.\" Copyright (c) 1990, 1991, 1993
.\"	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.
.\" 4. 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.
.\"
.\"     @(#)strcat.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: src/lib/libc/string/strcat.3,v 1.17 2009/12/01 07:28:56 brueffer Exp $
.\"
.Dd December 1, 2009
.Dt STRCAT 3
.Os
.Sh NAME
.Nm strcat ,
.Nm strncat
.Nd concatenate strings
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In string.h
.Ft char *
.Fo strcat
.Fa "char *restrict s1"
.Fa "const char *restrict s2"
.Fc
.Ft char *
.Fo strncat
.Fa "char *restrict s1"
.Fa "const char *restrict s2"
.Fa "size_t n"
.Fc
.Sh DESCRIPTION
The
.Fn strcat
and
.Fn strncat
functions
append a copy of the null-terminated string
.Fa s2
to the end of the null-terminated string
.Fa s1 ,
then add a terminating
.Ql \e0 .
The string
.Fa s1
must have sufficient space to hold the result.
.Pp
The
.Fn strncat
function
appends not more than
.Fa n
characters from
.Fa s2 ,
and then adds a terminating
.Ql \e0 .
.Pp
The source and destination strings should not overlap, as the
behavior is undefined.
.Sh RETURN VALUES
The
.Fn strcat
and
.Fn strncat
functions
return the pointer
.Fa s1 .
.Sh SECURITY CONSIDERATIONS
The
.Fn strcat
function is easily misused in a manner
which enables malicious users to arbitrarily change
a running program's functionality through a buffer overflow attack.
(See
the FSA.)
.Pp
Avoid using
.Fn strcat .
Instead, use
.Fn strncat
or
.Fn strlcat
and ensure that no more characters are copied to the destination buffer
than it can hold.
.Pp
Note that
.Fn strncat
can also be problematic.
It may be a security concern for a string to be truncated at all.
Since the truncated string will not be as long as the original,
it may refer to a completely different resource
and usage of the truncated resource
could result in very incorrect behavior.
Example:
.Bd -literal
void
foo(const char *arbitrary_string)
{
	char onstack[8] = "";

#if defined(BAD)
	/*
	 * This first strcat is bad behavior.  Do not use strcat!
	 */
	(void)strcat(onstack, arbitrary_string);	/* BAD! */
#elif defined(BETTER)
	/*
	 * The following two lines demonstrate better use of
	 * strncat().
	 */
	(void)strncat(onstack, arbitrary_string,
	    sizeof(onstack) - strlen(onstack) - 1);
#elif defined(BEST)
	/*
	 * These lines are even more robust due to testing for
	 * truncation.
	 */
	if (strlen(arbitrary_string) + 1 >
	    sizeof(onstack) - strlen(onstack))
		err(1, "onstack would be truncated");
	(void)strncat(onstack, arbitrary_string,
	    sizeof(onstack) - strlen(onstack) - 1);
#endif
}
.Ed
.Sh SEE ALSO
.Xr bcopy 3 ,
.Xr memccpy 3 ,
.Xr memcpy 3 ,
.Xr memmove 3 ,
.Xr strcpy 3 ,
.Xr strlcat 3 ,
.Xr strlcpy 3 ,
.Xr wcscat 3
.Sh STANDARDS
The
.Fn strcat
and
.Fn strncat
functions
conform to
.St -isoC .