UNVIS(3)                       Library Routines                       UNVIS(3)

NAME

       unvis, strunvis - decode a visual representation of characters

SYNOPSIS

       #include <vis.h>

       int unvis (char *cp, int c, int *astate, int flag)
       int strunvis (char *dst, const char *src)

DESCRIPTION

       The  unvis and strunvis functions are used to decode a visual represen‐
       tation 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 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 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 initial‐
       ize  an  integer  to zero.  Call unvis with each successive byte, along
       with a pointer to this integer, and a pointer to a destination  charac‐
       ter.   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  be‐
              tween 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 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 first appeared in 4.4BSD.



GNO                             3 January 1999                        UNVIS(3)