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 June 4, 1993 36.Dt STRCAT 3 37.Os 38.Sh NAME 39.Nm strcat 40.Nd concatenate strings 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In string.h 45.Ft char * 46.Fn strcat "char * restrict s" "const char * restrict append" 47.Ft char * 48.Fn strncat "char * restrict s" "const char * restrict append" "size_t count" 49.Sh DESCRIPTION 50The 51.Fn strcat 52and 53.Fn strncat 54functions 55append a copy of the null-terminated string 56.Fa append 57to the end of the null-terminated string 58.Fa s , 59then add a terminating 60.Ql \e0 . 61The string 62.Fa s 63must have sufficient space to hold the result. 64.Pp 65The 66.Fn strncat 67function 68appends not more than 69.Fa count 70characters from 71.Fa append , 72and then adds a terminating 73.Ql \e0 . 74.Sh RETURN VALUES 75The 76.Fn strcat 77and 78.Fn strncat 79functions 80return the pointer 81.Fa s . 82.Sh SECURITY CONSIDERATIONS 83The 84.Fn strcat 85function is easily misused in a manner 86which enables malicious users to arbitrarily change 87a running program's functionality through a buffer overflow attack. 88(See 89the FSA.) 90.Pp 91Avoid using 92.Fn strcat . 93Instead, use 94.Fn strncat 95or 96.Fn strlcat 97and ensure that no more characters are copied to the destination buffer 98than it can hold. 99.Pp 100Note that 101.Fn strncat 102can also be problematic. 103It may be a security concern for a string to be truncated at all. 104Since the truncated string will not be as long as the original, 105it may refer to a completely different resource 106and usage of the truncated resource 107could result in very incorrect behavior. 108Example: 109.Bd -literal 110void 111foo(const char *arbitrary_string) 112{ 113 char onstack[8]; 114 115#if defined(BAD) 116 /* 117 * This first strcat is bad behavior. Do not use strcat! 118 */ 119 (void)strcat(onstack, arbitrary_string); /* BAD! */ 120#elif defined(BETTER) 121 /* 122 * The following two lines demonstrate better use of 123 * strncat(). 124 */ 125 (void)strncat(onstack, arbitrary_string, 126 sizeof(onstack) - strlen(onstack) - 1); 127#elif defined(BEST) 128 /* 129 * These lines are even more robust due to testing for 130 * truncation. 131 */ 132 if (strlen(arbitrary_string) + 1 > 133 sizeof(onstack) - strlen(onstack)) 134 err(1, "onstack would be truncated"); 135 (void)strncat(onstack, arbitrary_string, 136 sizeof(onstack) - strlen(onstack) - 1); 137#endif 138} 139.Ed 140.Sh SEE ALSO 141.Xr bcopy 3 , 142.Xr memccpy 3 , 143.Xr memcpy 3 , 144.Xr memmove 3 , 145.Xr strcpy 3 , 146.Xr strlcat 3 , 147.Xr strlcpy 3 148.Sh STANDARDS 149The 150.Fn strcat 151and 152.Fn strncat 153functions 154conform to 155.St -isoC . 156