.\" $OpenBSD: strlcpy.3,v 1.5 1999/06/06 15:17:32 aaron Exp $ .\" .\" Copyright (c) 1998 Todd C. Miller <Todd.Miller@courtesan.com> .\" 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. .\" .\" $FreeBSD: src/lib/libc/string/strlcpy.3,v 1.13 2004/07/02 23:52:13 ru Exp $ .\" .Dd June 22, 1998 .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 strings respectively. They are designed to be safer, more consistent, and less error prone replacements for .Xr strncpy 3 and .Xr strncat 3 . Unlike those functions, .Fn strlcpy and .Fn strlcat take the full size of the buffer (not just the length) and guarantee to NUL-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 NUL in .Fa size . Also note that .Fn strlcpy and .Fn strlcat only operate on true .Dq C strings. This means that for .Fn strlcpy .Fa src must be NUL-terminated and for .Fn strlcat both .Fa src and .Fa dst must be NUL-terminated. .Pp The .Fn strlcpy function copies up to .Fa size - 1 characters from the NUL-terminated string .Fa src to .Fa dst , NUL-terminating the result. .Pp The .Fn strlcat function appends the NUL-terminated string .Fa src to the end of .Fa dst . It will append at most .Fa size - strlen(dst) - 1 bytes, NUL-terminating the result. .Pp The source and destination strings should not overlap, as the behavior is undefined. .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 characters without finding a NUL, the length of the string is considered to be .Fa size and the destination string will not be NUL-terminated (since there was no space for the 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]; \&... (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 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: .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 .Sh HISTORY The .Fn strlcpy and .Fn strlcat functions first appeared in .Ox 2.4 , and made their appearance in .Fx 3.3 .