1STRLCPY(3bsd) LOCAL STRLCPY(3bsd)
2
4 strlcpy, strlcat — size-bounded string copying and concatenation
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7 Utility functions from BSD systems (libbsd, -lbsd)
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10 #include <string.h>
11 (See libbsd(7) for include usage.)
12 size_t
14 strlcpy(char *dst, const char *src, size_t size);
15 size_t
17 strlcat(char *dst, const char *src, size_t size);
18
20 The strlcpy() and strlcat() functions copy and concatenate strings
21 respectively. They are designed to be safer, more consistent, and less
22 error prone replacements for strncpy(3) and strncat(3). Unlike those
23 functions, strlcpy() and strlcat() take the full size of the buffer (not
24 just the length) and guarantee to NUL-terminate the result (as long as
25 size is larger than 0 or, in the case of strlcat(), as long as there is
26 at least one byte free in dst). Note that a byte for the NUL should be
27 included in size. Also note that strlcpy() and strlcat() only operate on
28 true “C” strings. This means that for strlcpy() src must be NUL-termi‐
29 nated and for strlcat() both src and dst must be NUL-terminated.
30 The strlcpy() function copies up to size - 1 characters from the NUL-ter‐
32 minated string src to dst, NUL-terminating the result.
33 The strlcat() function appends the NUL-terminated string src to the end
35 of dst. It will append at most size - strlen(dst) - 1 bytes, NUL-termi‐
36 nating the result.
37
39 The strlcpy() and strlcat() functions return the total length of the
40 string they tried to create. For strlcpy() that means the length of src.
41 For strlcat() that means the initial length of dst plus the length of
42 src. While this may seem somewhat confusing, it was done to make trunca‐
43 tion detection simple.
44 Note, however, that if strlcat() traverses size characters without find‐
46 ing a NUL, the length of the string is considered to be size and the des‐
47 tination string will not be NUL-terminated (since there was no space for
48 the NUL). This keeps strlcat() from running off the end of a string. In
49 practice this should not happen (as it means that either size is incor‐
50 rect or that dst is not a proper “C” string). The check exists to pre‐
51 vent potential security problems in incorrect code.
52
54 The following code fragment illustrates the simple case:
55 char *s, *p, buf[BUFSIZ];
57 ...
59 (void)strlcpy(buf, s, sizeof(buf));
61 (void)strlcat(buf, p, sizeof(buf));
62 To detect truncation, perhaps while building a pathname, something like
64 the following might be used:
65 char *dir, *file, pname[MAXPATHLEN];
67 ...
69 if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
71 goto toolong;
72 if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
73 goto toolong;
74 Since it is known how many characters were copied the first time, things
76 can be sped up a bit by using a copy instead of an append:
77 char *dir, *file, pname[MAXPATHLEN];
79 size_t n;
80 ...
82 n = strlcpy(pname, dir, sizeof(pname));
84 if (n >= sizeof(pname))
85 goto toolong;
86 if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
87 goto toolong;
88 However, one may question the validity of such optimizations, as they
90 defeat the whole purpose of strlcpy() and strlcat(). As a matter of
91 fact, the first version of this manual page got it wrong.
92
94 snprintf(3), strncat(3), strncpy(3)
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97 The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, and
98 made their appearance in FreeBSD 3.3.
99BSD May 10, 2020 BSD