strncpy man page on Oracle

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STRCPY(3)		   Linux Programmer's Manual		     STRCPY(3)

       strcpy, strncpy - copy a string

       #include <string.h>

       char *strcpy(char *dest, const char *src);

       char *strncpy(char *dest, const char *src, size_t n);

       The  strcpy()  function	copies the string pointed to by src, including
       the terminating null byte ('\0'), to the buffer	pointed	 to  by	 dest.
       The  strings  may  not overlap, and the destination string dest must be
       large enough to receive the copy.  Beware  of  buffer  overruns!	  (See

       The  strncpy()  function is similar, except that at most n bytes of src
       are copied.  Warning: If there is no null byte among the first n	 bytes
       of src, the string placed in dest will not be null-terminated.

       If  the	length of src is less than n, strncpy() writes additional null
       bytes to dest to ensure that a total of n bytes are written.

       A simple implementation of strncpy() might be:

	   char *
	   strncpy(char *dest, const char *src, size_t n)
	       size_t i;

	       for (i = 0; i < n && src[i] != '\0'; i++)
		   dest[i] = src[i];
	       for ( ; i < n; i++)
		   dest[i] = '\0';

	       return dest;

       The strcpy() and strncpy() functions return a pointer to	 the  destina‐
       tion string dest.

       SVr4, 4.3BSD, C89, C99.

       Some  programmers consider strncpy() to be inefficient and error prone.
       If the programmer knows (i.e., includes code to test!)  that  the  size
       of dest is greater than the length of src, then strcpy() can be used.

       One  valid  (and	 intended) use of strncpy() is to copy a C string to a
       fixed-length buffer while ensuring both that the buffer	is  not	 over‐
       flowed  and that unused bytes in the target buffer are zeroed out (per‐
       haps to prevent information leaks if the buffer is  to  be  written  to
       media  or transmitted to another process via an interprocess communica‐
       tion technique).

       If there is no terminating null byte in	the  first  n  bytes  of  src,
       strncpy()  produces an unterminated string in dest.  You can force ter‐
       mination using something like the following:

	   strncpy(buf, str, n);
	   if (n > 0)
	       buf[n - 1]= '\0';

       (Of course, the above technique ignores the fact that information  con‐
       tained in src is lost in the copying to dest.)

       Some  systems  (the  BSDs,  Solaris,  and others) provide the following

	   size_t strlcpy(char *dest, const char *src, size_t size);

       This function is similar to strncpy(), but it  copies  at  most	size-1
       bytes  to  dest,	 always adds a terminating null byte, and does not pad
       the target with (further) null bytes.  This function fixes some of  the
       problems	 of  strcpy()  and strncpy(), but the caller must still handle
       the possibility of data loss if size is too small.  The return value of
       the function is the length of src, which allows truncation to be easily
       detected: if the return value is greater than or equal to size, trunca‐
       tion  occurred.	 If loss of data matters, the caller must either check
       the arguments before the call,  or  test	 the  function	return	value.
       strlcpy() is not present in glibc and is not standardized by POSIX, but
       is available on Linux via the libbsd library.

       If the destination string of a strcpy() is not large enough, then  any‐
       thing  might  happen.   Overflowing  fixed-length  string  buffers is a
       favorite cracker technique for taking complete control of the  machine.
       Any  time  a  program  reads  or copies data into a buffer, the program
       first needs to check that there's enough space.	This may  be  unneces‐
       sary  if you can show that overflow is impossible, but be careful: pro‐
       grams can get changed over time, in ways that may make  the  impossible

       bcopy(3),  memccpy(3),  memcpy(3),  memmove(3),	stpcpy(3), stpncpy(3),
       strdup(3), string(3), wcscpy(3), wcsncpy(3)

       This page is part of release 3.53 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

GNU				  2012-07-19			     STRCPY(3)

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