NAME
strlcpy, strlcat - size-bounded string copying and concatenation
LIBRARY
Utility functions from BSD systems (libbsd, -lbsd)
SYNOPSIS
#include <string.h>
size_t
strlcpy(char *dst, const char *src, size_t size);
size_t
strlcat(char *dst, const char *src, size_t size);
DESCRIPTION
The strlcpy() and strlcat() functions copy and concatenate strings
respectively. They are designed to be safer, more consistent, and less
error prone replacements for strncpy(3) and strncat(3). Unlike those
functions, strlcpy() and strlcat() take the full size of the buffer (not
just the length) and guarantee to NUL-terminate the result (as long as
size is larger than 0 or, in the case of strlcat(), as long as there is
at least one byte free in dst). Note that a byte for the NUL should be
included in size. Also note that strlcpy() and strlcat() only operate on
true “C” strings. This means that for strlcpy() src must be NUL-
terminated and for strlcat() both src and dst must be NUL-terminated.
The strlcpy() function copies up to size - 1 characters from the NUL-
terminated string src to dst, NUL-terminating the result.
The strlcat() function appends the NUL-terminated string src to the end
of dst. It will append at most size - strlen(dst) - 1 bytes, NUL-
terminating the result.
RETURN VALUES
The strlcpy() and strlcat() functions return the total length of the
string they tried to create. For strlcpy() that means the length of src.
For strlcat() that means the initial length of dst plus the length of
src. While this may seem somewhat confusing, it was done to make
truncation detection simple.
Note, however, that if strlcat() traverses size characters without
finding a NUL, the length of the string is considered to be size and the
destination string will not be NUL-terminated (since there was no space
for the NUL). This keeps strlcat() from running off the end of a string.
In practice this should not happen (as it means that either size is
incorrect or that dst is not a proper “C” string). The check exists to
prevent potential security problems in incorrect code.
EXAMPLES
The following code fragment illustrates the simple case:
char *s, *p, buf[BUFSIZ];
...
(void)strlcpy(buf, s, sizeof(buf));
(void)strlcat(buf, p, sizeof(buf));
To detect truncation, perhaps while building a pathname, something like
the following might be used:
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;
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:
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;
However, one may question the validity of such optimizations, as they
defeat the whole purpose of strlcpy() and strlcat(). As a matter of
fact, the first version of this manual page got it wrong.
SEE ALSO
snprintf(3), strncat(3), strncpy(3)
HISTORY
The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, and
made their appearance in FreeBSD 3.3.