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

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

     Standard C	Library	(libc, -lc)

     #include <string.h>

     strlcpy(char *dst,	const char *src, size_t	size);

     strlcat(char *dst,	const char *src, size_t	size);

     The strlcpy() and strlcat() functions copy	and concatenate	strings	re-
     spectively.  They are designed to be safer, more consistent, and less er-
     ror prone replacements for	strncpy(3) and strncat(3).  Unlike those func-
     tions, 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-ter-
     minated 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-termi-
     nating the	result.

     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 trunca-
     tion 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 destina-
     tion 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 incor-
     rect or that dst is not a proper "C" string).  The	check exists to	pre-
     vent potential security problems in incorrect code.

     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 de-
     feat the whole purpose of strlcpy() and strlcat().	 As a matter of	fact,
     the first version of this manual page got it wrong.

     snprintf(3), strncat(3), strncpy(3)

     The strlcpy() and strlcat() functions first appeared in OpenBSD 2.4, and
     made their	appearance in FreeBSD 3.3.

BSD				 June 22, 1998				   BSD


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