xref: /freebsd/lib/libc/string/strcat.3 (revision 50a717a67bc36ba6fb26b70476918f46b8ddf293)
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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.
65If
66.Fa s
67and
68.Fa append
69overlap, the results are undefined.
70.Pp
71The
72.Fn strncat
73function
74appends not more than
75.Fa count
76characters from
77.Fa append ,
78and then adds a terminating
79.Ql \e0 .
80If
81.Fa s
82and
83.Fa append
84overlap, the results are undefined.
85.Sh RETURN VALUES
86The
87.Fn strcat
88and
89.Fn strncat
90functions
91return the pointer
92.Fa s .
93.Sh SEE ALSO
94.Xr bcopy 3 ,
95.Xr memccpy 3 ,
96.Xr memcpy 3 ,
97.Xr memmove 3 ,
98.Xr strcpy 3 ,
99.Xr strlcat 3 ,
100.Xr strlcpy 3 ,
101.Xr wcscat 3
102.Sh STANDARDS
103The
104.Fn strcat
105and
106.Fn strncat
107functions
108conform to
109.St -isoC .
110.Sh SECURITY CONSIDERATIONS
111The
112.Fn strcat
113function is easily misused in a manner
114which enables malicious users to arbitrarily change
115a running program's functionality through a buffer overflow attack.
116.Pp
117Avoid using
118.Fn strcat .
119Instead, use
120.Fn strncat
121or
122.Fn strlcat
123and ensure that no more characters are copied to the destination buffer
124than it can hold.
125.Pp
126Note that
127.Fn strncat
128can also be problematic.
129It may be a security concern for a string to be truncated at all.
130Since the truncated string will not be as long as the original,
131it may refer to a completely different resource
132and usage of the truncated resource
133could result in very incorrect behavior.
134Example:
135.Bd -literal
136void
137foo(const char *arbitrary_string)
138{
139	char onstack[8];
140
141#if defined(BAD)
142	/*
143	 * This first strcat is bad behavior.  Do not use strcat!
144	 */
145	(void)strcat(onstack, arbitrary_string);	/* BAD! */
146#elif defined(BETTER)
147	/*
148	 * The following two lines demonstrate better use of
149	 * strncat().
150	 */
151	(void)strncat(onstack, arbitrary_string,
152	    sizeof(onstack) - strlen(onstack) - 1);
153#elif defined(BEST)
154	/*
155	 * These lines are even more robust due to testing for
156	 * truncation.
157	 */
158	if (strlen(arbitrary_string) + 1 >
159	    sizeof(onstack) - strlen(onstack))
160		err(1, "onstack would be truncated");
161	(void)strncat(onstack, arbitrary_string,
162	    sizeof(onstack) - strlen(onstack) - 1);
163#endif
164}
165.Ed
166