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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
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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
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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
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28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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31.\"
32.\" @(#)strcat.3 8.1 (Berkeley) 6/4/93
33.\" $FreeBSD: src/lib/libc/string/strcat.3,v 1.17 2009/12/01 07:28:56 brueffer Exp $
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.Fo strcat
48.Fa "char *restrict s1"
49.Fa "const char *restrict s2"
50.Fc
51.Ft char *
52.Fo strncat
53.Fa "char *restrict s1"
54.Fa "const char *restrict s2"
55.Fa "size_t n"
56.Fc
57.Sh DESCRIPTION
58The
59.Fn strcat
60and
61.Fn strncat
62functions
63append a copy of the null-terminated string
64.Fa s2
65to the end of the null-terminated string
66.Fa s1 ,
67then add a terminating
68.Ql \e0 .
69The string
70.Fa s1
71must have sufficient space to hold the result.
72.Pp
73The
74.Fn strncat
75function
76appends not more than
77.Fa n
78characters from
79.Fa s2 ,
80and then adds a terminating
81.Ql \e0 .
82.Pp
83The source and destination strings should not overlap, as the
84behavior is undefined.
85.Sh RETURN VALUES
86The
87.Fn strcat
88and
89.Fn strncat
90functions
91return the pointer
92.Fa s1 .
93.Sh SECURITY CONSIDERATIONS
94The
95.Fn strcat
96function is easily misused in a manner
97which enables malicious users to arbitrarily change
98a running program's functionality through a buffer overflow attack.
99(See
100the FSA.)
101.Pp
102Avoid using
103.Fn strcat .
104Instead, use
105.Fn strncat
106or
107.Fn strlcat
108and ensure that no more characters are copied to the destination buffer
109than it can hold.
110.Pp
111Note that
112.Fn strncat
113can also be problematic.
114It may be a security concern for a string to be truncated at all.
115Since the truncated string will not be as long as the original,
116it may refer to a completely different resource
117and usage of the truncated resource
118could result in very incorrect behavior.
119Example:
120.Bd -literal
121void
122foo(const char *arbitrary_string)
123{
124 char onstack[8] = "";
125
126#if defined(BAD)
127 /*
128 * This first strcat is bad behavior. Do not use strcat!
129 */
130 (void)strcat(onstack, arbitrary_string); /* BAD! */
131#elif defined(BETTER)
132 /*
133 * The following two lines demonstrate better use of
134 * strncat().
135 */
136 (void)strncat(onstack, arbitrary_string,
137 sizeof(onstack) - strlen(onstack) - 1);
138#elif defined(BEST)
139 /*
140 * These lines are even more robust due to testing for
141 * truncation.
142 */
143 if (strlen(arbitrary_string) + 1 >
144 sizeof(onstack) - strlen(onstack))
145 err(1, "onstack would be truncated");
146 (void)strncat(onstack, arbitrary_string,
147 sizeof(onstack) - strlen(onstack) - 1);
148#endif
149}
150.Ed
151.Sh SEE ALSO
152.Xr bcopy 3 ,
153.Xr memccpy 3 ,
154.Xr memcpy 3 ,
155.Xr memmove 3 ,
156.Xr strcpy 3 ,
157.Xr strlcat 3 ,
158.Xr strlcpy 3 ,
159.Xr wcscat 3
160.Sh STANDARDS
161The
162.Fn strcat
163and
164.Fn strncat
165functions
166conform to
167.St -isoC .