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The
and functions are used to decode a visual representation of characters,
as produced by the function, back into the original form. Unvis is called
with successive characters in until a valid sequence is recognized, at
which time the decoded character is available at the character pointed
to by Strunvis decodes the characters pointed to by into the buffer pointed
to by The function simply copies to decoding any escape sequences
along the way, and returns the number of characters placed into or -1 if
an invalid escape sequence was detected. The size of should be equal to
the size of (that is, no expansion takes place during decoding). The
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 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 with each
successive byte, along with a pointer to this integer, and a pointer to
a destination character. The function has several return codes that must
be handled properly. They are: Another character is necessary; nothing
has been recognized yet. A valid character has been recognized and is available
at the location pointed to by cp. 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. A valid sequence was detected,
but no character was produced. This return code is necessary to indicate
a logical break between characters. 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 one more
time with flag set to to extract any remaining character (the character
passed in is ignored). The following code fragment illustrates a proper
use of 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)
;
The function first appeared in 4.4BSD.
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