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

NAME
     ssttrrllccppyy, ssttrrllccaatt - size-bounded string copying and concatenation

LLIIBBRRAARRYY
     Standard C Library (libc, -lc)

SYNOPSIS
     ##iinncclluuddee <>

     sizet
     ssttrrllccppyy(char *dst, const char *src, sizet size);

     sizet
     ssttrrllccaatt(char *dst, const char *src, sizet size);

DESCRIPTION
     The ssttrrllccppyy() and ssttrrllccaatt() 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, ssttrrllccppyy() and ssttrrllccaatt() 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 ssttrrllccaatt(), 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 ssttrrllccppyy() and ssttrrllccaatt() only operate on
     true ``C'' strings.  This means that for ssttrrllccppyy() src must be NUL-termi-
     nated and for ssttrrllccaatt() both src and dst must be NUL-terminated.

     The ssttrrllccppyy() function copies up to size - 1 characters from the NUL-ter-
     minated string src to dst, NUL-terminating the result.

     The ssttrrllccaatt() function appends the NUL-terminated string src to the end
     of dst.  It will append at most size - strlen(dst) - 1 bytes, NUL-termi-
     nating the result.

RETURN VALUES
     The ssttrrllccppyy() and ssttrrllccaatt() functions return the total length of the
     string they tried to create.  For ssttrrllccppyy() that means the length of src.
     For ssttrrllccaatt() that means the initial length of dst plus the length of
     src.  While this may seem somewhat confusing it was done to make trunca-
     tion detection simple.

     Note however, that if ssttrrllccaatt() traverses size characters without finding
     a NUL, the length of the string is considered to be size and the destina-
     tion string will not be NUL-terminated (since there was no space for the
     NUL).  This keeps ssttrrllccaatt() from running off the end of a string.  In
     practice this should not happen (as it means that either size is incor-
     rect or that dst is not a proper ``C'' string).  The check exists to pre-
     vent potential security problems in incorrect code.

EEXXAAMMPPLLEESS
     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];
           sizet 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 ssttrrllccppyy() and ssttrrllccaatt().  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 ssttrrllccppyy() and ssttrrllccaatt() functions first appeared in OpenBSD 2.4, and
     made their appearance in FreeBSD 3.3.

BSD                              June 22, 1998                             BSD