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man:strncpy

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
     attributes(7).
     allbox; lbw19 lb lb l  l  l.   Interface Attribute Value  T{  strcpy(),
     strncpy() T}   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 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.  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, 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.

BUGS

     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
     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 4.16 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 2017-09-15 STRCPY(3)

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