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UNVIS(3)                    BSD Programmer's Manual                   UNVIS(3)

     unvis, strunvis - decode a visual representation of characters

     #include <&lt;vis.h>&gt;

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

     strunvis(char *dst, char *src);

     The unvis() and strunvis() functions are used to decode a visual repre-
     sentation 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 charac-
     ters 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 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.

     UNVIS_NOCHAR     A valid sequence was detected, but no character was pro-
                      duced.  This return code is necessary to indicate a log-
                      ical 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 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:
                           (void) putchar(out);
                   case UNVIS_VALIDPUSH:
                           (void) putchar(out);
                           goto again;
                   case UNVIS_SYNBAD:
                           (void)fprintf(stderr, "bad sequence!0);
           if (unvis(&out, (char)0, &state, UNVIS_END) == UNVIS_VALID)
                   (void) putchar(out);


     The unvis function first appeared in 4.4BSD.

4.4BSD                         December 11, 1993                             2