Why should you use strncpy instead of strcpy

Question

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

User · Answer

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

User · Answer

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  lt  dstsize-1   len   dstsize-1         char  memcpy dst  src  bl   bl    0        return len

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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 fixed sized array  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  gt  0        dest 0      0       strncat dest  source  dest size - 1       That will always nul-terminate the result  and won t copy more than necessary

User · Answer

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  quot str quot  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

[c] Why should you use strncpy instead of strcpy?

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