how to use strncpy correctly? - c

I know strncpy is a safer version of strcpy as said here.
However, when I want to copy from src to dst and dst is not a clean buffer, I get unwanted results, which can be avoided by strcpy:
char *p = "123";
char a[10] = "aaaaaa";
strncpy(a,p,strlen(p));
printf("%s\n", a); // 123aaa
strcpy(a,p);
printf("%s\n", a); // 123 <- desired output, as the trailing a's are garbage
In my actual case, I know strlen(src) < sizeof(dst) (at least, if that isn't the case, the program will crash a lot sooner), so I can safely strcpy.
However, if strncpy is what I should use, then must I add after dst[strlen(src)] = '\0' to avoid garbage (or maybe better yet, init the buffer beforehand?)?

The third argument of strncpy is meant to represent the size of the target buffer. And when it fills it up, it doesn't add a null terminating character by design.
If you have sufficient space for the terminator and you insist on strncpy, just pass strlen(p) + 1 so it will not assume it exhausted the target buffer.
Like many already noted by now. This use of strncpy defeats the purpose, and is really no better than than a simple call to strcpy. The only practical use for strncpy, is if you want to overwrite a part of the string in-place (which is the use case you stumbled upon). Though that too is questionable use...

how to use strncpy correctly?
When code needs to copy a string to a destination and tolerates the result being not null character terminated nor fully copied, use the sizeof destination for the size argument:
char a[10];
// strncpy(a,p,strlen(p));
strncpy(a, p, sizeof a);
// printf("%s\n", a);
printf("%.*s\n", (int) sizeof a, a);
When code wants to copy a string and detect insufficient memory problems via strncpy() or needs null character '\0' padding, also use the sizeof destination.
char a[10];
strncpy(a, p, sizeof a);
if (a[sizeof a - 1] != '\0') {
// insufficient memory
// Maybe set last last character to the null character
a[sizeof a - 1] == '\0';
// or other more robust handling
return ERROR_INSUFFICIENT_MEMORY;
}
Otherwise do not use strncpy()
There are more efficient ways to detect insufficient memory than via strncpy() when null character '\0' padding is not needed. strncpy() zero fills the rest, if any, of the un-copied buffer. The below consumes much time zero-filling just to provide a insufficiency check.
char a[1000];
strncpy(a, "abc", sizeof a);
if (a[sizeof a - 1] != '\0') {
....
Better alternatives employ strlen(), strlcpy(), memcpy(). #Deduplicator.
See also strncpy or strlcpy in my case.
For a standard C lib one-liner, code could use snprintf() and a line of error detection. A good compiler would be expected to analyse snprintf(a, sizeof a, "%s", p) and emit efficient code.
// Copy with no overflow.
// 'a' is always null character terminated.
int len = snprintf(a, sizeof a, "%s", p);
if (len < 0 || (unsigned) len >= sizeof a) Report_truncated_copy();

strncpy() isn't actually a string-function; instead, it deals with which zero-padded sequences of non-zero characters, which together have a known fixed length. It's for example the right tool to fill in data-structures which are thereafter sent to other programs, to the outside, or persisted, to avoid data-leaks.
Trying to use it outside its very specialized niche leads to hideous contortions, and inefficient code.
There are more appropriate ways to go about it, like strlcpy() and/or manually using strlen() and memcpy().

Related

What is the advantage of using `strncpy` over `snprintf`?

I just realized that I am in the habit of using snprintf for taking slices of strings, which now seems unconventional to me for several reasons (chiefly, the need to #include <stdio.h>). But after looking at strncpy more closely, I'm struggling to see why anyone would prefer it over snprintf.
To be clear, I'm comparing strncpy(dest, src, count) with snprintf(dest, count + 1, "%s", src). snprintf(dest, count + 1, src) obviously causes problems whenever src contains '%'.
What stood out to me most was how much more snprintf acts like an str- function than strncpy, which by comparison feels more like a mem- function to me.
If strncpy reaches the end of src before it copies count characters, it will write count - strlen(src) null characters in addition to the terminating null character copied from src.
This practice seems unnecessarily wasteful: functions expecting null-terminated strings (virtually every string function in C, for better or worse) would encounter the terminating null character and halt. Only mem- functions would even notice the additional characters, but I can't imagine the difference would ever be particularly useful. Also, if you're actually dealing with byte arrays, why are you using string functions?
From what I can tell, this is by-and-large not how C tends to deal with either arrays or strings. For example, char foo[6] = "hi"; doesn't fill the remaining 3 bytes with null characters! Furthermore, in my experience, arrays are typically pre-initialized in this manner using... you guessed it, memset. A mem- function.
If strncpy copies count characters before it reaches the end of src, the resulting character array won't be null-terminated. That's right, the string copying function sometimes doesn't produce a valid string. What?
strncpy's count argument doesn't include the terminating null character. This makes memory allocation a bit awkward since the character array must be allocated with at least count + 1 bytes in order to store count characters as a proper null-terminated string. Even worse, the last character in the array still needs to be set to the null character, which you can only do with strncpy if you know that strlen(src) == count + 1.
Compare all of this to snprintf:
snprintf copies at most count - 1 characters and a null character, meaning you need an array of count elements to store it all. Sweet!
If snprintf reaches the end of src before it copies count - 1 characters, it just adds a terminating null character to dest and halts - no questions asked.
If snprintf copies count - 1 characters before it reaches the end of src, it still adds a terminating null character to the end of dest.
Why on earth is strncpy this way?
What is the advantage of using strncpy over snprintf?
Well, it doesn't really make sense to compare two functions that are really intended to do different things.
strncpy is designed to write exactly n characters to dst. Some or all (but at least 1) of these n characters may be copied from the string src and the remaining characters (if any) will be null characters. Unfortunately many C programmers think that strncpy is a "safe" version of strcpy but it's not and it's not what was intended. The name strncpy is probably what is misleading.
snprintf is designed for a completely different task, i.e. formatted printing to a string.
Special use cases of sprintf like sprintf(dst, n, "%s", src) may seem equivalent to strncpy(dst, src, n) because the result dst when viewed as a "traditional C style" string looks the same. But it's not truely identical calls. They do different things to generate the result dst and dst may be different when looking at all array elements (see example in the end of this answer).
Why on earth is strncpy this way?
strncpy is handy when working with strings stored in fixed size arrays where no termination character is used and null padding is used for strings shorter than the array size.
Example of snprintf appearing to do the same as strncpy while it's not
int main ()
{
char src[] = "Hi";
char dst[6];
strcpy(dst, "Hello");
puts(dst);
strncpy(dst, src, 6);
puts(dst);
puts("------------------");
strcpy(dst, "Hello");
puts(dst);
snprintf(dst, 6, "%s", src);
puts(dst);
return 0;
}
Output
Hello
Hi
------------------
Hello
Hi
From the output it seems that the strncpy call and the snprintf call did the same, i.e. changed dst from "Hello" to "Hi".
However, a closer look at dst - like
void pp(const char* p)
{
for (int i=0; i < 6; ++i) printf("%02X ", p[i]);
puts("");
}
int main ()
{
char src[] = "Hi";
char dst[6];
strcpy(dst, "Hello");
puts(dst);
strncpy(dst, src, 6);
puts(dst);
pp(dst);
puts("------------------");
strcpy(dst, "Hello");
puts(dst);
snprintf(dst, 6, "%s", src);
puts(dst);
pp(dst);
return 0;
}
Output
Hello
Hi
48 69 00 00 00 00
------------------
Hello
Hi
48 69 00 6C 6F 00
shows that the two calls actually resulted in different dst values.
What is the advantage of using strncpy over snprintf?
Virtually none. The only advantage of strncpy (over anything) is if you want to create what the C FAQ list calls a "now-obsolete data structure, the fixed-length, not-necessarily-\0-terminated 'string.'"
If your goal is to copy the string src to the destination dst, as a proper (but possibly truncated) string, and without overflowing dst's size dstsize, then
snprintf(dst, dstsize, "%s", src);
is actually a pretty good way to do it. (Not the only way, and not a bad way, but perhaps overkill.)
On the other hand, strncpy(dst, src, dstsize), is no solution to this problem at all.
Other possible solutions to this problem are
*dst = '\0'; strncat(dst, src, dstsize);
strlcpy(dst, src, dstsize);
strcpy_s(dst, dstsize, src);
Why on earth is strncpy this way?
In the early days of C and Unix, it was somewhat common to use short, fixed-length pseudo-strings which were not necessarily null-terminated, but which were padded out to their "full length" with extra \0's if necessary. This meant that you could efficiently compare them using the equivalent of memcmp; you didn't have to check for \0 at all. See also the footnote to the aforementioned C FAQ list entry.
strncpy is pretty much obsolete today; many programmers recommend avoiding it entirely.

Creating C substrings: looping with assignment operator VS strncopy, which is better?

This might be somewhat pointless, but I'm curious what you guys think about it. I'm iterating over a string with pointers and want to pull a short substring out of it (placing the substring into a pre-allocated temporary array). Are there any reasons to use assignment over strncopy, or vice-versa? I.e.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main()
{ char orig[] = "Hello. I am looking for Molly.";
/* Strings to store the copies
* Pretend that strings had some prior value, ensure null-termination */
char cpy1[4] = "huh\0";
char cpy2[4] = "huh\0";
/* Pointer to simulate iteration over a string */
char *startptr = orig + 2;
int length = 3;
int i;
/* Using strncopy */
strncpy(cpy1, startptr, length);
/* Using assignment operator */
for (i = 0; i < length; i++)
{ cpy2[i] = *(startptr + i);
}
/* Display Results */
printf("strncpy result:\n");
printf("%s\n\n", cpy1);
printf("loop result:\n");
printf("%s\n", cpy2);
}
It seems to me that strncopy is both less typing and more easily readable, but I've seen people advocate looping instead. Is there a difference? Does it even matter? Assume that this is for small values of i (0 < i < 5), and null-termination is assured.
Refs: Strings in c, how to get subString, How to get substring in C, Difference between strncpy and memcpy?
strncpy(char * dst, char *src, size_t len) has two peculiar properties:
if (strlen(src) >= len) : the resulting string will not be nul-terminated.
if (strlen(src) < len) : the end of the string will be filled/padded with '\0'.
The first property will force you to actually check if (strlen(src) >= len) and act appropiately. (or brutally set the final character to nul with dst[len-1] = '\0';, like #Gilles does above) The other property is not particular dangerous, but can spill a lot of cycles. Imagine:
char buff[10000];
strncpy(buff, "Hello!", sizeof buff);
which touches 10000 bytes, where only 7 need to be touched.
My advice:
A: if you know the sizes, just do memcpy(dst,src,len); dst[len] = 0;
B: if you don't know the sizes, get them somehow (using strlen and/or sizeof and/or the allocated size for dynamically allocced memory). Then: goto A above.
Since for safe operation the strncpy() version already needs to know the sizes, (and the checks on them!), the memcpy() version is not more complex or more dangerous than the strncpy() version. (technically it is even marginally faster; because memcpy() does not have to check for the '\0' byte)
While this may seem counter-intuitive, there are more optimized ways to copy a string than by using the assignment operator in a loop. For instance, IA-32 provides the REP prefix for MOVS, STOS, CMPS etc for string handling, and these can be much faster than a loop that copies one char at a time. The implementation of strncpy or strcpy may choose to use such hardware-optimized code to achieve better performance.
As long as you know your lengths are "in range" and everything is correctly nul terminated, then strncpy is better.
If you need to get length checks etc in there, looping could be more convenient.
A loop with assignment is a bad idea because you're reinventing the wheel. You might make a mistake, and your code is likely to be less efficient than the code in the standard library (some processors have optimized instructions for memory copies, and optimized implementations usually at least copy word by word if possible).
However, note that strncpy is not a well-rounded wheel. In particular, if the string is too long, it does not append a null byte to the destination. The BSD function strlcpy is better designed, but not available everywhere. Even strlcpy is not a panacea: you need to get the buffer size right, and be aware that it might truncate the string.
A portable way to copy a string, with truncation if the string is too long, is to call strncpy and always add the terminating null byte. If the buffer is an array:
char buffer[BUFFER_SIZE];
strncpy(buffer, source, sizeof(buffer)-1);
buf[sizeof(buffer)-1] = 0;
If the buffer is given by a pointer and size:
strncpy(buf, source, buffer_size-1);
buf[buffer_size-1] = 0;

snprintf vs. strcpy (etc.) in C

For doing string concatenation, I've been doing basic strcpy, strncpy of char* buffers. Then I learned about the snprintf and friends.
Should I stick with my strcpy, strcpy + \0 termination? Or should I just use snprintf in the future?
For most purposes I doubt the difference between using strncpy and snprintf is measurable.
If there's any formatting involved I tend to stick to only snprintf rather than mixing in strncpy as well.
I find this helps code clarity, and means you can use the following idiom to keep track of where you are in the buffer (thus avoiding creating a Shlemiel the Painter algorithm):
char sBuffer[iBufferSize];
char* pCursor = sBuffer;
pCursor += snprintf(pCursor, sizeof(sBuffer) - (pCursor - sBuffer), "some stuff\n");
for(int i = 0; i < 10; i++)
{
pCursor += snprintf(pCursor, sizeof(sBuffer) - (pCursor - sBuffer), " iter %d\n", i);
}
pCursor += snprintf(pCursor, sizeof(sBuffer) - (pCursor - sBuffer), "into a string\n");
snprintf is more robust if you want to format your string. If you only want to concatenate, use strncpy (don't use strcpy) since it's more efficient.
As others did point out already: Do not use strncpy.
strncpy will not zero terminate in case of truncation.
strncpy will zero-pad the whole buffer if string is shorter than buffer. If buffer is large, this may be a performance drain.
snprintf will (on POSIX platforms) zero-terminate. On Windows, there is only _snprintf, which will not zero-terminate, so take that into account.
Note: when using snprintf, use this form:
snprintf(buffer, sizeof(buffer), "%s", string);
instead of
snprintf(buffer, sizeof(buffer), string);
The latter is insecure and - if string depends on user input - can lead to stack smashes, etc.
sprintf has an extremely useful return value that allows for efficient appending.
Here's the idiom:
char buffer[HUGE] = {0};
char *end_of_string = &buffer[0];
end_of_string += sprintf( /* whatever */ );
end_of_string += sprintf( /* whatever */ );
end_of_string += sprintf( /* whatever */ );
You get the idea. This works because sprintf returns the number of characters it wrote to the buffer, so advancing your buffer by that many positions will leave you pointing to the '\0' at the end of what's been written so far. So when you hand the updated position to the next sprintf, it can start writing new characters right there.
Constrast with strcpy, whose return value is required to be useless. It hands you back the same argument you passed it. So appending with strcpy implies traversing the entire first string looking for the end of it. And then appending again with another strcpy call implies traversing the entire first string, followed by the 2nd string that now lives after it, looking for the '\0'. A third strcpy will re-traverse the strings that have already been written yet again. And so forth.
So for many small appends to a very large buffer, strcpy approches (O^n) where n is the number of appends. Which is terrible.
Plus, as others mentioned, they do different things. sprintf can be used to format numbers, pointer values, etc, into your buffer.
I think there is another difference between strncpy and snprintf.
Think about this:
const int N=1000000;
char arr[N];
strncpy(arr, "abce", N);
Usually, strncpy will set the rest of the destination buffer to '\0'. This will cost lots of CPU time. While when you call snprintf,
snprintf(a, N, "%s", "abce");
it will leave the buffer unchanged.
I don't know why strncpy will do that, but in this case, I will choose snprintf instead of strncpy.
All *printf functions check formatting and expand its corresponding argument, thus it is slower than a simple strcpy/strncpy, which only copy a given number of bytes from linear memory.
My rule of thumb is:
Use snprintf whenever formatting is needed.
Stick to strncpy/memcpy when only need to copy a block of linear memory.
You can use strcpy whenever you know exatcly the size of buffers you're copying. Don't use that if you don't have full control over the buffers size.
strcpy, strncpy, etc. only copies strings from one memory location to another. But, with snprint, you can do more stuff like formatting the string. Copying integers into buffer, etc.
It purely depends on your requirement which one to use. If as per your logic, strcpy & strncpy is already working for you, there is no need to jump to snprintf.
Also, remember to use strncpy for better safety as suggested by others.
The difference between strncpy and snprintf is that strncpy basically lays on you responsibility of terminating string with '\0'. It may terminate dst with '\0' but only if src is short enough.
Typical examples are:
strncpy(dst, src, n);
// if src is longer than n dst will not contain null
// terminated string at this point
dst[n - 1] = '\0';
snprintf(dst, n, "%s", src); // dst will 100% contain null terminated string

Why does strncpy not null terminate?

strncpy() supposedly protects from buffer overflows. But if it prevents an overflow without null terminating, in all likelihood a subsequent string operation is going to overflow. So to protect against this I find myself doing:
strncpy( dest, src, LEN );
dest[LEN - 1] = '\0';
man strncpy gives:
The strncpy() function is similar, except that not more than n bytes of src are copied. Thus, if there is no null byte among the first n bytes of src, the result will not be null-terminated.
Without null terminating something seemingly innocent like:
printf( "FOO: %s\n", dest );
...could crash.
Are there better, safer alternatives to strncpy()?
strncpy() is not intended to be used as a safer strcpy(), it is supposed to be used to insert one string in the middle of another.
All those "safe" string handling functions such as snprintf() and vsnprintf() are fixes that have been added in later standards to mitigate buffer overflow exploits etc.
Wikipedia mentions strncat() as an alternative to writing your own safe strncpy():
*dst = '\0';
strncat(dst, src, LEN);
EDIT
I missed that strncat() exceeds LEN characters when null terminating the string if it is longer or equal to LEN char's.
Anyway, the point of using strncat() instead of any homegrown solution such as memcpy(..., strlen(...))/whatever is that the implementation of strncat() might be target/platform optimized in the library.
Of course you need to check that dst holds at least the nullchar, so the correct use of strncat() would be something like:
if (LEN) {
*dst = '\0'; strncat(dst, src, LEN-1);
}
I also admit that strncpy() is not very useful for copying a substring into another string, if the src is shorter than n char's, the destination string will be truncated.
Originally, the 7th Edition UNIX file system (see DIR(5)) had directory entries that limited file names to 14 bytes; each entry in a directory consisted of 2 bytes for the inode number plus 14 bytes for the name, null padded to 14 characters, but not necessarily null-terminated. It's my belief that strncpy() was designed to work with those directory structures - or, at least, it works perfectly for that structure.
Consider:
A 14 character file name was not null terminated.
If the name was shorter than 14 bytes, it was null padded to full length (14 bytes).
This is exactly what would be achieved by:
strncpy(inode->d_name, filename, 14);
So, strncpy() was ideally fitted to its original niche application. It was only coincidentally about preventing overflows of null-terminated strings.
(Note that null padding up to the length 14 is not a serious overhead - if the length of the buffer is 4 KB and all you want is to safely copy 20 characters into it, then the extra 4075 nulls is serious overkill, and can easily lead to quadratic behaviour if you are repeatedly adding material to a long buffer.)
There are already open source implementations like strlcpy that do safe copying.
http://en.wikipedia.org/wiki/Strlcpy
In the references there are links to the sources.
Strncpy is safer against stack overflow attacks by the user of your program, it doesn't protect you against errors you the programmer do, such as printing a non-null-terminated string, the way you've described.
You can avoid crashing from the problem you've described by limiting the number of chars printed by printf:
char my_string[10];
//other code here
printf("%.9s",my_string); //limit the number of chars to be printed to 9
Some new alternatives are specified in ISO/IEC TR 24731 (Check https://buildsecurityin.us-cert.gov/daisy/bsi/articles/knowledge/coding/317-BSI.html for info). Most of these functions take an additional parameter that specifies the maximum length of the target variable, ensure that all strings are null-terminated, and have names that end in _s (for "safe" ?) to differentiate them from their earlier "unsafe" versions.1
Unfortunately, they're still gaining support and may not be available with your particular tool set. Later versions of Visual Studio will throw warnings if you use the old unsafe functions.
If your tools don't support the new functions, it should be fairly easy to create your own wrappers for the old functions. Here's an example:
errCode_t strncpy_safe(char *sDst, size_t lenDst,
const char *sSrc, size_t count)
{
// No NULLs allowed.
if (sDst == NULL || sSrc == NULL)
return ERR_INVALID_ARGUMENT;
// Validate buffer space.
if (count >= lenDst)
return ERR_BUFFER_OVERFLOW;
// Copy and always null-terminate
memcpy(sDst, sSrc, count);
*(sDst + count) = '\0';
return OK;
}
You can change the function to suit your needs, for example, to always copy as much of the string as possible without overflowing. In fact, the VC++ implementation can do this if you pass _TRUNCATE as the count.
1Of course, you still need to be accurate about the size of the target buffer: if you supply a 3-character buffer but tell strcpy_s() it has space for 25 chars, you're still in trouble.
Use strlcpy(), specified here: http://www.courtesan.com/todd/papers/strlcpy.html
If your libc doesn't have an implementation, then try this one:
size_t strlcpy(char* dst, const char* src, size_t bufsize)
{
size_t srclen =strlen(src);
size_t result =srclen; /* Result is always the length of the src string */
if(bufsize>0)
{
if(srclen>=bufsize)
srclen=bufsize-1;
if(srclen>0)
memcpy(dst,src,srclen);
dst[srclen]='\0';
}
return result;
}
(Written by me in 2004 - dedicated to the public domain.)
Instead of strncpy(), you could use
snprintf(buffer, BUFFER_SIZE, "%s", src);
Here's a one-liner which copies at most size-1 non-null characters from src to dest and adds a null terminator:
static inline void cpystr(char *dest, const char *src, size_t size)
{ if(size) while((*dest++ = --size ? *src++ : 0)); }
strncpy works directly with the string buffers available, if you are working directly with your memory, you MUST now buffer sizes and you could set the '\0' manually.
I believe there is no better alternative in plain C, but its not really that bad if you are as careful as you should be when playing with raw memory.
Without relying on newer extensions, I have done something like this in the past:
/* copy N "visible" chars, adding a null in the position just beyond them */
#define MSTRNCPY( dst, src, len) ( strncpy( (dst), (src), (len)), (dst)[ (len) ] = '\0')
and perhaps even:
/* pull up to size - 1 "visible" characters into a fixed size buffer of known size */
#define MFBCPY( dst, src) MSTRNCPY( (dst), (src), sizeof( dst) - 1)
Why the macros instead of newer "built-in" (?) functions? Because there used to be quite a few different unices, as well as other non-unix (non-windows) environments that I had to port to back when I was doing C on a daily basis.
I have always preferred:
memset(dest, 0, LEN);
strncpy(dest, src, LEN - 1);
to the fix it up afterwards approach, but that is really just a matter of preference.
These functions have evolved more than being designed, so there really is no "why".
You just have to learn "how". Unfortunately the linux man pages at least are
devoid of common use case examples for these functions, and I've noticed lots
of misuse in code I've reviewed. I've made some notes here:
http://www.pixelbeat.org/programming/gcc/string_buffers.html

Why should you use strncpy instead of strcpy?

Edit: I've added the source for the example.
I came across this example:
char source[MAX] = "123456789";
char source1[MAX] = "123456789";
char destination[MAX] = "abcdefg";
char destination1[MAX] = "abcdefg";
char *return_string;
int index = 5;
/* This is how strcpy works */
printf("destination is originally = '%s'\n", destination);
return_string = strcpy(destination, source);
printf("after strcpy, dest becomes '%s'\n\n", destination);
/* This is how strncpy works */
printf( "destination1 is originally = '%s'\n", destination1 );
return_string = strncpy( destination1, source1, index );
printf( "After strncpy, destination1 becomes '%s'\n", destination1 );
Which produced this output:
destination is originally = 'abcdefg'
After strcpy, destination becomes '123456789'
destination1 is originally = 'abcdefg'
After strncpy, destination1 becomes '12345fg'
Which makes me wonder why anyone would want this effect. It looks like it would be confusing. This program makes me think you could basically copy over someone's name (eg. Tom Brokaw) with Tom Bro763.
What are the advantages of using strncpy() over strcpy()?
The strncpy() function was designed with a very particular problem in mind: manipulating strings stored in the manner of original UNIX directory entries. These used a short fixed-sized array (14 bytes), and a nul-terminator was only used if the filename was shorter than the array.
That's what's behind the two oddities of strncpy():
It doesn't put a nul-terminator on the destination if it is completely filled; and
It always completely fills the destination, with nuls if necessary.
For a "safer strcpy()", you are better off using strncat() like so:
if (dest_size > 0)
{
dest[0] = '\0';
strncat(dest, source, dest_size - 1);
}
That will always nul-terminate the result, and won't copy more than necessary.
strncpy combats buffer overflow by requiring you to put a length in it. strcpy depends on a trailing \0, which may not always occur.
Secondly, why you chose to only copy 5 characters on 7 character string is beyond me, but it's producing expected behavior. It's only copying over the first n characters, where n is the third argument.
The n functions are all used as defensive coding against buffer overflows. Please use them in lieu of older functions, such as strcpy.
While I know the intent behind strncpy, it is not really a good function. Avoid both. Raymond Chen explains.
Personally, my conclusion is simply to avoid strncpy and all its friends if you are dealing with null-terminated strings. Despite the "str" in the name, these functions do not produce null-terminated strings. They convert a null-terminated string into a raw character buffer. Using them where a null-terminated string is expected as the second buffer is plain wrong. Not only do you fail to get proper null termination if the source is too long, but if the source is short you get unnecessary null padding.
See also Why is strncpy insecure?
strncpy is NOT safer than strcpy, it just trades one type of bugs with another. In C, when handling C strings, you need to know the size of your buffers, there is no way around it. strncpy was justified for the directory thing mentioned by others, but otherwise, you should never use it:
if you know the length of your string and buffer, why using strncpy ? It is a waste of computing power at best (adding useless 0)
if you don't know the lengths, then you risk silently truncating your strings, which is not much better than a buffer overflow
What you're looking for is the function strlcpy() which does terminate always the string with 0 and initializes the buffer. It also is able to detect overflows. Only problem, it's not (really) portable and is present only on some systems (BSD, Solaris). The problem with this function is that it opens another can of worms as can be seen by the discussions on
http://en.wikipedia.org/wiki/Strlcpy
My personal opinion is that it is vastly more useful than strncpy() and strcpy(). It has better performance and is a good companion to snprintf(). For platforms which do not have it, it is relatively easy to implement.
(for the developement phase of a application I substitute these two function (snprintf() and strlcpy()) with a trapping version which aborts brutally the program on buffer overflows or truncations. This allows to catch quickly the worst offenders. Especially if you work on a codebase from someone else.
EDIT: strlcpy() can be implemented easily:
size_t strlcpy(char *dst, const char *src, size_t dstsize)
{
size_t len = strlen(src);
if(dstsize) {
size_t bl = (len < dstsize-1 ? len : dstsize-1);
((char*)memcpy(dst, src, bl))[bl] = 0;
}
return len;
}
The strncpy() function is the safer one: you have to pass the maximum length the destination buffer can accept. Otherwise it could happen that the source string is not correctly 0 terminated, in which case the strcpy() function could write more characters to destination, corrupting anything which is in the memory after the destination buffer. This is the buffer-overrun problem used in many exploits
Also for POSIX API functions like read() which does not put the terminating 0 in the buffer, but returns the number of bytes read, you will either manually put the 0, or copy it using strncpy().
In your example code, index is actually not an index, but a count - it tells how many characters at most to copy from source to destination. If there is no null byte among the first n bytes of source, the string placed in destination will not be null terminated
strncpy fills the destination up with '\0' for the size of source, eventhough the size of the destination is smaller....
manpage:
If the length of src is less than n, strncpy() pads the remainder of
dest with null bytes.
and not only the remainder...also after this until n characters is
reached. And thus you get an overflow... (see the man page
implementation)
This may be used in many other scenarios, where you need to copy only a portion of your original string to the destination. Using strncpy() you can copy a limited portion of the original string as opposed by strcpy(). I see the code you have put up comes from publib.boulder.ibm.com.
That depends on our requirement.
For windows users
We use strncpy whenever we don't want to copy entire string or we want to copy only n number of characters. But strcpy copies the entire string including terminating null character.
These links will help you more to know about strcpy and strncpy
and where we can use.
about strcpy
about strncpy
the strncpy is a safer version of strcpy as a matter of fact you should never use strcpy because its potential buffer overflow vulnerability which makes you system vulnerable to all sort of attacks

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