.\" $NetBSD: strlcpy.3,v 1.14.2.1 2024/09/20 11:22:29 martin Exp $ .\" from OpenBSD: strlcpy.3,v 1.11 2000/11/16 23:27:41 angelos Exp .\" .\" Copyright (c) 1998, 2000 Todd C. Miller .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission. .\" .\" 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. .\" .Dd August 11, 2023 .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 *dst" "const char *src" "size_t size" .Ft size_t .Fn strlcat "char *dst" "const char *src" "size_t size" .Sh DESCRIPTION The .Fn strlcpy and .Fn strlcat functions copy and concatenate .Tn NUL Ns -terminated strings respectively. .Pp The .Fn strlcpy function copies up to .Fa size Li "- 1" bytes from the .Tn NUL Ns -terminated string .Fa src to .Fa dst , .Tn NUL Ns -terminating the result. .Pp The .Fn strlcat function appends the .Tn NUL Ns -terminated string .Fa src to the end of .Fa dst . It will append at most .Fa size Li "-" Fn strlen dst Li "- 1" bytes, .Tn NUL Ns -terminating the result. .Ss Relation to Xr strncpy 3 and Xr strncat 3 Unlike .Xr strncpy 3 and .Xr strncat 3 , .Fn strlcpy and .Fn strlcat are guaranteed to .Tn NUL Ns -terminate the result (as long as .Fa size is larger than 0 or, in the case of .Fn strlcat , as long as there is at least one byte free in .Fa dst ) . Note that you should include a byte for the .Tn NUL in .Fa size . .Pp .Sy WARNING : Also unlike .Xr strncpy 3 and .Xr strncat 3 , .Fn strlcpy and .Fn strlcat are not guaranteed to initialize all .Fa size bytes of .Fa dst \(em bytes past .Fa dst Ns Li "[" Fn strlen src Li "+ 1" Ns Li "]" are left uninitialized. This can lead to security vulnerabilities such as leaking secrets from uninitialized stack or heap buffers. .Pp .Sy WARNING : .Fn strlcpy and .Fn strlcat only operate on true .Dq C strings. This means that for .Fn strlcpy .Fa src must be .Tn NUL Ns -terminated and for .Fn strlcat both .Fa src and .Fa dst must be .Tn NUL Ns -terminated . Applications handling fixed-width fields with .Pq possibly empty .Tn NUL padding, instead of .Tn NUL Ns -terminated C strings, MUST use .Xr strncpy 3 and .Xr strncat 3 instead. Attempting to use .Fn strlcpy or .Fn strlcat for these cases can lead to crashes or security vulnerabilities from buffer overruns. .Sh RETURN VALUES 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 . While this may seem somewhat confusing it was done to make truncation detection simple. .Pp Note however, that if .Fn strlcat traverses .Fa size bytes without finding a .Tn NUL , the length of the string is considered to be .Fa size and the destination string will not be .Tn NUL Ns -terminated .Pq since there was no space for the Tn NUL . This keeps .Fn strlcat from running off the end of a string. In practice this should not happen (as it means that either .Fa size is incorrect or that .Fa dst is not a proper .Dq C string). The check exists to prevent potential security problems in incorrect code. .Sh EXAMPLES The following code fragment illustrates the simple case: .Bd -literal -offset indent char *s, *p, buf[BUFSIZ]; \&... strlcpy(buf, s, sizeof(buf)); 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 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: .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 . .Sh SEE ALSO .Xr snprintf 3 , .Xr strncat 3 , .Xr strncpy 3 .Rs .%A Todd C. Miller .%A Theo de Raadt .%T strlcpy and strlcat -- Consistent, Safe, String Copy and Concatenation .%I USENIX Association .%B Proceedings of the FREENIX Track: 1999 USENIX Annual Technical Conference .%D June 6-11, 1999 .%U http://www.usenix.org/publications/library/proceedings/usenix99/full_papers/millert/millert.pdf .Re .Sh HISTORY The .Fn strlcpy and .Fn strlcat functions first appeared in .Ox 2.4 , then in .Nx 1.4.3 and .Fx 3.3 .