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 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 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 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:

           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 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 5.08 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)