strncpy(3)

NAME

   strcpy, strncpy - copy a string

SYNOPSIS

   #include <string.h>

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

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

DESCRIPTION

   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
   BUGS.)

   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;
       }

RETURN VALUE

   The  strcpy()  and  strncpy()  functions  return  a  pointer   to   the
   destination string dest.

ATTRIBUTES

   For   an   explanation   of   the  terms  used  in  this  section,  see
   attributes(7).

   ┌────────────────────┬───────────────┬─────────┐
   │Interface           │ Attribute     │ Value   │
   ├────────────────────┼───────────────┼─────────┤
   │strcpy(), strncpy() │ Thread safety │ MT-Safe │
   └────────────────────┴───────────────┴─────────┘

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

NOTES

   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
   overflowed and that unused bytes in the target buffer  are  zeroed  out
   (perhaps to prevent information leaks if the buffer is to be written to
   media  or  transmitted  to  another   process   via   an   interprocess
   communication technique).

   If  there  is  no  terminating  null  byte in the first n bytes of src,
   strncpy() produces an unterminated string in dest.  If buf  has  length
   buflen, you can force termination using something like the following:

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

   (Of  course, the above technique ignores the fact that, if src contains
   more than buflen - 1 bytes, information  is  lost  in  the  copying  to
   dest.)

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

       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,
   truncation  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.

BUGS

   If the destination string of a  strcpy()  is  not  large  enough,  then
   anything  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
   unnecessary  if  you  can  show  that  overflow  is  impossible, but be
   careful: programs can get changed over time, in ways that may make  the
   impossible possible.

SEE ALSO

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

COLOPHON

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.

---

Free and Open Source Software