STRCPY(3)                           Linux Programmer's Manual                           STRCPY(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  des‐
       tination 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.   Warn‐
       ing:  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
   Multithreading (see pthreads(7))
       The strcpy() and strncpy() functions are thread-safe.

CONFORMING TO
       SVr4, 4.3BSD, C89, C99.

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  func‐
       tion  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  com‐
       plete  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 3.74 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 http://www.kernel.org/doc/man-pages/.



GNU                                         2014-05-21                                  STRCPY(3)