|STRLCPY(3)||Library Functions Manual||STRLCPY(3)|
— size-bounded string copying and
*dst, const char
*dst, const char
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
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
functions return the total length of the string they tried to create. For
strlcpy() that means the length of
strlcat() that means
the initial length of dst plus the length of
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
strlcat(). As a matter of fact, the first version of
this manual page got it wrong.
strlcat() first appeared in OpenBSD
strlcat() were created by Todd C.
|January 25, 2019||OpenBSD-current|