xref: /dragonfly/lib/libc/string/strlcpy.3 (revision 984263bc) 
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3.\" Copyright (c) 1998 Todd C. Miller <Todd.Miller@courtesan.com>
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28.\" $FreeBSD: src/lib/libc/string/strlcpy.3,v 1.4.2.8 2002/01/19 12:29:40 yar Exp $
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.  They are designed
51to be safer, more consistent, and less error prone replacements for
52.Xr strncpy 3
53and
54.Xr strncat 3 .
55Unlike those functions,
56.Fn strlcpy
57and
58.Fn strlcat
59take the full size of the buffer (not just the length) and guarantee to
60NUL-terminate the result (as long as
61.Fa size
62is larger than 0 or, in the case of
63.Fn strlcat ,
64as long as there is at least one byte free in
65.Fa dst ) .
66Note that you should include a byte for the NUL in
67.Fa size .
68Also note that
69.Fn strlcpy
70and
71.Fn strlcat
72only operate on true
73.Dq C
74strings.
75This means that for
76.Fn strlcpy
77.Fa src
78must be NUL-terminated and for
79.Fn strlcat
80both
81.Fa src
82and
83.Fa dst
84must be NUL-terminated.
85.Pp
86The
87.Fn strlcpy
88function copies up to
89.Fa size
90- 1 characters from the NUL-terminated string
91.Fa src
92to
93.Fa dst ,
94NUL-terminating the result.
95.Pp
96The
97.Fn strlcat
98function appends the NUL-terminated string
99.Fa src
100to the end of
101.Fa dst .
102It will append at most
103.Fa size
104- strlen(dst) - 1 bytes, NUL-terminating the result.
105.Sh RETURN VALUES
106The
107.Fn strlcpy
108and
109.Fn strlcat
110functions return the total length of the string they tried to
111create.  For
112.Fn strlcpy
113that means the length of
114.Fa src .
115For
116.Fn strlcat
117that means the initial length of
118.Fa dst
119plus
120the length of
121.Fa src .
122While this may seem somewhat confusing it was done to make
123truncation detection simple.
124.Pp
125Note however, that if
126.Fn strlcat
127traverses
128.Fa size
129characters without finding a NUL, the length of the string is considered
130to be
131.Fa size
132and the destination string will not be NUL-terminated (since there was
133no space for the NUL).
134This keeps
135.Fn strlcat
136from running off the end of a string.
137In practice this should not happen (as it means that either
138.Fa size
139is incorrect or that
140.Fa dst
141is not a proper
142.Dq C
143string).
144The check exists to prevent potential security problems in incorrect code.
145.Sh EXAMPLES
146The following code fragment illustrates the simple case:
147.Bd -literal -offset indent
148char *s, *p, buf[BUFSIZ];
149
150\&...
151
152(void)strlcpy(buf, s, sizeof(buf));
153(void)strlcat(buf, p, sizeof(buf));
154.Ed
155.Pp
156To detect truncation, perhaps while building a pathname, something
157like the following might be used:
158.Bd -literal -offset indent
159char *dir, *file, pname[MAXPATHLEN];
160
161\&...
162
163if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
164	goto toolong;
165if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
166	goto toolong;
167.Ed
168.Pp
169Since we know how many characters we copied the first time, we can
170speed things up a bit by using a copy instead of an append:
171.Bd -literal -offset indent
172char *dir, *file, pname[MAXPATHLEN];
173size_t n;
174
175\&...
176
177n = strlcpy(pname, dir, sizeof(pname));
178if (n >= sizeof(pname))
179	goto toolong;
180if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
181	goto toolong;
182.Ed
183.Pp
184However, one may question the validity of such optimizations, as they
185defeat the whole purpose of
186.Fn strlcpy
187and
188.Fn strlcat .
189As a matter of fact, the first version of this manual page got it wrong.
190.Sh SEE ALSO
191.Xr snprintf 3 ,
192.Xr strncat 3 ,
193.Xr strncpy 3
194.Sh HISTORY
195.Fn strlcpy
196and
197.Fn strlcat
198functions first appeared in
199.Ox 2.4 ,
200and made their appearance in
201.Fx 3.3 .
202