NAME

     strlcpy, strlcat — size-bounded string copying and concatenation

LIBRARY

     library “libbsd”

SYNOPSIS

     #include <string.h>
     (See libbsd(7) for include usage.)

     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.