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.
The
unvis() function 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.
The
strunvis() function 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 supported flags are:
VIS_HTTPSTYLE and
VIS_MIMESTYLE.
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 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
flag argument is also used to specify the encoding style of the source. If set to
VIS_HTTPSTYLE,
unvis() will decode URI strings as specified in RFC 1808. If set to
VIS_MIMESTYLE,
unvis() will decode MIME Quoted-Printable strings as specified in RFC 2045.
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:
errx(EXIT_FAILURE, "Bad character sequence!");
}
}
if (unvis(&out, '\0', &state, UNVIS_END) == UNVIS_VALID)
(void)putchar(out);