of a few options for working with strings:
Use fixed buffers and snprintf, as long as you don't mind fixed buffers.
Use bounded string functions like strlcpy and strlcat. These are BSD extensions.
Use a custom string point which tracks string lengths, and write your own functions using memcpy and malloc.
If that code is a verbatim copy from the book, the author either does not know C or is not a security specialist or both.
It could also be that he's using a misconfigured compiler, explicitly prohibiting the use of certain known to be potentially unsafe functions. And it's a questionable practice when the compiler cannot distinguish safe from unsafe from potentially unsafe and is just getting in the way all the time.
The reason for the strn family is to prevent you from overflowing buffers. If you're passed data from a caller and blindly trust that it's properly null-terminated and won't overflow the buffer you're copying it to, your going to got owned by a buffer overrun attack.
The efficiency difference is negligible. The strn family might be slightly slower as it needs to keep checking that you're not overflowing.
strncpy(dest, src, strlen(src)); is the same as strcpy(dest, src); so there's no difference here, but..
Strcpy can lead to security holes, since you don't know how long the strings are. Somebody clever can overwrite something on your server using this.
Use of strncpy against strcpy has little to do with its execution speed but strncpy strongly lowers the possibility of buffer overflow attacks.
In C, the pointers are most powerful but they also make the system vulnerable.
Following excerpt from a good Hack-Proofing book might help you.
Many overflow bugs are a result of bad string manipulation. Calls such as
strcpy() do not check the length of a string before copying it. The result is
that a buffer overflow may occur. It is expected that a NULL terminator will
be present. In one sense, the attacker relies on this bug in order to exploit a
machine; however, it also means that the attacker’s injected buffer also must
be free of NULL characters. If the attacker inserts a NULL character, the
string copy will be terminated before the entire payload can be inserted.
Are functions like strcpy, gets, etc. always dangerous? What if I write a code like this:
int main(void)
{
char *str1 = "abcdefghijklmnop";
char *str2 = malloc(100);
strcpy(str2, str1);
}
This way the function doesn't accept arguments(parameters...) and the str variable will always be the same length...which is here 16 or slightly more depending on the compiler version...but yeah 100 will suffice as of march, 2011 :).
Is there a way for a hacker to take advantage of the code above?
10x!
Absolutely not. Contrary to Microsoft's marketing campaign for their non-standard functions, strcpy is safe when used properly.
The above is redundant, but mostly safe. The only potential issue is that you're not checking the malloc return value, so you may be dereferencing null (as pointed out by kotlinski). In practice, this likely to cause an immediate SIGSEGV and program termination.
An improper and dangerous use would be:
char array[100];
// ... Read line into uncheckedInput
// Extract substring without checking length
strcpy(array, uncheckedInput + 10);
This is unsafe because the strcpy may overflow, causing undefined behavior. In practice, this is likely to overwrite other local variables (itself a major security breach). One of these may be the return address. Through a return to lib C attack, the attacker may be able to use C functions like system to execute arbitrary programs. There are other possible consequences to overflows.
However, gets is indeed inherently unsafe, and will be removed from the next version of C (C1X). There is simply no way to ensure the input won't overflow (causing the same consequences given above). Some people would argue it's safe when used with a known input file, but there's really no reason to ever use it. POSIX's getline is a far better alternative.
Also, the length of str1 doesn't vary by compiler. It should always be 17, including the terminating NUL.
You are forcefully stuffing completely different things into one category.
Functions gets is indeed always dangerous. There's no way to make a safe call to gets regardless of what steps you are willing to take and how defensive you are willing to get.
Function strcpy is perfectly safe if you are willing to take the [simple] necessary steps to make sure that your calls to strcpy are safe.
That already puts gets and strcpy in vastly different categories, which have nothing in common with regard to safety.
The popular criticisms directed at safety aspects of strcpy are based entirely on anecdotal social observations as opposed to formal facts, e.g. "programmers are lazy and incompetent, so don't let them use strcpy". Taken in the context of C programming, this is, of course, utter nonsense. Following this logic we should also declare the division operator exactly as unsafe for exactly the same reasons.
In reality, there are no problems with strcpy whatsoever. gets, on the other hand, is a completely different story, as I said above.
yes, it is dangerous. After 5 years of maintenance, your code will look like this:
int main(void)
{
char *str1 = "abcdefghijklmnop";
{enough lines have been inserted here so as to not have str1 and str2 nice and close to each other on the screen}
char *str2 = malloc(100);
strcpy(str2, str1);
}
at that point, someone will go and change str1 to
str1 = "THIS IS A REALLY LONG STRING WHICH WILL NOW OVERRUN ANY BUFFER BEING USED TO COPY IT INTO UNLESS PRECAUTIONS ARE TAKEN TO RANGE CHECK THE LIMITS OF THE STRING. AND FEW PEOPLE REMEMBER TO DO THAT WHEN BUGFIXING A PROBLEM IN A 5 YEAR OLD BUGGY PROGRAM"
and forget to look where str1 is used and then random errors will start happening...
Your code is not safe. The return value of malloc is unchecked, if it fails and returns 0 the strcpy will give undefined behavior.
Besides that, I see no problem other than that the example basically does not do anything.
strcpy isn't dangerous as far as you know that the destination buffer is large enough to hold the characters of the source string; otherwise strcpy will happily copy more characters than your target buffer can hold, which can lead to several unfortunate consequences (stack/other variables overwriting, which can result in crashes, stack smashing attacks & co.).
But: if you have a generic char * in input which hasn't been already checked, the only way to be sure is to apply strlen to such string and check if it's too large for your buffer; however, now you have to walk the entire source string twice, once for checking its length, once to perform the copy.
This is suboptimal, since, if strcpy were a little bit more advanced, it could receive as a parameter the size of the buffer and stop copying if the source string were too long; in a perfect world, this is how strncpy would perform (following the pattern of other strn*** functions). However, this is not a perfect world, and strncpy is not designed to do this. Instead, the nonstandard (but popular) alternative is strlcpy, which, instead of going out of the bounds of the target buffer, truncates.
Several CRT implementations do not provide this function (notably glibc), but you can still get one of the BSD implementations and put it in your application. A standard (but slower) alternative can be to use snprintf with "%s" as format string.
That said, since you're programming in C++ (edit I see now that the C++ tag has been removed), why don't you just avoid all the C-string nonsense (when you can, obviously) and go with std::string? All these potential security problems vanish and string operations become much easier.
The only way malloc may fail is when an out-of-memory error occurs, which is a disaster by itself. You cannot reliably recover from it because virtually anything may trigger it again, and the OS is likely to kill your process anyway.
As you point out, under constrained circumstances strcpy isn't dangerous. It is more typical to take in a string parameter and copy it to a local buffer, which is when things can get dangerous and lead to a buffer overrun. Just remember to check your copy lengths before calling strcpy and null terminate the string afterward.
Aside for potentially dereferencing NULL (as you do not check the result from malloc) which is UB and likely not a security threat, there is no potential security problem with this.
gets() is always unsafe; the other functions can be used safely.
gets() is unsafe even when you have full control on the input -- someday, the program may be run by someone else.
The only safe way to use gets() is to use it for a single run thing: create the source; compile; run; delete the binary and the source; interpret results.
Background:
I have a small routine that mimics fgets(character, 2, fp) except it takes a character from a string instead of a stream. newBuff is dynamically allocated string passed as a parameter and character is declared as char character[2].
Routine:
character[0] = newBuff[0];
character[1] = '\0';
strcpy(newBuff, newBuff+1);
The strcpy replicates the loss of information as each character is read from it.
Problem: Valgrind does warns me about
this activity, "Source and destination
overlap in strcpy(0x419b818,
0x419b819)".
Should I worry about this warning?
Probably the standard does not specify what happens when these buffers overlap. So yes, valgrind is right to complain about this.
In practical terms you will most likely find that your strcpy copies in order from left-to-right (eg. while (*dst++ = *src++);) and that it's not an issue. But it it still incorrect and may have issues when running with other C libraries.
One standards-correct way to write this would be:
memmove(newBuff, newBuff+1, strlen(newBuff));
Because memmove is defined to handle overlap. (Although here you would end up traversing the string twice, once to check the length and once to copy. I also took a shortcut, since strlen(newBuff) should equal strlen(newBuff+1)+1, which is what I originally wrote.)
Yes, and you should also worry that your function has pathologically bad performance (O(n^2) for a task that should be O(n)). Moving the entire contents of the string back by a character every time you read a character is a huge waste of time. Instead you should just keep a pointer to the current position and increment that pointer.
Situations where you find yourself needing memmove or the equivalent (copying between buffers that overlap) almost always indicate a design flaw. Often it's not just a flaw in the implementation but in the interface.
Yes -- the behavior of strcpy is only defined if the source and dest don't overlap. You might consider a combination of strlen and memmove instead.
Yes, you should worry. The C standard states that the behavior of strcpy is undefined when the source and destination objects overlap. Undefined behavior means it may work sometimes, or it may fail, or it may appear to succeed but manifest failure elsewhere in the program.
The behavior of strcpy() is officially undefined if source and destination overlap.
From the manpage for memcpy comes a suggestion:
The memcpy() function copies n bytes from memory area s2 to memory area s1. If s1 and s2 overlap, behavior is undefined. Applications in which s1 and s2 might overlap should use memmove(3) instead.
The answer is yes: with certain compiler/library implementations, newest ones I guess, you'll end up with a bogus result. See How is strcpy implemented? for an example.