SYNOPSIS

     #include <vis.h>

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

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

     int
     strunvisx(char *dst, 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(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
     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 only supported flag is VIS_HTTPSTYLE.

     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 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.

     specified in RFC 1808.

     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:
                           (void)fprintf(stderr, "bad sequence!\n");
                   exit(1);
                   }
           }
           if (unvis(&out, (char)0, &state, UNVIS_END) == UNVIS_VALID)
                   (void) putchar(out);

SEE ALSO

     vis(1), vis(3)

     R. Fielding, Relative Uniform Resource Locators, RFC1808.

HISTORY

     The unvis() function first appeared in 4.4BSD.

BSD                            December 11, 1993                           BSD