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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);

     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
     encoded with.  Currently, the supported flags are: VIS_HTTPSTYLE and

     The unvis() function implements a state machine that can be used to
     decode 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
                      recognized 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
                      produced.  This return code is necessary to indicate a
                      logical 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:
                   case UNVIS_VALIDPUSH:
                           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
                        decoder 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.

     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