strlcpy(3)

NAME

Standard C Library (libc, -lc)

#include <string.h>
size_t
strlcpy(char *dst, const char *src, size_t size);
size_t
strlcat(char *dst, const char *src, size_t size);

The strlcpy() and strlcat() functions copy and concatenate
strings respectively. They are designed to be safer, more consis
tent, and less error prone replacements for strncpy(3) and strncat(3). Un
like those
functions, strlcpy() and strlcat() take the full size of the
buffer (not just the length) and guarantee to NUL-terminate the result
(as long as
size is larger than 0 or, in the case of strlcat(), as long
as there is at least one byte free in dst). Note that you should in
clude a byte for
the NUL in size. Also note that strlcpy() and strlcat() on
ly operate on
true ``C'' strings. This means that for strlcpy() src must
be NUL-terminated and for strlcat() both src and dst must be NUL-termi
nated.
The strlcpy() function copies up to size - 1 characters from
the NUL-terminated string src to dst, NUL-terminating the result.
The strlcat() function appends the NUL-terminated string src
to the end of dst. It will append at most size - strlen(dst) - 1
bytes, NUL-terminating the result.

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. While this may seem somewhat confusing it was done to
make truncation detection simple.
Note however, that if strlcat() traverses size characters
without finding a NUL, the length of the string is considered to be size and
the destination string will not be NUL-terminated (since there was no
space for the
NUL). This keeps strlcat() from running off the end of a
string. In practice this should not happen (as it means that either
size is incorrect or that dst is not a proper ``C'' string). The check
exists to prevent potential security problems in incorrect code.

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[MAXPATHLEN];; ...; if (strlcpy(pname, dir, sizeof(pname)) >= size; of(pname))
goto toolong;; if (strlcat(pname, file, sizeof(pname)) >= size; of(pname))
goto toolong;
Since we know how many characters we copied the first time,
we can speed things up a bit by using a copy instead of an append:: char *dir, *file, pname[MAXPATHLEN];
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 optimiza
tions, 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.