1.\" $OpenBSD: strlcpy.3,v 1.5 1999/06/06 15:17:32 aaron Exp $
2.\"
3.\" Copyright (c) 1998 Todd C. Miller <Todd.Miller@courtesan.com>
4.\" All rights reserved.
5.\"
6.\" Redistribution and use in source and binary forms, with or without
7.\" modification, are permitted provided that the following conditions
8.\" are met:
9.\" 1. Redistributions of source code must retain the above copyright
10.\" notice, this list of conditions and the following disclaimer.
11.\" 2. Redistributions in binary form must reproduce the above copyright
12.\" notice, this list of conditions and the following disclaimer in the
13.\" documentation and/or other materials provided with the distribution.
14.\" 3. The name of the author may not be used to endorse or promote products
15.\" derived from this software without specific prior written permission.
16.\"
17.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
18.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
19.\" AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
20.\" THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
21.\" EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
22.\" PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
23.\" OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
24.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
25.\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
26.\" ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27.\"
28.\" $FreeBSD: head/lib/libc/string/strlcpy.3 131504 2004-07-02 23:52:20Z ru $
29.\"
30.Dd June 22, 1998
31.Dt STRLCPY 3
32.Os
33.Sh NAME
34.Nm strlcpy ,
35.Nm strlcat
36.Nd size-bounded string copying and concatenation
37.Sh LIBRARY
38.Lb libc
39.Sh SYNOPSIS
40.In string.h
41.Ft size_t
42.Fn strlcpy "char *dst" "const char *src" "size_t size"
43.Ft size_t
44.Fn strlcat "char *dst" "const char *src" "size_t size"
45.Sh DESCRIPTION
46The
47.Fn strlcpy
48and
49.Fn strlcat
50functions copy and concatenate strings respectively.
51They are designed
52to be safer, more consistent, and less error prone replacements for
53.Xr strncpy 3
54and
55.Xr strncat 3 .
56Unlike those functions,
57.Fn strlcpy
58and
59.Fn strlcat
60take the full size of the buffer (not just the length) and guarantee to
61NUL-terminate the result (as long as
62.Fa size
63is larger than 0 or, in the case of
64.Fn strlcat ,
65as long as there is at least one byte free in
66.Fa dst ) .
67Note that you should include a byte for the NUL in
68.Fa size .
69Also note that
70.Fn strlcpy
71and
72.Fn strlcat
73only operate on true
74.Dq C
75strings.
76This means that for
77.Fn strlcpy
78.Fa src
79must be NUL-terminated and for
80.Fn strlcat
81both
82.Fa src
83and
84.Fa dst
85must be NUL-terminated.
86.Pp
87The
88.Fn strlcpy
89function copies up to
90.Fa size
91- 1 characters from the NUL-terminated string
92.Fa src
93to
94.Fa dst ,
95NUL-terminating the result.
96.Pp
97The
98.Fn strlcat
99function appends the NUL-terminated string
100.Fa src
101to the end of
102.Fa dst .
103It will append at most
104.Fa size
105- strlen(dst) - 1 bytes, NUL-terminating the result.
106.Sh RETURN VALUES
107The
108.Fn strlcpy
109and
110.Fn strlcat
111functions return the total length of the string they tried to
112create.
113For
114.Fn strlcpy
115that means the length of
116.Fa src .
117For
118.Fn strlcat
119that means the initial length of
120.Fa dst
121plus
122the length of
123.Fa src .
124While this may seem somewhat confusing it was done to make
125truncation detection simple.
126.Pp
127Note however, that if
128.Fn strlcat
129traverses
130.Fa size
131characters without finding a NUL, the length of the string is considered
132to be
133.Fa size
134and the destination string will not be NUL-terminated (since there was
135no space for the NUL).
136This keeps
137.Fn strlcat
138from running off the end of a string.
139In practice this should not happen (as it means that either
140.Fa size
141is incorrect or that
142.Fa dst
143is not a proper
144.Dq C
145string).
146The check exists to prevent potential security problems in incorrect code.
147.Sh EXAMPLES
148The following code fragment illustrates the simple case:
149.Bd -literal -offset indent
150char *s, *p, buf[BUFSIZ];
151
152\&...
153
154(void)strlcpy(buf, s, sizeof(buf));
155(void)strlcat(buf, p, sizeof(buf));
156.Ed
157.Pp
158To detect truncation, perhaps while building a pathname, something
159like the following might be used:
160.Bd -literal -offset indent
161char *dir, *file, pname[MAXPATHLEN];
162
163\&...
164
165if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
166 goto toolong;
167if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
168 goto toolong;
169.Ed
170.Pp
171Since we know how many characters we copied the first time, we can
172speed things up a bit by using a copy instead of an append:
173.Bd -literal -offset indent
174char *dir, *file, pname[MAXPATHLEN];
175size_t n;
176
177\&...
178
179n = strlcpy(pname, dir, sizeof(pname));
180if (n >= sizeof(pname))
181 goto toolong;
182if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
183 goto toolong;
184.Ed
185.Pp
186However, one may question the validity of such optimizations, as they
187defeat the whole purpose of
188.Fn strlcpy
189and
190.Fn strlcat .
191As a matter of fact, the first version of this manual page got it wrong.
192.Sh SEE ALSO
193.Xr snprintf 3 ,
194.Xr strncat 3 ,
195.Xr strncpy 3
196.Sh HISTORY
197The
198.Fn strlcpy
199and
200.Fn strlcat
201functions first appeared in
202.Ox 2.4 ,
203and made their appearance in
204.Fx 3.3 .
2.\"
3.\" Copyright (c) 1998 Todd C. Miller <Todd.Miller@courtesan.com>
4.\" All rights reserved.
5.\"
6.\" Redistribution and use in source and binary forms, with or without
7.\" modification, are permitted provided that the following conditions
8.\" are met:
9.\" 1. Redistributions of source code must retain the above copyright
10.\" notice, this list of conditions and the following disclaimer.
11.\" 2. Redistributions in binary form must reproduce the above copyright
12.\" notice, this list of conditions and the following disclaimer in the
13.\" documentation and/or other materials provided with the distribution.
14.\" 3. The name of the author may not be used to endorse or promote products
15.\" derived from this software without specific prior written permission.
16.\"
17.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
18.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
19.\" AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
20.\" THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
21.\" EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
22.\" PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
23.\" OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
24.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
25.\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
26.\" ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27.\"
28.\" $FreeBSD: head/lib/libc/string/strlcpy.3 131504 2004-07-02 23:52:20Z ru $
29.\"
30.Dd June 22, 1998
31.Dt STRLCPY 3
32.Os
33.Sh NAME
34.Nm strlcpy ,
35.Nm strlcat
36.Nd size-bounded string copying and concatenation
37.Sh LIBRARY
38.Lb libc
39.Sh SYNOPSIS
40.In string.h
41.Ft size_t
42.Fn strlcpy "char *dst" "const char *src" "size_t size"
43.Ft size_t
44.Fn strlcat "char *dst" "const char *src" "size_t size"
45.Sh DESCRIPTION
46The
47.Fn strlcpy
48and
49.Fn strlcat
50functions copy and concatenate strings respectively.
51They are designed
52to be safer, more consistent, and less error prone replacements for
53.Xr strncpy 3
54and
55.Xr strncat 3 .
56Unlike those functions,
57.Fn strlcpy
58and
59.Fn strlcat
60take the full size of the buffer (not just the length) and guarantee to
61NUL-terminate the result (as long as
62.Fa size
63is larger than 0 or, in the case of
64.Fn strlcat ,
65as long as there is at least one byte free in
66.Fa dst ) .
67Note that you should include a byte for the NUL in
68.Fa size .
69Also note that
70.Fn strlcpy
71and
72.Fn strlcat
73only operate on true
74.Dq C
75strings.
76This means that for
77.Fn strlcpy
78.Fa src
79must be NUL-terminated and for
80.Fn strlcat
81both
82.Fa src
83and
84.Fa dst
85must be NUL-terminated.
86.Pp
87The
88.Fn strlcpy
89function copies up to
90.Fa size
91- 1 characters from the NUL-terminated string
92.Fa src
93to
94.Fa dst ,
95NUL-terminating the result.
96.Pp
97The
98.Fn strlcat
99function appends the NUL-terminated string
100.Fa src
101to the end of
102.Fa dst .
103It will append at most
104.Fa size
105- strlen(dst) - 1 bytes, NUL-terminating the result.
106.Sh RETURN VALUES
107The
108.Fn strlcpy
109and
110.Fn strlcat
111functions return the total length of the string they tried to
112create.
113For
114.Fn strlcpy
115that means the length of
116.Fa src .
117For
118.Fn strlcat
119that means the initial length of
120.Fa dst
121plus
122the length of
123.Fa src .
124While this may seem somewhat confusing it was done to make
125truncation detection simple.
126.Pp
127Note however, that if
128.Fn strlcat
129traverses
130.Fa size
131characters without finding a NUL, the length of the string is considered
132to be
133.Fa size
134and the destination string will not be NUL-terminated (since there was
135no space for the NUL).
136This keeps
137.Fn strlcat
138from running off the end of a string.
139In practice this should not happen (as it means that either
140.Fa size
141is incorrect or that
142.Fa dst
143is not a proper
144.Dq C
145string).
146The check exists to prevent potential security problems in incorrect code.
147.Sh EXAMPLES
148The following code fragment illustrates the simple case:
149.Bd -literal -offset indent
150char *s, *p, buf[BUFSIZ];
151
152\&...
153
154(void)strlcpy(buf, s, sizeof(buf));
155(void)strlcat(buf, p, sizeof(buf));
156.Ed
157.Pp
158To detect truncation, perhaps while building a pathname, something
159like the following might be used:
160.Bd -literal -offset indent
161char *dir, *file, pname[MAXPATHLEN];
162
163\&...
164
165if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
166 goto toolong;
167if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
168 goto toolong;
169.Ed
170.Pp
171Since we know how many characters we copied the first time, we can
172speed things up a bit by using a copy instead of an append:
173.Bd -literal -offset indent
174char *dir, *file, pname[MAXPATHLEN];
175size_t n;
176
177\&...
178
179n = strlcpy(pname, dir, sizeof(pname));
180if (n >= sizeof(pname))
181 goto toolong;
182if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
183 goto toolong;
184.Ed
185.Pp
186However, one may question the validity of such optimizations, as they
187defeat the whole purpose of
188.Fn strlcpy
189and
190.Fn strlcat .
191As a matter of fact, the first version of this manual page got it wrong.
192.Sh SEE ALSO
193.Xr snprintf 3 ,
194.Xr strncat 3 ,
195.Xr strncpy 3
196.Sh HISTORY
197The
198.Fn strlcpy
199and
200.Fn strlcat
201functions first appeared in
202.Ox 2.4 ,
203and made their appearance in
204.Fx 3.3 .