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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.
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36 .\" @(#)strcat.3 8.1 (Berkeley) 6/4/93
37 .\" $FreeBSD: src/lib/libc/string/strcat.3,v 1.13 2002/09/06 11:24:06 tjr Exp $
38 .\"
39 .Dd June 4, 1993
40 .Dt STRCAT 3
41 .Os
42 .Sh NAME
43 .Nm strcat ,
44 .Nm strncat
45 .Nd concatenate strings
46 .Sh LIBRARY
47 .Lb libc
48 .Sh SYNOPSIS
49 .In string.h
50 .Ft char *
51 .Fo strcat
52 .Fa "char *restrict s1"
53 .Fa "const char *restrict s2"
54 .Fc
55 .Ft char *
56 .Fo strncat
57 .Fa "char *restrict s1"
58 .Fa "const char *restrict s2"
59 .Fa "size_t n"
60 .Fc
61 .Sh DESCRIPTION
62 The
63 .Fn strcat
64 and
65 .Fn strncat
66 functions
67 append a copy of the null-terminated string
68 .Fa s2
69 to the end of the null-terminated string
70 .Fa s1 ,
71 then add a terminating
72 .Ql \e0 .
73 The string
74 .Fa s1
75 must have sufficient space to hold the result.
76 .Pp
77 The
78 .Fn strncat
79 function
80 appends not more than
81 .Fa n
82 characters from
83 .Fa s2 ,
84 and then adds a terminating
85 .Ql \e0 .
86 .Pp
87 The source and destination strings should not overlap, as the
88 behavior is undefined.
89 .Sh RETURN VALUES
90 The
91 .Fn strcat
92 and
93 .Fn strncat
94 functions
95 return the pointer
96 .Fa s1 .
97 .Sh SECURITY CONSIDERATIONS
98 The
99 .Fn strcat
100 function is easily misused in a manner
101 which enables malicious users to arbitrarily change
102 a running program's functionality through a buffer overflow attack.
103 (See
104 the FSA.)
105 .Pp
106 Avoid using
107 .Fn strcat .
108 Instead, use
109 .Fn strncat
110 or
111 .Fn strlcat
112 and ensure that no more characters are copied to the destination buffer
113 than it can hold.
114 .Pp
115 Note that
116 .Fn strncat
117 can also be problematic.
118 It may be a security concern for a string to be truncated at all.
119 Since the truncated string will not be as long as the original,
120 it may refer to a completely different resource
121 and usage of the truncated resource
122 could result in very incorrect behavior.
123 Example:
124 .Bd -literal
125 void
126 foo(const char *arbitrary_string)
127 {
128 char onstack[8] = "";
129
130 #if defined(BAD)
131 /*
132 * This first strcat is bad behavior. Do not use strcat!
133 */
134 (void)strcat(onstack, arbitrary_string); /* BAD! */
135 #elif defined(BETTER)
136 /*
137 * The following two lines demonstrate better use of
138 * strncat().
139 */
140 (void)strncat(onstack, arbitrary_string,
141 sizeof(onstack) - strlen(onstack) - 1);
142 #elif defined(BEST)
143 /*
144 * These lines are even more robust due to testing for
145 * truncation.
146 */
147 if (strlen(arbitrary_string) + 1 >
148 sizeof(onstack) - strlen(onstack))
149 err(1, "onstack would be truncated");
150 (void)strncat(onstack, arbitrary_string,
151 sizeof(onstack) - strlen(onstack) - 1);
152 #endif
153 }
154 .Ed
155 .Sh SEE ALSO
156 .Xr bcopy 3 ,
157 .Xr memccpy 3 ,
158 .Xr memcpy 3 ,
159 .Xr memmove 3 ,
160 .Xr strcpy 3 ,
161 .Xr strlcat 3 ,
162 .Xr strlcpy 3
163 .Sh STANDARDS
164 The
165 .Fn strcat
166 and
167 .Fn strncat
168 functions
169 conform to
170 .St -isoC .