xref: /dragonfly/lib/libc/string/strlcpy.3 (revision 0dace59e) 
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28.\" $FreeBSD: head/lib/libc/string/strlcpy.3 235286 2012-05-11 20:06:46Z gjb $
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 * restrict dst" "const char * restrict src" "size_t size"
43.Ft size_t
44.Fn strlcat "char * restrict dst" "const char * restrict 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 a byte for the NUL should be included 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 it is known how many characters were copied the first time, things
172can be sped 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.Xr wcslcpy 3
197.Sh HISTORY
198The
199.Fn strlcpy
200and
201.Fn strlcat
202functions first appeared in
203.Ox 2.4 ,
204and made their appearance in
205.Fx 3.3 .
206