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UNVIS(3)		 BSD Library Functions Manual		      UNVIS(3)

     unvis, strunvis --	decode a visual	representation 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);

     strunvisx(char *dst, 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.  Unvis 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.	Strunvis de-
     codes the characters pointed to by	src into the buffer pointed to by dst.

     The strunvis() function simply copies src to dst, decoding	any escape se-
     quences 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 only supported	flag is	VIS_HTTPSTYLE.

     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, unvis() will decode URI strings as
     specified in RFC 1808.

     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:
			   (void) putchar(out);
			   (void) putchar(out);
			   goto	again;
		   case	UNVIS_SYNBAD:
			   (void)fprintf(stderr, "bad sequence!\n");
	   if (unvis(&out, (char)0, &state, UNVIS_END) == UNVIS_VALID)
		   (void) putchar(out);

     vis(1), vis(3)

     R.	Fielding, Relative Uniform Resource Locators, RFC1808.

     The unvis() function first	appeared in 4.4BSD.

BSD			       December	11, 1993			   BSD


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