xref: /freebsd/lib/libc/string/strlcpy.3 (revision 069ac184) 
1.\"	$OpenBSD: strlcpy.3,v 1.26 2013/09/30 12:02:35 millert Exp $
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3.\" Copyright (c) 1998, 2000 Todd C. Miller <Todd.Miller@courtesan.com>
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28.Dd October 27, 2023
29.Dt STRLCPY 3
30.Os
31.Sh NAME
32.Nm strlcpy ,
33.Nm strlcat
34.Nd size-bounded string copying and concatenation
35.Sh LIBRARY
36.Lb libc
37.Sh SYNOPSIS
38.In string.h
39.Ft size_t
40.Fn strlcpy "char * restrict dst" "const char * restrict src" "size_t dstsize"
41.Ft size_t
42.Fn strlcat "char * restrict dst" "const char * restrict src" "size_t dstsize"
43.Sh DESCRIPTION
44The
45.Fn strlcpy
46and
47.Fn strlcat
48functions copy and concatenate strings with the same input parameters and output result as
49.Xr strcpy 3
50and
51.Xr strcat 3
52with proper overflow protection.
53They are designed to be safer, more consistent, and less error
54prone replacements for the easily misused functions
55.Xr strncpy 3
56and
57.Xr strncat 3 .
58.Pp
59.Fn strlcpy
60and
61.Fn strlcat
62take the full size of the destination buffer and guarantee
63NUL-termination if there is room.
64Note that room for the NUL should be included in
65.Fa dstsize .
66.Pp
67.Fn strlcpy
68copies up to
69.Fa dstsize
70\- 1 characters from the string
71.Fa src
72to
73.Fa dst ,
74NUL-terminating the result if
75.Fa dstsize
76is not 0.
77.Pp
78.Fn strlcat
79appends string
80.Fa src
81to the end of
82.Fa dst .
83It will append at most
84.Fa dstsize
85\- strlen(dst) \- 1 characters.
86It will then NUL-terminate, unless
87.Fa dstsize
88is 0 or the original
89.Fa dst
90string was longer than
91.Fa dstsize
92(in practice this should not happen
93as it means that either
94.Fa dstsize
95is incorrect or that
96.Fa dst
97is not a proper string).
98.Pp
99If the
100.Fa src
101and
102.Fa dst
103strings overlap, the behavior is undefined.
104.Sh RETURN VALUES
105The
106.Fn strlcpy
107and
108.Fn strlcat
109functions return the total length of the string they tried to create.
110For
111.Fn strlcpy
112that means the length of
113.Fa src .
114For
115.Fn strlcat
116that means the initial length of
117.Fa dst
118plus
119the length of
120.Fa src .
121.Pp
122If the return value is
123.Cm >=
124.Va dstsize ,
125the output string has been truncated.
126It is the caller's responsibility to handle this.
127.Sh EXAMPLES
128The following code fragment illustrates the simple case:
129.Bd -literal -offset indent
130char *s, *p, buf[BUFSIZ];
131
132\&...
133
134(void)strlcpy(buf, s, sizeof(buf));
135(void)strlcat(buf, p, sizeof(buf));
136.Ed
137.Pp
138To detect truncation, perhaps while building a pathname, something
139like the following might be used:
140.Bd -literal -offset indent
141char *dir, *file, pname[MAXPATHLEN];
142
143\&...
144
145if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
146	goto toolong;
147if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
148	goto toolong;
149.Ed
150.Pp
151Since it is known how many characters were copied the first time, things
152can be sped up a bit by using a copy instead of an append:
153.Bd -literal -offset indent
154char *dir, *file, pname[MAXPATHLEN];
155size_t n;
156
157\&...
158
159n = strlcpy(pname, dir, sizeof(pname));
160if (n >= sizeof(pname))
161	goto toolong;
162if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
163	goto toolong;
164.Ed
165.Pp
166However, one may question the validity of such optimizations, as they
167defeat the whole purpose of
168.Fn strlcpy
169and
170.Fn strlcat .
171As a matter of fact, the first version of this manual page got it wrong.
172.Sh SEE ALSO
173.Xr snprintf 3 ,
174.Xr strncat 3 ,
175.Xr strncpy 3 ,
176.Xr wcslcpy 3
177.Rs
178.%A Todd C. Miller
179.%A Theo de Raadt
180.%T strlcpy and strlcat -- Consistent, Safe, String Copy and Concatenation
181.%I USENIX Association
182.%B Proceedings of the FREENIX Track: 1999 USENIX Annual Technical Conference
183.%D June 6-11, 1999
184.%U http://www.usenix.org/publications/library/proceedings/usenix99/full_papers/millert/millert.pdf
185.Re
186.Sh HISTORY
187The
188.Fn strlcpy
189and
190.Fn strlcat
191functions first appeared in
192.Ox 2.4 ,
193and
194.Fx 3.3 .
195