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STRLCPY(3)               BSD Library Functions Manual               STRLCPY(3)

NAME
     strlcpy, strlcat -- size-bounded string copying and concatenation

LIBRARY
     Standard C Library (libc, -lc)

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 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 NULterminating
     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 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().  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.

BSD                              June 22, 1998                             BSD