— size-bounded string copying
*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 strncat(3).
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.
copies up to dstsize - 1 characters from the string
src to dst, NUL-terminating the
result if dstsize is not 0.
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
strlcat() functions return the total length of the
string they tried to create. For
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
strlcat(). As a matter of fact, the first version of
this manual page got it wrong.
snprintf(3), strncat(3), strncpy(3), wcslcpy(3)
strlcat() first appeared in OpenBSD
strlcat() were created by Todd C.