NAME

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

LIBRARY

     library “libbsd”

SYNOPSIS

     #include <bsd/vis.h>

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

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

     ssize_t
     strnunvis(char *dst, char *src, size_t size);

DESCRIPTION

     The unvis(), strunvis() and strnunvis() 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.

     strnunvis() decodes the characters pointed to by src into the buffer pointed to by dst,
     writing a maximum of size bytes.  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).  strunvis() terminates the
     destination string with a trailing NUL byte; strnunvis() does so if size is larger than 0.

     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.

RETURN VALUES

     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 strunvis() function returns the number of bytes written (not counting the trailing NUL
     byte) or -1 if an error occurred.

     The strnunvis() function returns the number of bytes (not counting the trailing NUL byte)
     that would be needed to fully convert the input string, or -1 if an error occurred.

EXAMPLES

     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

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

HISTORY

     The unvis() function first appeared in 4.4BSD.