1.\" Copyright (c) 1990, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" Chris Torek and the American National Standards Committee X3, 6.\" on Information Processing Systems. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)strcat.3 8.1 (Berkeley) 6/4/93 33.\" $FreeBSD$ 34.\" 35.Dd December 1, 2009 36.Dt STRCAT 3 37.Os 38.Sh NAME 39.Nm strcat , 40.Nm strncat 41.Nd concatenate strings 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In string.h 46.Ft char * 47.Fn strcat "char * restrict s" "const char * restrict append" 48.Ft char * 49.Fn strncat "char * restrict s" "const char * restrict append" "size_t count" 50.Sh DESCRIPTION 51The 52.Fn strcat 53and 54.Fn strncat 55functions 56append a copy of the null-terminated string 57.Fa append 58to the end of the null-terminated string 59.Fa s , 60then add a terminating 61.Ql \e0 . 62The string 63.Fa s 64must have sufficient space to hold the result. 65.Pp 66The 67.Fn strncat 68function 69appends not more than 70.Fa count 71characters from 72.Fa append , 73and then adds a terminating 74.Ql \e0 . 75.Sh RETURN VALUES 76The 77.Fn strcat 78and 79.Fn strncat 80functions 81return the pointer 82.Fa s . 83.Sh SEE ALSO 84.Xr bcopy 3 , 85.Xr memccpy 3 , 86.Xr memcpy 3 , 87.Xr memmove 3 , 88.Xr strcpy 3 , 89.Xr strlcat 3 , 90.Xr strlcpy 3 , 91.Xr wcscat 3 92.Sh STANDARDS 93The 94.Fn strcat 95and 96.Fn strncat 97functions 98conform to 99.St -isoC . 100.Sh SECURITY CONSIDERATIONS 101The 102.Fn strcat 103function is easily misused in a manner 104which enables malicious users to arbitrarily change 105a running program's functionality through a buffer overflow attack. 106(See 107the FSA.) 108.Pp 109Avoid using 110.Fn strcat . 111Instead, use 112.Fn strncat 113or 114.Fn strlcat 115and ensure that no more characters are copied to the destination buffer 116than it can hold. 117.Pp 118Note that 119.Fn strncat 120can also be problematic. 121It may be a security concern for a string to be truncated at all. 122Since the truncated string will not be as long as the original, 123it may refer to a completely different resource 124and usage of the truncated resource 125could result in very incorrect behavior. 126Example: 127.Bd -literal 128void 129foo(const char *arbitrary_string) 130{ 131 char onstack[8]; 132 133#if defined(BAD) 134 /* 135 * This first strcat is bad behavior. Do not use strcat! 136 */ 137 (void)strcat(onstack, arbitrary_string); /* BAD! */ 138#elif defined(BETTER) 139 /* 140 * The following two lines demonstrate better use of 141 * strncat(). 142 */ 143 (void)strncat(onstack, arbitrary_string, 144 sizeof(onstack) - strlen(onstack) - 1); 145#elif defined(BEST) 146 /* 147 * These lines are even more robust due to testing for 148 * truncation. 149 */ 150 if (strlen(arbitrary_string) + 1 > 151 sizeof(onstack) - strlen(onstack)) 152 err(1, "onstack would be truncated"); 153 (void)strncat(onstack, arbitrary_string, 154 sizeof(onstack) - strlen(onstack) - 1); 155#endif 156} 157.Ed 158