206 lines
5.1 KiB
Groff
206 lines
5.1 KiB
Groff
.\" $OpenBSD: strlcpy.3,v 1.19 2007/05/31 19:19:32 jmc Exp $
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.\"
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.\" Copyright (c) 1998, 2000 Todd C. Miller <Todd.Miller@courtesan.com>
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.\"
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.\" Permission to use, copy, modify, and distribute this software for any
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.\" purpose with or without fee is hereby granted, provided that the above
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.\" copyright notice and this permission notice appear in all copies.
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.\"
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.\"
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
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.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
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.\" AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
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.\" THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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.\" EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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.\" PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
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.\" OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
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.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
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.\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
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.\" ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd June 22, 1998
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.Dt STRLCPY 3
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.Os
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.Sh NAME
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.Nm strlcpy ,
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.Nm strlcat
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.Nd size-bounded string copying and concatenation
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In string.h
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.Ft size_t
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.Fn strlcpy "char * restrict dst" "const char * restrict src" "size_t size"
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.Ft size_t
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.Fn strlcat "char * restrict dst" "const char * restrict src" "size_t size"
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.Sh DESCRIPTION
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The
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.Fn strlcpy
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and
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.Fn strlcat
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functions copy and concatenate strings respectively.
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They are designed
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to be safer, more consistent, and less error prone replacements for
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.Xr strncpy 3
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and
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.Xr strncat 3 .
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Unlike those functions,
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.Fn strlcpy
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and
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.Fn strlcat
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take the full size of the buffer (not just the length) and guarantee to
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NUL-terminate the result (as long as
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.Fa size
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is larger than 0 or, in the case of
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.Fn strlcat ,
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as long as there is at least one byte free in
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.Fa dst ) .
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Note that a byte for the NUL should be included in
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.Fa size .
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Also note that
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.Fn strlcpy
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and
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.Fn strlcat
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only operate on true
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.Dq C
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strings.
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This means that for
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.Fn strlcpy
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.Fa src
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must be NUL-terminated and for
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.Fn strlcat
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both
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.Fa src
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and
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.Fa dst
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must be NUL-terminated.
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.Pp
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The
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.Fn strlcpy
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function copies up to
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.Fa size
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- 1 characters from the NUL-terminated string
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.Fa src
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to
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.Fa dst ,
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NUL-terminating the result.
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.Pp
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The
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.Fn strlcat
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function appends the NUL-terminated string
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.Fa src
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to the end of
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.Fa dst .
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It will append at most
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.Fa size
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- strlen(dst) - 1 bytes, NUL-terminating the result.
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.Sh RETURN VALUES
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The
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.Fn strlcpy
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and
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.Fn strlcat
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functions return the total length of the string they tried to
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create.
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For
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.Fn strlcpy
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that means the length of
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.Fa src .
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For
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.Fn strlcat
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that means the initial length of
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.Fa dst
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plus
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the length of
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.Fa src .
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While this may seem somewhat confusing, it was done to make
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truncation detection simple.
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.Pp
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Note however, that if
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.Fn strlcat
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traverses
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.Fa size
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characters without finding a NUL, the length of the string is considered
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to be
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.Fa size
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and the destination string will not be NUL-terminated (since there was
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no space for the NUL).
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This keeps
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.Fn strlcat
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from running off the end of a string.
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In practice this should not happen (as it means that either
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.Fa size
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is incorrect or that
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.Fa dst
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is not a proper
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.Dq C
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string).
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The check exists to prevent potential security problems in incorrect code.
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.Sh EXAMPLES
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The following code fragment illustrates the simple case:
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.Bd -literal -offset indent
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char *s, *p, buf[BUFSIZ];
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\&...
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(void)strlcpy(buf, s, sizeof(buf));
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(void)strlcat(buf, p, sizeof(buf));
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.Ed
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.Pp
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To detect truncation, perhaps while building a pathname, something
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like the following might be used:
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.Bd -literal -offset indent
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char *dir, *file, pname[MAXPATHLEN];
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\&...
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if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
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goto toolong;
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if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
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goto toolong;
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.Ed
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.Pp
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Since it is known how many characters were copied the first time, things
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can be sped up a bit by using a copy instead of an append.
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.Bd -literal -offset indent
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char *dir, *file, pname[MAXPATHLEN];
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size_t n;
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\&...
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n = strlcpy(pname, dir, sizeof(pname));
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if (n >= sizeof(pname))
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goto toolong;
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if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
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goto toolong;
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.Ed
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.Pp
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However, one may question the validity of such optimizations, as they
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defeat the whole purpose of
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.Fn strlcpy
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and
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.Fn strlcat .
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As a matter of fact, the first version of this manual page got it wrong.
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.Sh SEE ALSO
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.Xr snprintf 3 ,
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.Xr strncat 3 ,
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.Xr strncpy 3 ,
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.Xr wcslcpy 3
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.Sh HISTORY
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The
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.Fn strlcpy
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and
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.Fn strlcat
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functions first appeared in
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.Ox 2.4 ,
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and made their appearance in
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.Fx 3.3 .
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