libc/string/strlcpy.3

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.\" Copyright (c) 1998, 2000 Todd C. Miller <Todd.Miller@courtesan.com>
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.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 NUL-terminated strings respectively.
.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.
.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
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 .
.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 NUL-terminated and for
.Fn strlcat
both
.Fa src
and
.Fa dst
must be NUL-terminated.
Applications handling fixed-width fields with
.Pq possibly empty
NUL padding, instead of NUL-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
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 \*[Ge] sizeof(pname))
	goto toolong;
if (strlcpy(pname + n, file, sizeof(pname) - n) \*[Ge] 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 .