freebsd-dev/lib/libc/string/strlcpy.3

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.Dd May 1, 2020
.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
.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/
.%U full_papers/millert/millert.pdf
.Re
.Sh HISTORY
The
.Fn strlcpy
and
.Fn strlcat
functions first appeared in
.Ox 2.4 ,
and
.Fx 3.3 .