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 5.05 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)