.\" $OpenBSD: strlcpy.3,v 1.26 2013/09/30 12:02:35 millert Exp $ .\" .\" Copyright (c) 1998, 2000 Todd C. Miller <Todd.Miller@courtesan.com> .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR .\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES .\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. .\" .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, .\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY .\" AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL .\" THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, .\" EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, .\" PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; .\" OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, .\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR .\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF .\" ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .\" $FreeBSD$ .\" .Dd February 26, 2016 .Dt STRLCPY 3 .Os .Sh NAME .Nm strlcpy , .Nm strlcat .Nd size-bounded string copying and concatenation .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In string.h .Ft size_t .Fn strlcpy "char * restrict dst" "const char * restrict src" "size_t dstsize" .Ft size_t .Fn strlcat "char * restrict dst" "const char * restrict src" "size_t dstsize" .Sh DESCRIPTION The .Fn strlcpy and .Fn strlcat functions copy and concatenate strings with the same input parameters and output result as .Xr snprintf 3 . They are designed to be safer, more consistent, and less error prone replacements for the easily misused functions .Xr strncpy 3 and .Xr strncat 3 . .Pp .Fn strlcpy and .Fn 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 .Fa dstsize . .Pp .Fn strlcpy copies up to .Fa dstsize \- 1 characters from the string .Fa src to .Fa dst , NUL-terminating the result if .Fa dstsize is not 0. .Pp .Fn strlcat appends string .Fa src to the end of .Fa dst . It will append at most .Fa dstsize \- strlen(dst) \- 1 characters. It will then NUL-terminate, unless .Fa dstsize is 0 or the original .Fa dst string was longer than .Fa dstsize (in practice this should not happen as it means that either .Fa dstsize is incorrect or that .Fa dst is not a proper string). .Pp If the .Fa src and .Fa dst strings overlap, the behavior is undefined. .Sh RETURN VALUES Besides quibbles over the return type .Pf ( Va size_t versus .Va int ) and signal handler safety .Pf ( Xr snprintf 3 is not entirely safe on some systems), the following two are equivalent: .Bd -literal -offset indent n = strlcpy(dst, src, len); n = snprintf(dst, len, "%s", src); .Ed .Pp Like .Xr snprintf 3 , the .Fn strlcpy and .Fn strlcat functions return the total length of the string they tried to create. For .Fn strlcpy that means the length of .Fa src . For .Fn strlcat that means the initial length of .Fa dst plus the length of .Fa src . .Pp If the return value is .Cm >= .Va dstsize , the output string has been truncated. It is the caller's responsibility to handle this. .Sh EXAMPLES The following code fragment illustrates the simple case: .Bd -literal -offset indent char *s, *p, buf[BUFSIZ]; \&... (void)strlcpy(buf, s, sizeof(buf)); (void)strlcat(buf, p, sizeof(buf)); .Ed .Pp To detect truncation, perhaps while building a pathname, something like the following might be used: .Bd -literal -offset indent 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; .Ed .Pp 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: .Bd -literal -offset indent 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; .Ed .Pp However, one may question the validity of such optimizations, as they defeat the whole purpose of .Fn strlcpy and .Fn strlcat . As a matter of fact, the first version of this manual page got it wrong. .Sh SEE ALSO .Xr snprintf 3 , .Xr strncat 3 , .Xr strncpy 3 , .Xr wcslcpy 3 .Sh HISTORY The .Fn strlcpy and .Fn strlcat functions first appeared in .Ox 2.4 , and .Fx 3.3 .