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NetBSD 6.1.5 - man page for strlcpy (netbsd section 3)

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 buf-
     fer (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));

     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 pur-
     pose of strlcpy() and strlcat().

SEE ALSO
     snprintf(3), strncat(3), strncpy(3)

     Todd C. Miller and Theo de Raadt, "strlcpy and strlcat -- Consistent, Safe, String Copy and
     Concatenation", Proceedings of the FREENIX Track: 1999 USENIX Annual Technical Conference,
     USENIX Association,
     http://www.usenix.org/publications/library/proceedings/usenix99/full_papers/millert/millert.pdf,
     June 6-11, 1999.

HISTORY
     The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, then in NetBSD 1.4.3
     and FreeBSD 3.3.

BSD					  March 1, 2001 				      BSD


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