Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
UNVIS(3)		 BSD Library Functions Manual		      UNVIS(3)

     unvis, strunvis, strnunvis, strunvisx, strnunvisx -- decode a visual rep-
     resentation of characters

     Standard C	Library	(libc, -lc)

     #include <vis.h>

     unvis(char	*cp, int c, int	*astate, int flag);

     strunvis(char *dst, const char *src);

     strnunvis(char *dst, size_t dlen, const char *src);

     strunvisx(char *dst, const	char *src, int flag);

     strnunvisx(char *dst, size_t dlen,	const char *src, int flag);

     The unvis(), strunvis() and strunvisx() functions are used	to decode a
     visual representation of characters, as produced by the vis(3) function,
     back into the original form.

     The unvis() function is called with successive characters in c until a
     valid sequence is recognized, at which time the decoded character is
     available at the character	pointed	to by cp.

     The strunvis() function decodes the characters pointed to by src into the
     buffer pointed to by dst.	The strunvis() function	simply copies src to
     dst, decoding any escape sequences	along the way, and returns the number
     of	characters placed into dst, or -1 if an	invalid	escape sequence	was
     detected.	The size of dst	should be equal	to the size of src (that is,
     no	expansion takes	place during decoding).

     The strunvisx() function does the same as the strunvis() function,	but it
     allows you	to add a flag that specifies the style the string src is en-
     coded with.  Currently, the supported flags are: VIS_HTTPSTYLE and

     The unvis() function implements a state machine that can be used to de-
     code an arbitrary stream of bytes.	 All state associated with the bytes
     being decoded is stored outside the unvis() function (that	is, a pointer
     to	the state is passed in), so calls decoding different streams can be
     freely intermixed.	 To start decoding a stream of bytes, first initialize
     an	integer	to zero.  Call unvis() with each successive byte, along	with a
     pointer to	this integer, and a pointer to a destination character.	 The
     unvis() function has several return codes that must be handled properly.
     They are:

     0 (zero)	      Another character	is necessary; nothing has been recog-
		      nized yet.

     UNVIS_VALID      A	valid character	has been recognized and	is available
		      at the location pointed to by cp.

     UNVIS_VALIDPUSH  A	valid character	has been recognized and	is available
		      at the location pointed to by cp;	however, the character
		      currently	passed in should be passed in again.

     UNVIS_NOCHAR     A	valid sequence was detected, but no character was pro-
		      duced.  This return code is necessary to indicate	a log-
		      ical break between characters.

     UNVIS_SYNBAD     An invalid escape	sequence was detected, or the decoder
		      is in an unknown state.  The decoder is placed into the
		      starting state.

     When all bytes in the stream have been processed, call unvis() one	more
     time with flag set	to UNVIS_END to	extract	any remaining character	(the
     character passed in is ignored).

     The flag argument is also used to specify the encoding style of the
     source.  If set to	VIS_HTTPSTYLE or VIS_HTTP1808, unvis() will decode URI
     strings as	specified in RFC 1808.	If set to VIS_HTTP1866,	unvis()	will
     decode entity references and numeric character references as specified in
     RFC 1866.	If set to VIS_MIMESTYLE, unvis() will decode MIME Quoted-
     Printable strings as specified in RFC 2045.  If set to VIS_NOESCAPE,
     unvis() will not decode `\' quoted	characters.

     The following code	fragment illustrates a proper use of unvis().

	   int state = 0;
	   char	out;

	   while ((ch =	getchar()) != EOF) {
		   switch(unvis(&out, ch, &state, 0)) {
		   case	0:
		   case	UNVIS_NOCHAR:
		   case	UNVIS_VALID:
			   goto	again;
		   case	UNVIS_SYNBAD:
			   errx(EXIT_FAILURE, "Bad character sequence!");
	   if (unvis(&out, '\0', &state, UNVIS_END) == UNVIS_VALID)

     The functions strunvis(), strnunvis(), strunvisx(), and strnunvisx() will
     return -1 on error	and set	errno to:

     [EINVAL]		An invalid escape sequence was detected, or the	de-
			coder is in an unknown state.

     In	addition the functions strnunvis() and strnunvisx() will can also set
     errno on error to:

     [ENOSPC]		Not enough space to perform the	conversion.

     unvis(1), vis(1), vis(3)

     R.	Fielding, Relative Uniform Resource Locators, RFC1808.

     The unvis() function first	appeared in 4.4BSD.  The strnunvis() and
     strnunvisx() functions appeared in	NetBSD 6.0 and FreeBSD 9.2.

     The names VIS_HTTP1808 and	VIS_HTTP1866 are wrong.	 Percent-encoding was
     defined in	RFC 1738, the original RFC for URL.  RFC 1866 defines HTML
     2.0, an application of SGML, from which it	inherits concepts of numeric
     character references and entity references.

BSD				March 12, 2011				   BSD


Want to link to this manual page? Use this URL:

home | help