NAME

     unvis, strunvis, strnunvis, strunvisx, strnunvisx — decode a visual representation of
     characters

LIBRARY

     Utility functions from BSD systems (libbsd, -lbsd)

SYNOPSIS

     #include <vis.h>
     (See libbsd(7) for include usage.)

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

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

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

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

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

DESCRIPTION

     The unvis(), strunvis() and strunvisx() functions are used to decode a visual representation
     of characters, as produced by the vis(3bsd) 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 VIS_MIMESTYLE.

     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) {
           again:
                   switch(unvis(&out, ch, &state, 0)) {
                   case 0:
                   case UNVIS_NOCHAR:
                           break;
                   case UNVIS_VALID:
                           (void)putchar(out);
                           break;
                   case UNVIS_VALIDPUSH:
                           (void)putchar(out);
                           goto again;
                   case UNVIS_SYNBAD:
                           errx(EXIT_FAILURE, "Bad character sequence!");
                   }
           }
           if (unvis(&out, '\0', &state, UNVIS_END) == UNVIS_VALID)
                   (void)putchar(out);

ERRORS

     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.

SEE ALSO

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

     R. Fielding, Relative Uniform Resource Locators, RFC1808.

HISTORY

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

BUGS

     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.