strlcpy(3C)strlcpy(3C)NAME
strlcpy, strlcat - size-bounded string copying and concatenation
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(3C) and strncat(3C) . 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 you should include a byte for the NUL 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));
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strlcpy(3C)strlcpy(3C)
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 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:
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
SEE ALSOsnprintf(3S), strncat(3C), strncpy(3C)HISTORYstrlcpy and strlcat first appeared in OpenBSD 2.4. They were added in
IRIX 6.5.21.
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