unvis(3)            UNIX Programmer's Manual             unvis(3)



   NAME
        unvis , strunvis - decode a visual representation of charac-
        ters


SYNOPSIS
     #include <vis.h>

     _i_n_t
     unvis(_u__c_h_a_r *_c_p, _u__c_h_a_r _c, _i_n_t *_a_s_t_a_t_e, _i_n_t _f_l_a_g)

     _i_n_t
     strunvis(_c_h_a_r *_d_s_t, _c_h_a_r *_s_r_c)

DESCRIPTION
     The unvis() and strunvis() 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 _c_p. Strunvis decodes the
     characters pointed to by _s_r_c into the buffer pointed to by _d_s_t.

     The strunvis() function simply copies _s_r_c to _d_s_t, decoding any escape se-
     quences along the way, and returns the number of characters placed into
     _d_s_t, or -1 if an invalid escape sequence was detected.  The size of _d_s_t
     should be equal to the size of _s_r_c (that is, no expansion takes place
     during decoding).

     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 an 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-





Printed 12/12/92                                                1






unvis(3)            UNIX Programmer's Manual             unvis(3)



                      ical break between characters.

     UNVIS_SYNBAD     An invalid esacpe 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 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!0);
                   exit(1);
                   }
           }
           if (unvis(&out, (char)0, &state, UNVIS_END) == UNVIS_VALID)
                   (void) putchar(out);

SEE ALSO
     vis(1)

HISTORY
     The unvis() function is currently under development.















Printed 12/12/92                                                2