strncpy (3)
Leading comments
Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. ...
NAME
strcpy, strncpy - copy a stringSYNOPSIS
#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 (aq\0aq), 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] != aq\0aq; i++)
dest[i] = src[i];
for ( ; i < n; i++)
dest[i] = aq\0aq;
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 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]= aq\0aq;
(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, 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.