.\"	$OpenBSD: strncpy.3,v 1.2 2014/04/19 11:30:40 deraadt Exp $
.\"
.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek and the American National Standards Committee X3,
.\" on Information Processing Systems.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd $Mdocdate: April 19 2014 $
.Dt STRNCPY 3
.Os
.Sh NAME
.Nm strncpy
.Nd copy part of a string to another
.Sh SYNOPSIS
.In string.h
.Ft char *
.Fn strncpy "char *dst" "const char *src" "size_t len"
.Sh DESCRIPTION
The
.Fn strncpy
function copies not more than
.Fa len
characters from the string
.Fa src
to the buffer
.Fa dst .
If
.Fa src
is less than
.Fa len
characters long,
it fills the remaining buffer with
.Ql \e0
characters.
If the length of
.Fa src
is greater than or equal to
.Fa len ,
.Fa dst
will
.Em not
be NUL-terminated.
.Pp
.Fn strncpy
.Em only
NUL terminates the destination string when the length of the source
string is less than the length parameter.
.Pp
If the
.Fa src
and
.Fa dst
strings overlap, the behavior is undefined.
.Sh RETURN VALUES
The
.Fn strncpy
function returns
.Fa dst .
.Sh EXAMPLES
The following sets
.Va chararray
to
.Dq abc\e0\e0\e0 :
.Bd -literal -offset indent
(void)strncpy(chararray, "abc", 6);
.Ed
.Pp
The following sets
.Va chararray
to
.Dq abcdef ,
without a NUL-terminator:
.Bd -literal -offset indent
(void)strncpy(chararray, "abcdefgh", 6);
.Ed
.Pp
The following sequence copies as many characters from
.Va input
to
.Va buf
as will fit, and then NUL terminates the result by hand:
.Bd -literal -offset indent
char buf[BUFSIZ];

(void)strncpy(buf, input, sizeof(buf) - 1);
buf[sizeof(buf) - 1] = '\e0';
.Ed
.Pp
By now it is clear that
.Nm strncpy
is dangerously easy to misuse.
The
.Xr strlcpy 3
function is safer for this kind of operation:
.Bd -literal -offset indent
if (strlcpy(buf, input, sizeof(buf)) >= sizeof(buf))
        goto toolong;
.Ed
.Sh SEE ALSO
.Xr strlcpy 3 ,
.Xr wcscpy 3 ,
.Xr wcslcpy 3
.Sh STANDARDS
The
.Fn strncpy
function conforms to
.St -ansiC .
.Sh HISTORY
The
.Fn strncpy
function first appeared in
.At v7 .