STRLCPY(3) | Library Functions Manual | STRLCPY(3) |
strlcpy
, strlcat
—
#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);
strlcpy
() and strlcat
()
functions copy and concatenate strings respectively. They are designed to be
safer, more consistent, and less error prone replacements for
strncpy(3) and
strncat(3). Unlike 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 include a byte for the NUL
in size. Also note that
strlcpy
() and strlcat
() only
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-terminated.
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.
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.
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)) ≥ sizeof(pname)) goto toolong; if (strlcat(pname, file, sizeof(pname)) ≥ sizeof(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 optimizations, as
they defeat the whole purpose of strlcpy
() and
strlcat
().
Todd C. Miller and Theo de Raadt, strlcpy and strlcat -- Consistent, Safe, String Copy and Concatenation, Proceedings of the FREENIX Track: 1999 USENIX Annual Technical Conference, USENIX Association, http://www.usenix.org/publications/library/proceedings/usenix99/full_papers/millert/millert.pdf, June 6-11, 1999.
strlcpy
() and strlcat
()
functions first appeared in OpenBSD 2.4, then in
NetBSD 1.4.3 and FreeBSD 3.3.
March 1, 2001 | NetBSD 9.0 |