strcpy

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 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  destina-
       tion string dest.

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(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 over-
       flowed and that unused bytes in the destination 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  communica-
       tion 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:

           if (buflen > 0) {
               strncpy(buf, str, buflen - 1);
               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 destination with (further) null bytes.  This function fixes some of
       the  problems of strcpy() and strncpy(), but the caller must still han-
       dle 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  any-
       thing  might  happen.  Overflowing fixed-length string buffers is a fa-
       vorite 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
       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 5.05 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/.

GNU                               2019-03-06                         STRCPY(3)