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.Dd December 11, 1993
.Dt UNVIS 3
.Os
.Sh NAME
.Nm unvis ,
.Nm strunvis
.Nd decode a visual representation of characters
.Sh SYNOPSIS
.Fd #include <vis.h>
.Ft int 
.Fn unvis "char *cp" "char c" "int *astate" "int flag"
.Ft int 
.Fn strunvis "char *dst" "char *src"
.Sh DESCRIPTION
The
.Fn unvis
and
.Fn strunvis
functions
are used to decode a visual representation of characters, as produced
by the
.Xr vis 3
function, back into
the original form.  Unvis is called with successive characters in
.Ar c 
until a valid
sequence is recognized, at which time the decoded character is
available at the character pointed to by
.Ar cp .
Strunvis decodes the
characters pointed to by
.Ar src
into the buffer pointed to by
.Ar dst .
.Pp
The
.Fn strunvis
function
simply copies
.Ar src
to
.Ar dst ,
decoding any escape sequences along the way,
and returns the number of characters placed into
.Ar dst ,
or \-1 if an
invalid escape sequence was detected.  The size of
.Ar dst
should be
equal to the size of
.Ar src
(that is, no expansion takes place during
decoding).
.Pp
The
.Fn 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
.Fn 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
.Fn unvis
with each successive byte, along with a pointer
to this integer, and a pointer to a destination character.
The
.Xr unvis
function
has several return codes that must be handled properly.  They are:
.Bl -tag -width UNVIS_VALIDPUSH
.It Li \&0 (zero)
Another character is necessary; nothing has been recognized yet.
.It Dv  UNVIS_VALID	
A valid character has been recognized and is available at the location
pointed to by cp.
.It Dv  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.
.It Dv  UNVIS_NOCHAR
A valid sequence was detected, but no character was produced.  This
return code is necessary to indicate a logical break between characters.
.It Dv  UNVIS_SYNBAD
An invalid escape sequence was detected, or the decoder is in an
unknown state.  The decoder is placed into the starting state.
.El
.Pp
When all bytes in the stream have been processed, call
.Fn unvis
one more time with flag set to
.Dv UNVIS_END
to extract any remaining character (the character passed in is ignored).
.Pp
The following code fragment illustrates a proper use of
.Fn unvis .
.Bd -literal -offset indent
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);
.Ed
.Sh SEE ALSO
.Xr unvis 1 ,
.Xr vis 1 ,
.Xr vis 3
.Sh HISTORY
The
.Nm unvis
function first appeared in
.Bx 4.4 .