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 NUL-terminated strings respectively.
The strlcpy
() function copies up to
size - 1
bytes 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
() are
guaranteed 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.
WARNING: Also unlike
strncpy(3) and
strncat(3),
strlcpy
() and strlcat
() are
not guaranteed to initialize all size bytes of
dst — bytes past
dst[strlen
(src)
+ 1
] are left uninitialized. This can lead to
security vulnerabilities such as leaking secrets from uninitialized stack or
heap buffers.
WARNING: 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. Applications handling
fixed-width fields with (possibly empty) NUL padding, instead of
NUL-terminated C strings, MUST use
strncpy(3) and
strncat(3) instead.
Attempting to use strlcpy
() or
strlcat
() for these cases can lead to crashes or
security vulnerabilities from buffer overruns.
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 bytes 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]; ... strlcpy(buf, s, sizeof(buf)); 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 bytes 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.
August 11, 2023 | NetBSD 10.1 |