strlcpy,
strlcat —
size-bounded string copying and concatenation
#include
<string.h>
size_t
strlcpy(
char
*dst,
const char
*src,
size_t
dstsize);
size_t
strlcat(
char
*dst,
const char
*src,
size_t
dstsize);
The
strlcpy() and
strlcat() functions copy and concatenate
strings with the same input parameters and output result as
snprintf(3). They are designed
to be safer, more consistent, and less error prone replacements for the easily
misused functions
strncpy(3)
and
strncat(3).
strlcpy() and
strlcat() take the full size of the
destination buffer and guarantee NUL-termination if there is room. Note that
room for the NUL should be included in
dstsize.
strlcpy() copies up to
dstsize - 1 characters from the string
src to
dst,
NUL-terminating the result if
dstsize is not
0.
strlcat() appends string
src to the end of
dst. It will append at most
dstsize - strlen(dst) - 1 characters. It will
then NUL-terminate, unless
dstsize is 0 or
the original
dst string was longer than
dstsize (in practice this should not happen
as it means that either
dstsize is incorrect
or that
dst is not a proper string).
If the
src and
dst strings overlap, the behavior is
undefined.
Besides quibbles over the return type (
size_t
versus
int) and signal handler safety
(
snprintf(3) is not entirely
safe on some systems), the following two are equivalent:
n = strlcpy(dst, src, len);
n = snprintf(dst, len, "%s", src);
Like
snprintf(3), the
strlcpy() and
strlcat() functions return the total length
of the string they tried to create. For
strlcpy() that means the length of
src. For
strlcat() that means the initial length of
dst plus the length of
src.
If the return value is
>=
dstsize, the output string has been
truncated. It is the caller's responsibility to handle this.
The following code fragment illustrates the simple case:
char *s, *p, buf[BUFSIZ];
...
(void)strlcpy(buf, s, sizeof(buf));
(void)strlcat(buf, p, sizeof(buf));
To detect truncation, perhaps while building a pathname, something like the
following might be used:
char *dir, *file, pname[PATH_MAX];
...
if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
goto toolong;
if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
goto toolong;
Since it is known how many characters were copied the first time, things can be
sped up a bit by using a copy instead of an append:
char *dir, *file, pname[PATH_MAX];
size_t n;
...
n = strlcpy(pname, dir, sizeof(pname));
if (n >= sizeof(pname))
goto toolong;
if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
goto toolong;
However, one may question the validity of such optimizations, as they defeat the
whole purpose of
strlcpy() and
strlcat(). As a matter of fact, the first
version of this manual page got it wrong.
snprintf(3),
strncat(3),
strncpy(3),
wcslcpy(3)
strlcpy() and
strlcat() first appeared in
OpenBSD 2.4.
strlcpy() and
strlcat() were created by
Todd C. Miller
<
Todd.Miller@courtesan.com>.
![[OpenBSD]](/openbsd.gif){kind=small}